I - P

I/E region Abbreviation for integration-excision region.

ICSI See: intracytoplasmic sperm injection.

identical twin See: monozygotic twin.

idiogram See: karyogram.

idiotype An identifying property or characteristic of an item or system. 1.A plant form expected on physiological grounds to represent an optimal type for the environment in which the plant is to be grown. 2. A classification of antibody molecules according to the antigenicity of the variable regions. Each idiotype is unique to a particular immunoglobulin raised to a particular antigen.

IgA/Igd/IgG/IgE/IgM See: antibody class.

IGS Abbreviation for 1. internal guide sequence. 2. intergenic spacer.

imaginal disc A mass of cells in the larvae of Drosophila melanogaster and other holometabolous insects that gives rise to particular adult organs, such as antennae, eyes or wings.

imbibition 1. The absorption of liquids or vapours into the ultramicroscopic spaces or pores found in materials. 2. The initial water uptake by seeds prior to germination.

immediate early gene A viral gene that is expressed promptly after infection.

immobilized cells Cells entrapped in matrices such as alginate, polyacrylamide and agarose, for use in membrane and filter bioreactors.

immortalization The genetic transformation of a cell type into a cell line which can proliferate indefinitely.

immortalizing oncogene A gene that, upon transfection, enables a primary cell to grow indefinitely in culture.

immune response The processes, including the synthesis of antibodies, that are used by vertebrates to respond to the presence of a foreign antigen. See: primary immune response; secondary immune response.

immunity The lack of susceptibility of an animal or plant to infection by a particular pathogen, or to the harmful effects of their toxins.

immunization The production of immunity in an individual by artificial means. Active immunization involves the introduction, either orally or by infection, of specially treated bacteria, viruses or their toxins so as to stimulate the production of antibodies. See: passive immunity.

immunoaffinity chromatography A purification technique in which an antibody is bound to a matrix and is used to isolate a protein from a complex mixture. See: affinity chromatography.

immunoassay An detection system for a particular molecule, which exploits the specific binding of an antibody raised against it. For measurement, the antibody can incorporate a radioactive or fluorescent label, or be linked to an enzyme which catalyses an easily monitored reaction such as a change in colour (see: ELISA). Synonym: immunodiagnostics.

immunochemical control See: immunotherapy.

immunodiagnostics See: immunoassay.

immunogen See: antigen.

immunogenicity The ability to elicit an immune response.

immunoglobulin See: antibody.

immunoprophylaxis The process of active or passive immunization. Active immunization with vaccines leads to long-term protection through mobilizing the organism's own immune system. Passive immunization is achieved by injection of antibody against a specific pathogen obtained either by fractionation of blood from an individual previously infected with the pathogen, or by monoclonal antibody technology. See: adoptive immunization, passive immunity.

immunosensor A biosensor having an antibody as the biological element.

immunosuppression The suppression of the immune response. Necessary following organ transplants from a genetically different donor in order to prevent the host rejecting the grafted organ.

immunosuppressor A substance, an agent or a condition that prevents or diminishes the immune response.

immunotherapy The use of an antibody or a fusion protein containing the antigen binding site of an antibody to cure a disease or enhance the well-being of a patient. Synonym: immunochemical control.

immunotoxin Protein drugs consisting of an antibody joined to a toxin molecule. Made by either chemically linking the molecules, or by fusing the genes for the toxin and the antibody, and expressing a fusion protein. The antibody portion of the molecule targets the toxin.

impeller An agitator that is used for mixing the contents of a bioreactor.

in silico In a computer file. In the present context, the use of data bases of DNA and protein sequence to help answer biological questions. This is growing area of biology as the amount of genomics and proteomics data continues to grow. See: bio-informatics.

in situ In the natural place or in the original place. 1. Experimental treatments performed on cells or tissue rather than on extracts from them. 2. Assays or manipulations performed with intact tissues.

in situ colony hybridization A procedure for screening bacterial colonies or plaques growing on plates or membranes for the presence of specific DNA sequences by the hybridization of nucleic acid probes to the DNA molecules present in these colonies or plaques. Synonym: in situ plaque hybridization.

in-situ conservation The conservation of ecosystems and natural habitats and the maintenance and recovery of viable populations of species in their natural surroundings and, in the case of domesticated or cultivated species, in the surroundings where they have developed their distinctive properties.

in situ hybridization The visualization of in vivo location of macromolecules (particularly polynucleotides and polypeptides) by the histological staining of tissue sections or cytological preparations via labelled probes/antibodies.

in situ plaque hybridization See: in situ colony hybridization.

in vitro Outside the organism, or in an artificial environment. Applied for example to cells, tissues or organs cultured in glass or plastic containers.

in vitro embryo production (Abbreviation: IVEP). The combination of ovum pickup, in vitro maturation of ova, and in vitro fertilization. A potential means of overcoming the variability between donors in number of ova collected in embryo-transfer programmes.

in vitro fertilization (Abbreviation: IVF). A widely used technique in human and animal science, whereby the egg is fertilized with sperm outside the body before re-implanting into the uterus.

in vitro maturation (Abbreviation: IVM). Culture of immature ova in the laboratory, usually until they are ready for in vitro fertilization.

in vitro mutagenesis See: directed mutagenesis.

in vitro transcription The cell-free synthesis from DNA of RNA in the test tube. Synonym: cell-free transcription.

in vitro translation The synthesis of proteins from isolated mRNA molecules in the test-tube. Synonym: cell-free translation.

in vivo The natural conditions in which organisms reside. Refers to biological processes that take place within a living organism or cell under normal conditions.

in vivo gene therapy The delivery of a gene or genes to a tissue or organ of a complete living individual to alleviate a genetic disorder.

inactivated agent A virus, bacterium or other organism that has been treated to prevent it from causing a disease. See: attenuated vaccine.

inbred line The product of inbreeding, i.e. the intercrossing of individuals that have ancestors in common. In plants and laboratory animals, it refers to populations resulting from at least 6 generations of selfing or 20 generations of brother-sister mating, so that they have become, for all practical purposes, completely homozygous. In farm animals, the term is sometimes used to describe populations that have resulted from several generations of the mating of close relatives, without having reached complete homozygosity.

inbreeding Matings between individuals that have one or more ancestors in common, the extreme condition being self-fertilization, which occurs naturally in many plants and some primitive animals. Synonym: endogamy.

inbreeding depression The reduction in vigour over generations of inbreeding. This affects species which are normally outbreeding and highly heterozygous. See: hybrid vigour.

inclusion body A protein that is overproduced in a recombinant bacterium and forms a crystalline structure inside the bacterial cell.

incompatibility 1. Genetically or physiologically determined prevention of intermating. 2. A physiological interaction resulting in graft rejection or failure. 3. A function of a related group of plasmids. Incompatible plasmids share similar replication functions, and this leads to the exclusion of one or the other plasmid if they are present in the same cell. Plasmids belonging to one incompatibility group are very closely related.

incompatibility group Plasmids must belong to different incompatibility groups to co-exist within the same cell. A plasmid cloning vector should always belong to an incompatibility group different from that of the host bacterium's endogenous plasmids.

incomplete digest See: partial digest.

incomplete dominance A gene action in which heterozygotes have a phenotype that is different from either homozygote, and is usually intermediate between them.

incomplete penetrance Where the phenotype does not allow perfect prediction of the genotype as a result of interference in gene expression by the environment.

incubation 1. The hatching of eggs by means of heat, either natural or artificial. 2. Period between infection and appearance of symptoms induced by a pathogen. 3. The culture of cells and organisms.

incubator An apparatus in which environmental conditions (light, photoperiod, temperature, humidity, etc.) are fully controlled; used for hatching eggs, multiplying micro-organisms, culturing plants, etc.

indehiscent Describing a fruit or fruiting body that does not open to release its seeds or spores when ripe.

independent assortment The random distribution during meiosis of alleles (at different genes) to the gametes that is the case when the genes in question are located on different chromosomes or are unlinked on the same chromosome. See: linkage.

indeterminate growth The condition in which the terminal bud persists and produces successive lateral branches over an indefinite period. Opposite: determinate growth.

indirect embryogenesis Plant embryo formation on callus tissues derived from explants, including zygotic or somatic embryos and seedlings. Opposite: direct embryogenesis.

indirect organogenesis Plant organ formation on callus tissues derived from explants. Opposite: direct organogenesis.

inducer A low-molecular-weight compound or a physical agent that associates with a repressor protein to produce a complex that can no longer bind to the operator. Thus, the presence of the inducer turns on the expression of the gene(s) controlled by the operator.

inducible A gene or gene product whose transcription or synthesis is increased by exposure of the cells to an inducer or to a condition, e.g. heat. Opposite: constitutive.

inducible enzyme An enzyme that is synthesized only in the presence of the substrate that acts as an inducer.

inducible gene A gene that is expressed only in the presence of a specific metabolite, the inducer.

inducible promoter The activation of a promoter in response to either the presence of a particular compound, i.e. the inducer, or to a defined external condition, e.g. elevated temperature.

induction The act or process of causing some specific effect to occur; for example the transcription of a specific gene or operon, or the production of a protein by an organism after it is exposed to a specific stimulus.

induction media 1. Media used to induce the formation of organs or other structures. 2. Media causing variation or mutation in the tissues exposed to it.

inembryonation See: artificial inembryonation.

infection The successful colonization of any living organism by a pathogen.

infectious agent Synonym of pathogen.

infiltrate The entry of liquid into pores or other spaces.

inflorescence The flowers of a plant, and the way those flowers are arranged.

inheritance The transmission of genes and phenotypes from generation to generation.

inhibitor 1. Any substance or object that retards a chemical reaction. 2. A metabolite or modifier gene that interferes with a reaction or with the expression of another gene.

initial Cells in a meristem that remain permanently capable of differentiation, and which develop into tissues of particular structure and function.

initiation Causing something to start. 1. Early steps or stages of a tissue culture process. 2. Early stages of biosynthesis.

initiation codon See: start codon.

initiation factor Soluble protein required for the initiation of translation.

inoculate Deliberately introduce, in contrast to contamination. 1. In bacteriology, tissue culture, etc., placing an inoculum into (or onto) medium to initiate a culture. 2. In immunology, to carry out immunization. 3. In plant pathology, application of pathogen spores etc. on to plants under conditions where infection should result in the absence of resistance.

inoculation cabinet Small room or cabinet for inoculation (of tissue or micro-organism cultures) operations, often with a current of sterile air to carry contaminants away from the work area.

inoculum (pl.: inocula) 1. A small piece of tissue cut from callus, or an explant from a tissue or organ, or a small amount of cell material from a suspension culture, transferred into fresh medium for continued growth of the culture. See: minimum inoculum size. 2. Microbial spores or parts (such as mycelium). 3. Vaccine.

inorganic compound Historically, chemicals that could not be derived from living processes. In modern usage, chemicals that do not contain carbon, although carbonates and a few other simple carbon compounds are generally regarded as inorganic.

inositol A cyclic acid (hexahydroxycyclohexane) that is a constituent of certain cell phosphoglycerides. It is a nutrient frequently referred to as a "vitamin" in plant tissue culture. Also acts as a growth factor in some animals and micro-organisms.

inositol lipid A membrane-anchored phospholipid that transduces hormonal signals by stimulating the release of any of several chemical messengers.

insecticide A substance that kills insects.

insert 1. To incorporate a DNA molecule into a cloning vector; also used as a noun to describe such a DNA molecule. 2. To introduce a gene or gene construct into a new genomic site or into a new genome.

insertion element Generic term for DNA sequences found in bacteria capable of genome insertion. Postulated to be responsible for site-specific phage and plasmid integration. Synonym: insertion sequence.

insertion mutation Changes in the base sequence of a DNA molecule resulting from the random integration of DNA from another source. See: mutation.

insertion sequence See: insertion element.

insertion site 1. A unique restriction site in a vector DNA molecule into which foreign DNA can readily be inserted. This is achieved by treating both the vector and the insert with the relevant restriction endonuclease and then ligating the two different molecules, both having the same sticky ends. Synonym: cloning site. 2. The position of integration of a transposon.

instability A lack of consistent phenotype, usually as a result of uncontrolled genetic changes. These may be due to transposon activity, or in cell lines, to changes in karyotype.

insulin A peptide hormone secreted by the Langerhans islets of the pancreas, and that regulates the level of sugar in the blood.

integrating vector A vector that is designed to integrate cloned DNA into the host's chromosomal DNA.

integration The recombination process which inserts a small DNA molecule (usually by homologous recombination) into a larger one. If the molecules are circular, integration involves only a single crossing-over; if linear, then two crossings-over are required.

integration-excision region (Abbreviation: I/E). The portion of bacteriophage lambda (ë) DNA that enables ë-DNA to be inserted into a specific site in the E. coli bacteriophage lambda chromosome or excised from this site.

integument One of the layers that enclosed the ovule, and is the precursor of the seed coat.

intellectual property rights (Abbreviation: IPR). The legal framework, which includes patenting and plant variety protection, by which inventors control the commercial application of their work.

intensifying screen A plastic sheet impregnated with a rare-earth compound, such as calcium tungstate, which reacts to radiation by emitting light. When placed on one side of a piece of X-ray film with a radioactive sample on the other side, the intensifying screen will capture some of the radioactive energy which has passed through the film, exposing the X-ray film and so enhancing the sensitivity of the detection. Often used in Southern and northern blotting procedures.

intercalary 1. Meristematic tissue or growth not restricted to the apex of an organ, i.e. growth at nodes. 2. Referring to internal segments of a chromosomes (i.e. not at the ends).

intercalary growth A pattern of stem elongation typical of grasses. Elongation proceeds from the lower internodes to the upper internodes through the differentiation of meristematic tissue at the base of each internode.

intercalating agent A chemical capable of inserting between adjacent base pairs in a double-stranded nucleic acid. A prominent example is ethidium bromide.

intercellular space The pore space between cells, especially typical of leaf tissues.

interfascicular cambium Cambium that arises between vascular bundles.

interference The effect of one crossing over event in altering the probability of another crossing over event occurring at a nearby location. This probability can be either increased (positive interference) or decreased (negative interference), but the latter is the more usual.

interferon One of a group of small proteins synthesized by certain T cells of vertebrates, which inhibit virus replication. There are three types of interferon in humans. See: cytokine.

intergeneric cross A hybrid made between parents belonging to two different genera.

intergenic regions Non-coding DNA located between genes; this comprises a variable but considerable proportion of all eukaryotic genomic DNA, and its function is largely unknown.

intergenic spacer (Abbreviation: IGS). Non-coding DNA separating tandemly arranged copies of a repeated gene sequence (typically ribosomal DNA). Of particular interest because, unlike the coding sequence itself, the spacers show high levels of interspecific sequence polymorphism, and are thus useful as assays for species identification.

interleukin A group of proteins that transmit signals between immune cells and are necessary for mounting normal immune responses. See: cytokine.

internal guide sequence (Abbreviation: IGS). See: guide sequence.

internal transcribed spacer (Abbreviation: ITS). Non-coding regions separating the individual components of the ribosomal DNA units. These regions show much more sequence polymorphism than the genic regions themselves, and therefore, like the intergenic spacers, are useful a source of genetic markers for the ribosomal DNA locus.

International Undertaking on Plant Genetic Resources The first comprehensive voluntary, international agreement (adopted in 1983) dealing with plant genetic resources for food and agriculture. Designed as an instrument to promote international harmony in matters regarding access to plant genetic resources for food and agriculture. Following extensive negotiations to revise the Undertaking in harmony with the Convention on Biological Diversity, the binding International Treaty on Plant Genetic Resources for Food and Agriculture was adopted by the 2001 FAO Conference.

International Treaty on Plant Genetic Resources for Food and Agriculture The international treaty resulting from the revision of the International Undertaking on Plant Genetic Resources was adopted by the 2001 FAO Conference as a binding international instrument to enter into force after ratification by 40 states. Its objectives are the conservation and sustainable use of plant genetic resources for food and agriculture and equitable sharing of the benefits of this use.

internode The region of a stem between two successive nodes.

interphase The stage in the cell cycle when the cell is not dividing and during part of which DNA replication occurs; it follows telophase of one mitotic division and extends to the beginning of prophase in the next division.

intersex Synonym of hermaphrodite.

inter-simple sequence repeat (Abbreviation: ISSR). A PCR-based molecular marker assay of genomic sequence lying between adjacent microsatellites. Primers carrying, at their 3'-end, sequence complementary to the repeat unit of the microsatellite will amplify this genomic DNA.

interspecific cross A hybrid made between parents belonging to two different species. See: intrageneric cross, intraspecific cross.

intervening sequence See: intron.

intracellular Occurring within a cell.

intracytoplasmic sperm injection (Abbreviation: ICSI). The micro-injection of a single sperm into the cytoplasm of a mature oocyte.

intrageneric Within a genus, such as an intrageneric cross, or intrageneric variation.

intrageneric cross A hybrid made between parents belonging to two species in the same genus. See: intraspecific cross, interspecific cross.

intragenic complementation Occurs when wild type phenotype is restored in an F₁ individual made by crossing two independent mutants, carrying different heteroalleles.

intraspecific Within a species, such as an intraspecific cross, or intraspecific variation.

intraspecific cross A hybrid made between parents belonging to the same species. See: intrageneric cross, interspecific cross.

introgression The introduction of new alleles or gene(s) into a population from an exotic source, usually another species. This is achieved by repeated backcrossing of the initial hybrid in order to eliminate all genetic changes except for the desired new gene(s).

intron A segment of the primary transcript of a eukaryotic gene, removed (before the mature mRNA is translated) in a process known as intron splicing. Some eukaryotic genes contain a large number of introns, which make up the bulk of the DNA sequence of the gene. Introns are also found in genes whose RNA transcripts are not translated, namely eukaryotic rRNA and tRNA genes. In these cases the intron sequence does not appear in the functional RNA molecule. Synonym: intervening sequence.

invasiveness The ability of a plant, particularly a weed, to spread beyond its presently established site, and become established in new locations.

inversion A chromosome re-arrangement, which involves the re-orientation of a segment so that the order of a linear array of genes within it is reversed.

inverted repeat Two sequences of nucleotides occurring in one strand, where, relative to the first sequence, the second has complementary bases but in the inverted order. Under appropriate conditions this allows formation of a hairpin loop in the single strand. See: palindrome.

ion channel A protein integral to a cell membrane, through which selective ion transport occurs.

IPR Abbreviation for intellectual property rights.

IPTG Abbreviation for isopropyl-3-D-thiogalactopyranoside. A synthetic inducer of beta-galactosidase activity in many bacteria. Used in combination with the synthetic chromogenic substrate Xgal to differentiate recombinant from non-recombinant bacterial colonies in cloning strategies using plasmid vectors containing the lacZ gene: blue coloured colonies are produced when ?-galactosidase activity is not disrupted by an insert; but when it is disrupted, the colonies are white. Hence white colonies are indicative of recombinant plasmids, and blue colonies of non-recombinant ones.

irradiation Illumination with electromagnetic radiation, typically of sufficiently high energy (low-wavelength UV or gamma, etc.) to disrupt biological macromolecules and hence induce mutations.

IS element Abbreviation for insertion sequence element. A short (800-1400 nucleotide pairs) DNA sequence found in bacteria that is capable of transposing to a new genomic location; DNA sequences contained within an IS element can be transposed along with the IS itself.

isoallele Multiple similar copies of a gene, usually located at independent positions in the genome, which encode similar gene products and produce the same, or a very similar phenotype. See: allele.

isochromosome A chromosome produced following an error in meiosis, in which the two arms are mirror images of each other. The presence of an isochromosome results in the duplication of all genes present on the originating chromosome arm.

isodiametric Commonly used to describe cells with equal diameters.

iso-electric focusing gel (Abbreviation: IEF gel). A variant of gel electrophoresis, in which macromolecules (usually proteins) are separated on the basis of differing iso-electric point, rather than on the basis of size.

isoenzyme See: isozyme.

isoform 1. A tissue-specific form of a protein. 2. Synonym of isoenzyme.

isogamy Fusion of gametes of similar size and structure.

isogenic A group of individuals that possesses the same genotype, irrespective of their being homozygous or heterozygous.

isogenic stock Strains of organisms that are genetically nearly identical, except with respect to identified genes. Generally produced by repeated backcrossing, or by transformation.

isolating mechanism The properties of an organism that prevent interbreeding (and therefore exchange of genetic material) between members of different species that inhabit the same geographical area.

isolation medium An optimum plant tissue culture medium suitable for explant survival, growth and development.

isomer 1. Structural isomers have the same chemical formula but different structures; e.g. leucine and isoleucine. 2. Stereoisomers are different topological forms of an otherwise single chemical structure, due to changes in bond configurations about some axis or plane of symmetry; eg, D- and L-glucose or cis- and trans-cinnamic acid.

isomerase Any of a class of enzymes that catalyse the re-arrangement of the atoms within a molecule, thereby converting one isomer into another.

iso-osmotic See: isotonic.

isotonic Solutions with the same osmotic potential, as a result of being of the same molar concentration. For protoplasts to avoid losing or gaining water, the medium they are suspended in must be isotonic with them. See: hypertonic, hypotonic, osmosis.

isotope One of two or more forms of an element that differ in the number of neutrons carried by the nucleus. Radioactive isotopes (radio-isotopes) are used as probes in many biochemical analyses.

isozyme A genetic variant of an enzyme. Isozymes for a given enzyme share the same function, but may differ in level of activity, as a result of minor differences in their amino acid sequence. Electrophoretic separation of isozymes has been used to distinguish between individuals and varieties.

ISSR Abbreviation for inter-simple sequence repeat.

ITS Abbreviation for internal transcribed spacer.

IVEP Abbreviation for in vitro embryo production.

IVF Abbreviation for in vitro fertilization.

IVM Abbreviation for in vitro maturation.

J See: joining segment.

Jiffy potä Pots made from wood pulp and peat, commonly used for transplanting tissue culture-derived plants into soil medium.

JIVET Abbreviation for juvenile in vitro embryo technology.

JIVT Abbreviation for juvenile in vitro embryo technology.

joining segment (Abbreviation: J). A small DNA segment that links genes in order to yield a functional gene encoding an immunoglobulin.

jumping gene See: transposable element.

jumping library See: chromosome jumping.

junk DNA See: repetitive DNA.

juvenile hormone A hormone secreted by insects from a pair of endocrine glands close to the brain. Its function is to inhibit metamorphosis so maintaining the larval features.

juvenile in vitro embryo technology (Abbreviation: JIVT or JIVET). A technology involving collection of immature eggs from young animals, their in vitro maturation and fertilization, and the transfer of the resultant embryos into recipient females. The method is designed to achieve rapid generation turnover.

juvenility Early phase of development in which an organism is incapable of sexual reproduction.

kanamycin An antibiotic of the aminoglycoside family that inhibits translation by binding to the ribosomes. Important as a substrate for selection of plant transformants.

kan^r Kanamycin-resistance gene. See: neo^r, selectable marker.

kappa chain One of two classes of antibody light chains. The other is a lambda chain.

karyogamy The fusion of nuclei or nuclear material that occurs at fertilization during sexual reproduction.

karyogram A diagrammatic representation of the full chromosome set of a species, highlighting characteristic physical features of individual chromosomes.

karyokinesis The division of a cell nucleus. See: meiosis; mitosis.

karyotype The chromosome constitution of a cell, an individual, or of a related group of individuals, as defined both by the number and the morphology of the chromosomes, usually in mitotic metaphase; chromosomes arranged in order of length and according to position of centromere; also, the abbreviated formula for the chromosome constitution, such as 47, + 21 for human trisomy-21 (Down's syndrome).

kb Abbreviation for kilobase (of single-stranded nucleic acid).

kbp Abbreviation for kilobase pairs (of double-stranded DNA).

k_cat The catalytic rate constant that characterizes an enzyme-catalysed reaction. The larger the k_cat value, the faster the conversion of substrate into product.

k_cat /K_m The catalytic efficiency of an enzyme-catalysed reaction. The greater the value of k_cat/K_m, the more rapidly and efficiently the substrate is converted into product.

K_d Abbreviation for dissociation constant. Describes the strength of binding (or affinity) between molecules and their ligands. See: avidity.

kDa Abbreviation for kiloDalton. A unit of molecular mass equal to 1000 Dalton.

killer T cell T cells that kill cells displaying recognized antigens.

kilobase (Abbreviation: kb). A length of single-stranded nucleic acid composed of 1000 bases. One kilobase of single-stranded DNA has a mass of about 330 kiloDalton (exact mass depends on base composition).

kilobase pairs (Abbreviation: kbp). A length of double-stranded DNA composed of 1000 base pairs.

kinase An enzyme that catalyses the transfer of a phosphate group from a high energy state (as in ATP) to another molecule.

kinetics Dynamic processes involving motion. Often used as a suffix to indicate studies involving movement or rates of reactions. See: pharmacokinetics, enzyme kinetics

kinetin A cytokinin.

kinetochore Structure at the centromere of eukaryotic chromosomes. The kinetochore consists of inner and outer electron dense plates and a central zone containing repetitive DNA elements. Kinetochores are involved in the control of chromosome movement in cell division.

kinetosome Granular cytoplasmic structure which forms the base of a cilium or flagellum. Synonym: basal body.

kinin A substance promoting cell division. In plant systems, the prefix cyto- has been added (cytokinin) to distinguish it from kinin in animal systems.

Klenow fragment A truncated form of DNA polymerase I from E. coli, used extensively for the production of synthetic DNA molecules as it retains polymerase and 3'-exonuclease activities, but not 5'-exonuclease activity.

K_m A dissociation constant that characterizes the binding of an enzyme to a substrate. The smaller the value of K_m, the tighter the binding of the enzyme to the substrate. Also called the Michaelis constant.

knockout A mutant individual, in which a single functional gene has been replaced by a non-functional form of the gene. Used to understand gene function via the comparison of the phenotypes of wild type and knockouts.

label A compound or atom that is attached to, or incorporated into, another molecule in order to allow detection of the latter's presence. Commonly, labels exploit radioactivity, fluorescence or antigenicity. Synonym: tag.

labelling The process of attaching or inserting a label into a molecule. Most often in the context of nucleic acids or proteins.

lac repressor-lac promoter system See: IPTG.

lactose A disaccharide sugar produced in milk, composed of one unit each of glucose and galactose.

lag phase 1. The state of apparent inactivity preceding a response to a treatment; also called a latent phase. 2. The initial growth phase, during which cell number remains relatively constant, prior to the onset of rapid cell division.

lagging strand The strand of DNA that is synthesized discontinuously during replication (because DNA synthesis can proceed only in the 5'?3' direction). See: Okazaki fragment.

lambda chain One of two classes of antibody light chains. The other is a kappa chain.

lambda phage A bacteriophage that infects E. coli, commonly used as a cloning vector. See: integration-excision region.

lamella A structure, plate or vesicle that is formed by two membranes lying parallel to each other.

lamina Blade or expanded part of a leaf.

laminar air-flow cabinet Cabinet designed for cell or tissue culture manipulations requiring a sterile environment. Achieved by a continuous, non-turbulent flow of filter-sterilized air over the working area. Synonym: laminar air-flow hood.

laminarin A storage polysaccharide of the brown algae.

lampbrush chromosome Large diplotene chromosomes present in oocyte nuclei, and particularly conspicuous in amphibians. These chromosomes have extended regions called loops, which are active sites of transcription. See: diplonema.

landrace In plant genetic resources, an early, cultivated form of a crop species, evolved from a wild population, and generally composed of a heterogeneous mixture of genotypes.

latent agent A pathogen, usually a virus, present in a host organism without producing any symptoms.

latent bud An inactive bud not held back by rest or dormant period, but which may start growth if stimulated.

latent phase See: lag phase.

lateral bud See: axillary bud.

lateral meristem A meristem giving rise to secondary plant tissues, such as the vascular and cork cambia.

lawn A uniform and uninterrupted layer of bacterial growth, typically on agar medium, in which individual colonies cannot be observed.

layering A technique for vegetative propagation, in which new plants produce adventitious roots before being severed from the parent plant.

LCR Abbreviation for ligase chain reaction.

LD₅₀ Abbreviation for lethal dose_50%. The amount of a substance required to kill 50% of the test population. The higher the LD₅₀, the lower the toxicity of the chemical in that specific test.

lead compound A chemical that has demonstrated promising biological activity in preliminary assays.

leader peptide See: signal sequence.

leader sequence A variable length sequence of nucleotides at the 5' end of an mRNA molecule that precedes the AUG initiation codon where translation begins and is not itself translated into protein.

leading strand The strand of DNA that is synthesized continuously during replication.

leaf blade The usually flattened portion of the leaf.

leaf bud cutting A cutting that includes a short section of stem with attached leaf.

leaf margin The edge of a leaf.

leaf primordium A lateral outgrowth from the apical meristem, which will become a leaf when fully developed and expanded.

leaf roll A symptom of some virus diseases, characterized by curling of the leaves. Can also occur as a response to water stress.

leaf scar Mark left on a stem after leaf abscission.

leaflet Expanded leaf-like part of a compound leaf.

leaky mutant A mutant in which the gene product still retains some biological activity.

lectin A group of plant proteins that can bind to specific oligosaccharides on the surface of cells, causing the cells to clump together.

leptonema Stage in meiosis immediately preceding synapsis and post DNA replication, in which the chromosomes appear as single, fine, threadlike structures.

leptotene (adj.) See: leptonema.

lethal allele A mutant form of a gene that, in the homozygous state, is fatal.

lethal gene See: lethal allele.

lethal mutation See: lethal allele.

leukocyte White blood cell, up to 0.02 mm in diameter, of which there are normally 4-11 million per millilitre of human blood. There are several kinds, all involved in the body's defence mechanisms. Granulocytes have granules in their cytoplasm; monocytes ingest and feed on bacteria and other micro-organisms that cause infection; lymphocytes include the B cells that are involved with the production of antibodies.

library A collection of cells, usually bacteria or yeast, that have been transformed with recombinant vectors carrying DNA derived from an unrelated organism. See: cDNA library, expression library, genomic library.

life cycle The sequence of events from a given developmental stage in one generation to the same stage in the following generation. In sexually reproduced organisms, the starting point is the fusion of gametes to form the zygote.

ligand A small molecule (e.g. activators, substrates and inhibitors of enzyme activity) bound to a protein by non-covalent forces; an ion or a molecule that binds to another chemical entity to form a larger complex.

ligase See: DNA ligase.

ligase chain reaction (Abbreviation: LCR). A technique for the detection and amplification of target DNA sequences. Two oligonucleotides are synthesized which between them are complementary to the entire target sequence, one to the 5'-side and one to the 3'-side. If the target sequence is present in the DNA sample under examination, the oligonucleotides will bind to it with their ends abutting in the centre, and a heat-stable ligase will join them into a complete polynucleotide. No ligation occurs if the target sequence is absent or if the match between synthetic oligonucleotides and target sequence is imperfect in the region where they abut. At a high temperature, the new polynucleotide dissociates from the original DNA template, and upon cooling, it and the original DNA serve as templates for a second cycle of hybridization, ligation and thermal dissociation. At each cycle there is a doubling of the number of new complete polynucleotides.

ligate, ligation The joining of two linear double-stranded DNA fragments by the formation of phosphodiester bonds.

lignification The thickening and strengthening of a plant cell wall with lignin.

lignin A group of high-molecular-weight amorphous polymers of phenylpropanoid compounds, giving strength to certain tissues. A major component of wood.

lignocellulose The combination of lignin, hemicellulose and cellulose that forms the structural framework of plant cell walls.

LINE Abbreviation for long interspersed nuclear element.

lineage A group of individuals, related by common descent, e.g. an in vitro cell line derived from a single cell.

linear phase The growth phase during cell culture when cell number increases arithmetically. The linear phase follows a period of exponential growth.

linearized vector A covalently closed circular DNA vector (typically a plasmid) which has been opened by restriction digestion to convert it to a linear molecule. In molecular cloning, DNA to be cloned is mixed with the linearized vector, and treated with ligase to join and recircularize the resulting hybrid molecule.

linkage The tendency of a set of genes to be inherited together more often than would be expected if they were assorting independently. exists between two genes when they are located sufficiently close to one another on the same chromosome that a proportion of gametes is produced without crossing-over occurring between them.

linkage disequilibrium See: gametic phase disequilibrium.

linkage equilibrium See: gametic phase equilibrium.

linkage map A linear or circular diagram that shows the relative positions of genes on a chromosome as determined by recombination fraction. See: genetic map.

linked gene, linked marker A gene or marker that is linked to another gene or marker.

linker A synthetic double-stranded oligonucleotide that carries the recognition sequence for one or more restriction endonucleases. The ligation of a linker to each end of a DNA fragment facilitates the preparation of the fragment for cloning into a vector. See: polylinker.

lipase A class of enzymes which break down lipids into their component fatty acids and glycerol. Lipases used in biotechnology are generally digestive, with a role in the break-down of fats in food into their components, so that these can be used to make other materials.

lipid Any of a group of fats or fat-like compounds insoluble in water and soluble in fat solvents.

lipofection Delivery into eukaryotic cells of DNA, RNA or other compounds that have been encapsulated in liposomes.

lipopolysaccharide (Abbreviation: LPS). A compound containing lipid bound to a polysaccharide; often a component of microbial cells walls.

liposome A synthetic microscopic spherical structure consisting of a phospholipid bilayer membrane containing a user-defined aqueous solution. Liposomes can be used to transport relatively toxic drugs into diseased cells, where they can exert their maximum effect. DNA molecules may be entrapped in, or bound to the surface of, the vesicles, and subsequent fusion of the liposome with the cell membrane will deliver the DNA into the cell. Liposomes have been used to develop an efficient transfection procedure for Streptomyces bacteria.

liquefaction Enzymatic digestion (often by alpha-amylase) of gelatinized starch to form lower molecular weight polysaccharides.

liquid medium Culture solution, without a solidifying agent, for in vitro cell growth.

liquid membrane Thin films made up of liquids (as opposed to solids) which are stable in another liquid (usually water). Thus the liquid must not dissolve in the water, but nevertheless must be prevented from collapsing into small droplets.

liquid nitrogen Nitrogen gas condensed to a liquid with a boiling point of about -196 °C. Commonly used as a medium for long-term storage of biological materials. See: cryopreservation.

litmus paper A pH indicator paper. It turns red in acidic and blue in alkaline solutions.

live recombinant vaccine A vaccine created by the expression of a pathogen antigen in a non-pathogenic organism.

live vaccine A living, non-virulent form of a pathogenic micro-organism or virus used to elicit an antibody response for the protection against infection by a virulent form of the same pathogen.

living modified organism (Abbreviation: LMO). "Living organism that possess a novel combination of genetic material obtained through the use of modern biotechnology" (Convention on Biological Diversity). Synonym of GMO, but restricted to organisms that can endanger biological diversity.

LMO Abbreviation for living modified organism

locus (pl.: loci) A site on a chromosome.

lod score The logarithm of the odds of linkage between two loci. Used to measure the statistical support for linkage.

logarithmic phase The growth phase in cell culture, during which cell number doubles every 20-30 minutes. Synonym: exponential phase.

log phase Abbreviation for logarithmic phase.

long interspersed nuclear element (Abbreviation: LINE). Families of common DNA elements, of average length 6.5 kb, which are dispersed at numerous locations within the genome. The human genome contains over 500,000 LINEs (representing ca. 16% of the genome). They appear to represent degenerate copies of transposable elements. See: SINE.

long template A DNA strand, synthesized during PCR, which has a primer sequence at one end but is extended beyond the site that is complementary to the second primer at the other end.

long terminal repeat (Abbreviation: LTR). A characteristic sequence of nucleotides that occurs at each end of a retrovirus element that has become integrated into the host genome. Involved in the integration process.

long-day plant Plants requiring a period of short nights before the switch from vegetative to reproductive growth can be initiated. See: Short-day plant.

loop bioreactor Fermenters in which material is cycled between a bulk tank and a smaller tank or loop of pipes. The circulation helps to mix the materials and to ensure that gas injected into the fermenter is well distributed in the liquid. Particularly useful for photosynthetic fermentations, where the photosynthesizing organisms are passed through a system of many small transparent pipes, which allow the access of light.

LPS Abbreviation for lipopolysaccharide.

LTR Abbreviation for long terminal repeat.

luteinizing hormone A pituitary hormone which causes growth of the yellow body of the ovary and also stimulates activity of the interstitial cells of the testis.

luxury consumption Nutrient absorption by an organism in excess of that required for optimum growth and productivity.

lyase Any of a class of enzymes that catalyse either the cleavage of a double bond and the addition of new groups to a substrate, or the formation of a double bond.

lymphocyte White blood cells that are important components of the immune system of vertebrates. See: B cell, T cell.

lymphokine Generic name for proteins that are released by lymphocytes to act on other cells involved in the immune response. The term includes interleukins and interferons. A sub-class of cytokines. See: monokine.

lymphoma Cancer originating in the lymph nodes, spleen and other lympho-reticular sites.

lyophilize See: freeze-drying.

lysis The destruction or breakage of cells either by viruses or by chemical or physical treatment.

lysogen A bacterial cell whose chromosome contains integrated bacteriophage DNA.

lysogenic Bacteria or bacteriophages undergoing lysogeny.

lysogenic bacterium Bacterium harbouring temperate (non-virulent, lysogenic) bacteriophages.

lysogeny A condition in which a bacteriophage genome (pro-phage) survives within a host bacterium, either as part of the host chromosome or as part of an extrachromosomal element, and does not initiate lysis.

lysosome A membrane-bound sac within the cytoplasm of animal cells that contains enzymes responsible for the digestion of material in food vacuoles, the dissolution of foreign particles entering the cell and, on the death of the cell, the breaking down of all cell structures. The digestive system of the cell.

lysozyme A naturally occurring enzyme extracted from egg white protein and other animal and plant sources, which attacks the cell wall of gram-positive bacteria leading to cell lysis and death.

lytic A phase of the virus life cycle during which the virus replicates within the host cell, releasing a new generation of viruses when the infected cell undergoes lysis.

lytic cycle The steps in viral production that lead to cell lysis.

M13 A single-stranded DNA bacteriophage used as a vector for DNA sequencing.

M13 strand The single-stranded DNA molecule that is present in the infective form of bacteriophage M13.

MAAP Abbreviation for multiple arbitrary amplicon profiling.

mAb Abbreviation for monoclonal antibody.

macerate To disintegrate tissue to disrupt cells. Commonly achieved via mechanical shearing, plasmolysis or enzymatic cell wall degradation.

macromolecule Any high molecular weight molecule. Often used as a synonym for polymers.

macronutrient A major chemical element essential for normal growth and development. In tissue culture media, macronutrients are those required in concentrations above 0.5 millimole/litre.

macrophage Large white blood cells that ingest foreign substances and display on their surfaces antigens which are recognized by other cells of the immune system.

macropropagation Production of plant clones from growing parts.

macrospore See: megaspore.

mad cow disease Colloquial term for bovine spongiform encephalopathy. See proteinaceous infectious particle

MADS box A highly conserved DNA sequence motif found in a large family of transcription factors, most of which play important roles in developmental processes. Most prominently, the MADS box genes known in flowering plants are intimately involved in the control of flower morphogenesis.

magenta A type of plastic container frequently used for plant micropropagation and tissue culture.

major histocompatibility antigen A cell-surface protein or glycoprotein that allows the immune system to distinguish foreign or "non-self" from "self". A better term is histoglobulin. These are the antigens that must be matched between donors and recipients during organ and tissue transplants to prevent rejection.

major histocompatibility complex (Abbreviation: MHC). The large cluster of genes that encode the major histocompatibility antigens in mammals.

malt extract A mixture of organic compounds prepared from malt, used as a culture medium additive.

malting Enzymatic reduction of starch to sugars in germinating grain, used in brewing.

mammary gland The milk-producing organ of female mammals.

management of farm animal genetic resources The sum total of technical, policy and logistical operations involved in understanding (characterization), using and developing (utilization), maintaining (conservation), accessing, and sharing the benefits of animal genetic resources.

mannitol A sugar alcohol widely distributed in plants. Commonly used as a nutrient and osmoticum in suspension media for plant protoplasts.

mannose A hexose component of many polysaccharides, occasionally used as a carbohydrate source in plant tissue culture media.

map 1. verb: to determine the relative positions of loci (genes or DNA sequences) on a chromosome. Linkage maps are obtained from the frequency of recombination between loci. Physical maps are obtained commonly by the use of in situ hybridization of cloned DNA fragments to metaphase chromosomes, or by somatic-cell hybrids or radiation hybrids. 2. noun: a diagram showing the relative position of, and distances between, loci on a chromosome.

map distance The standard measure of genetic distance between loci, expressed in centiMorgans (cM) or map units. Estimated from recombination fraction via a mapping function. For small recombination fractions, map distance in cM equals the recombination fraction in %.

map unit One centiMorgan (1 cM) See: map distance; crossing-over unit.

mapping The construction of a localized (around a gene), or broad-based (whole genome) genetic map. More generally, determining the location of a locus (gene or genetic marker) on a chromosome.

mapping function A mathematical expression relating observed recombination fraction to map distance.

mariculture See: aquaculture.

marker An identifiable DNA sequence that is inherited in Mendelian fashion, and which facilitates the study of inheritance of a trait or a linked gene.

marker gene A gene of known function or known location, used for marker-assisted selection or genetic studies.

marker peptide A portion of fusion protein that facilitates its identification or purification.

marker-assisted introgression The use of DNA markers to increase the speed and efficiency of introgression of a new allele(s) or gene(s) into a breeding population. The markers will be closely linked to the gene(s) in question.

marker-assisted selection (Abbreviation: MAS). The use of DNA markers to improve response to selection in a population. The markers will be closely linked to one or more target loci, which may often be quantitative trait loci.

MAS Abbreviation for marker-assisted selection.

mass selection As practised in plant and animal breeding, the selection ofa number of individuals, on the basis of their individual phenotypes, to interbreed to form the next generation.

maternal effect An effect attributable to a genetic contribution of the female parent of the individual being evaluated.

maternal inheritance Inheritance controlled by non-nuclear genes (e.g. mitochrondria, chloroplast) that are transmitted only through the female line.

matric potential A water potential component, always of negative value, resulting from the presence of solid (often finely divided) surfaces; primarily responsible for water uptake by a dry seed prior to germination.

maturation The formation of gametes or spores.

MCS Abbreviation for multiple cloning site. See: polylinker.

MDA Abbreviation for multiple drop array. See: microdroplet array.

mean In statistics, the arithmetic average; the sum of all measurements or values in a sample divided by the sample size.

media See: culture medium; medium.

median In a set of measurements, the central value above and below which there are an equal number of measurements.

medium (pl.: media) 1. In plant tissue culture, a term for the liquid or solid formulation upon which plant cells, tissues or organs develop. See: culture medium. 2. In general terms, a substrate for plant growth, such as nutrient solution, soil, sand, etc., e.g. potting medium.

medium formulation In tissue culture, the particular constituents for the culture medium, commonly comprising macro- and micro-elements, vitamins, plant hormones, and a carbohydrate source. Some formulations are very specific to the kind of explant or plant species that can be maintained; some are very general.

mega yeast artificial chromosome A yeast artificial chromosome (YAC) which can carry particularly large inserts (up to 1Mbp) - standard YACs typically carry inserts of up to 500kbp.

megabase (Abbreviation: Mb). A length of DNA consisting of 10⁶ bases.

megabase cloning The cloning of large DNA fragments of the order of 1Mb.

megaDalton (Abbreviation: MDa). One megaDalton is equal to 10⁶ Dalton.

megagametophyte The female gametophyte; the plant that develops from a megaspore.

megaspore The female gametophyte in heterosporous plants. Synonym: macrospore.

meiosis The two-stage process in sexual reproduction by which the chromosome number is reduced from the somatic to the haploid number. The first division, in which homologous chromosomes pair and exchange genetic material, is followed by amitotic division. The nucleus divides twice, but the chromosomes only once, generating haploid nuclei, which develop into the gametes (egg and sperm in animals; egg and s in plants).

meiotic analysis The use of patterns of chromosome pairing at meiotic prophase and metaphase to detect relationships between chromosomes, from which can be deduced the relationship between the parents of the organism studied..

meiotic drive Any mechanism that causes a particular allele or chromosome to be over-represented in a population of gametes.

meiotic product See: gamete.

melanin Dark pigment, produced by specialized epidermal cells called melanocytes.

melting temperature (Abbreviation: T_m). The temperature at which a double-stranded DNA molecule denatures into separate single strands. T_m is determined by the length of the molecule and its base composition. DNAs rich in G:C base pairs have higher T_m than A:T rich DNA, because since three hydrogen bonds are formed between G and C, but only two between A and T.

membrane bioreactor A vessel in which cells are cultured on or behind a permeable membrane, which allows the diffusion of nutrients to the cells, but retains the cells themselves. A variation is the hollow-fibre reactor.

memory cell Long-lived B cells and T cells that mediate rapid secondary immune responses to a previously encountered antigen.

Mendel's Laws Two laws summarizing Gregor Mendel's theory of inheritance. The Law of Segregation states that each hereditary characteristic is controlled by two 'factors' (now called alleles), which segregate and pass into separate germ cells. The Law of Independent Assortment states that pairs of 'factors' segregate independently of each other when germ cells are formed. See: independent assortment; linkage.

Mendelian population A natural, interbreeding unit of sexually reproducing plants or animals sharing a common gene pool.

Mendelian segregation Occurs when alleles are inherited according to Mendel's Laws.

mericlinal Refers to a chimera with tissue of one genotype partly surrounded by that of another genotype.

mericloning A propagation method using shoot tips in culture to proliferate multiple buds, which can then be separated, rooted and planted out.

meristele The branch of a stele supplying the leaf.

meristem Undifferentiated but determined plant tissue, in which the cells are capable of active division and differentiation into specialized tissues such as shoots and roots.

meristem culture A tissue culture containing meristematic dome tissue without adjacent leaf primordia or stem tissue. The term may also imply the culture of meristemoidal regions of plants, or meristematic growth in culture.

meristem tip An explant comprising the meristem (meristematic dome) and usually one pair of leaf primordia. Also refers to explants originating from apical meristem tip or lateral or axillary meristem tip.

meristem tip culture Cultures derived from meristem tip explants. Used widely to achieve virus elimination and axillary shoot proliferation, less commonly for callus production.

meristemoid A localized group of callus cells, characterized by their accumulation of starch, RNA and protein, and giving rise to adventitious shoots or roots.

merozygote Partial zygote produced by a process of partial genetic exchange, such as transformation in bacteria.

mesh bioreactor See: filter bioreactor.

mesoderm The middle germ layer that forms in the early animal embryo and gives rise to parts such as bone and connective tissue.

mesophile A micro-organism able to grow in the temperature range 20-50 °C; optimal growth often occurs at about 37 °C. See: psychrophile, thermophile.

mesophyll Leaf parenchyma tissue found between epidermal layers.

messenger RNA Abbreviation: mRNA.

metabolic cell A cell that is not dividing. See: metabolism

metabolism The biochemical processes whereby nutritive material is converted to living matter, or aids in building living matter, or by which complex substances and food are broken down into simple substances.

metabolite A low-molecular-weight biological compound that is usually synthesized enzymically.

metabolomics The large-scale study of the full complement of secondary metabolites produced by a given species in all its tissues and growth stages.

metacentric chromosome A chromosome in which the centromere is located in the middle and, consequently, the chromosome arms are of about equal length.

metal affinity chromatography A chromatographic technique, in which a compound interacting with a specific metal ion can be captured by immobilizing the relevant ion on the column's solid matrix.

metalloenzyme An enzyme which requires the presence of a metal in order to be catalytically active.

metallothionein A protective protein that binds heavy metals such as cadmium and lead.

metaphase Stage of mitosis or meiosis (following prophase and preceding anaphase) during which the chromosomes, or at least the kinetochores, lie in the central plane of the spindle. The stage of maximum chromosome condensation, at which karyotypes are generally described. In the first division of meiosis, metaphase represents the stage at which meiotic analysis is generally performed.

metastasis The spread of cancer cells to previously unaffected organs.

methylation The addition of a methyl group (-CH₃) to a molecule, most commonly in the context of DNA where cytosine and, less often, adenine residues can be modified in this way, sometimes resulting in a change in transcription. See: epigenetic variation.

MHC Abbreviation for major histocompatibility complex.

Michaelis constant See: K_m.

microalgal culture Culture in bioreactors of microalgae (including seaweeds).

micro-array A large set of cloned DNA molecules immobilized as a compact and orderly pattern of sub-microlitre spots onto a solid matrix (typically a glass slide). Used to analyse patterns of gene expression, presence of markers, or nucleotide sequence. The major advantage of micro-arrays is the extent to which the process of genotyping can be automated, thereby enabling large numbers of individuals to be simultaneously genotyped at many loci. A similar approach may be used with other immobilized components for other purposes. Synonym: chip or DNA chip. See: somatic cell hybrid panel, radiation hybrid cell panel

microbe See: micro-organism.

microbial mat Layered microbial populations, usually growing on the surface of a solid medium or on a membrane.

microbody A frequently spherical cellular organelle, bound by a single membrane, 20-60 nm in diameter, and containing a variety of enzymes.

micro-carrier Small particles used as a support material for (particularly mammalian) cells, which are too fragile to be pumped and stirred as bacterial cells are in a large-scale culture.

microdroplet array (Abbreviation: MDA). A technique used to simultaneously evaluate large numbers of media modifications, employing small quantities of medium into which are placed small numbers of cells or protoplasts. These form a monolayer at the droplet meniscus and can easily be examined. Synonyms: multiple drop array, hanging droplet technique

micro-element A nutritional element required in very small quantities.

micro-encapsulation A process of enclosing a substance in very small sealed capsules from which material is released by heat, solution or other means.

micro-environment A small-scale environment in which the conditions (temperature, humidity, pH etc.) are distinct; typically used in connection with the surroundings of a living object.

microfibril Microscopic fibres visible only at the high magnification of the electron microscope.

microgametophyte See: pollen.

micrograft See: shoot-tip graft.

micro-injection The introduction of small amounts of (usually) liquid material (DNA, RNA, enzymes, cytotoxic agents) into a defined tissue or single cell with a fine, microscopic needle.

micro-isolating system Mechanical separation of single cells or protoplasts thus allowing them to proliferate individually.

micronucleus A nucleus, distinct from and smaller than the main nucleus, but lying within the same cell. They usually arise following abnormal meiotic or mitotic telophases, where individual chromosomes or chromosome fragments do not reach the pole.

micronutrient An essential element normally required in cell culture at concentrations under 0.5 millimole/litre.

micro-organism Organism visible only under magnification.

microplast Vesicle produced by subdivision and fragmentation of protoplasts or thin-walled cells.

microprojectile bombardment See: biolistics.

micropropagation Miniaturized in vitro multiplication and/or regeneration of plant material under aseptic and controlled environmental conditions.

micropyle 1. A small opening in the surface of a plant ovule through which the pollen tube passes prior to fertilization. 2. A small pore in some animal cells or tissues.

microsatellite A segment of DNA characterized by a variable number of copies(typically 5-50) of a sequence of around 5 or fewer bases (called a repeat unit). At any one locus (genomic site), there are usually several different "alleles" in a population, each allele identifiable according to the number of repeat units. This existence of multiple alleles (high level of polymorphism) has enabled microsatellites to be developed as powerful markers in many different species. They are detected by the polymerase chain reaction..

microspore The immature male gametophyte in seed plants; the haploid male cell that ripens into a pollen grain.

microtuber Miniature tuber, produced in tissue culture, which is readily regenerable into a normal tuberous plant.

microtubule Self-assembling components of the cytoskeleton. Microtubules are cylindrical protein polymers, interconnected by cross-bridging proteins, which structurally and dynamically organize functional activities in living cells. They form the spindle during mitosis.

middle lamella A thin exclusively plant membrane separating two adjacent protoplasts and remaining as a distinct cementing layer between adjacent cell walls.

mid-parent value The average of the phenotypic measure, with respect to a given trait, of the two parents used to generate the population being analysed.

mineralization The conversion of organic compounds into inorganic (mineral) ones. For example, the conversion of ethanol into carbon dioxide and water.

minimum effective cell density The cell density below which reproducible cell growth fails. The minimum density is determined by both the source function of the tissue (species, explant, cell line) and by the culture phase of the initial inoculum.

minimum inoculum size The critical volume of inoculum required to initiate culture growth, due to the diffusive loss of cell materials into the medium. The subsequent culture growth cycle is dependent on the inoculum size, which is determined by the volume of medium and size of the culture vessel.

mini-prep A small-scale preparation of plasmid or phage DNA. Used to purify cloned DNA from the DNA of the vector.

minisatellite A form of variable number tandem repeats in which the repeat unit size ranges from 10-100 bp. Used for DNA fingerprinting following Southern hybridization. Generally concentrated at the ends of chromosomes and in regions with a high frequency of recombination.

minituber Small tubers (5-15 mm in diameter) formed on shoot cultures or cuttings of tuber-forming crops, such as potato.

mismatch The occurrence of a non-complementary pairs of bases in a double helix of DNA, e.g. A:C, G:T.

mismatch repair A DNA repair process that corrects mismatched base pairs.

missense mutation A mutation that changes a codon for one amino acid into a codon specifying another amino acid.

mist propagation Application of fine droplets of water to maintain humidity around plantlets or cuttings, which have not yet developed effective roots.

mite Free-living and parasitic arachnids. Infestation of plant crops reduces yield through the destruction of leaf tissue. Also can infest plant tissue culture work areas, contaminating culture vessels, thereby spreading bacteria and fungi.

mitochondrial DNA (Abbreviation: mtDNA). A circular DNA found in mitochondria. In mammals, mtDNA makes up less than 1% of the total DNA, but in plants the amount is variable. It encodes rRNA and tRNA and some mitochondrial proteins (up to 30 in animals).

mitochondrion (pl.: mitochondria) Organelle possessing its own DNA which appear in all eukaryotic cells (and never in prokaryotic cells) and produce adenosine triphosphate as an energy source for the cell via oxidative phosphorylation. Mitochondria contain many enzymes of the respiratory cycle, although most of these proteins are nuclear encoded.

mitogen A substance that can cause cells to initiate mitosis.

mitosis Splitting of replicated chromosomes, and the division of the cytoplasm to produce two genetically identical daughter cells. On the basis of the appearance of the chromosomes, it is separated into five stages: interphase, prophase, metaphase, anaphase and telophase.

mixed bud A bud containing both rudimentary leaves and flowers.

mixoploid Groups of cells with variable (a mix of euploid, aneuploid, polyploid) chromosome numbers.

mobilization 1. The transfer between bacteria of a non-conjugative plasmid by a conjugative plasmid. 2. The transfer between bacteria of chromosomal genes by a conjugative plasmid.

mobilizing function The genes on a plasmid that give it the ability to facilitate the transfer of either a non-conjugative or a conjugative plasmid from one bacterium to another.

mode In a frequency distribution, the class having the greatest frequency.

model 1. A mathematical description of a biological phenomenon. 2. A simplified biological system used to test hypotheses (e.g. Arabidopsis thaliana as a model plant).

modern biotechnology The application of:

a. In vitro nucleic acid techniques, including recombinant deoxyribonucleic acid (DNA) and direct injection of nucleic acid into cells or organelles, or

b. Fusion of cells beyond the taxonomic family,

that overcome natural physiological reproductive or recombination barriers and that are not techniques used in traditional breeding and selection (Convention on Biological Diversity).

modifying gene A gene that affects the expression of some other gene.

MOET Abbreviation for multiple ovulation and embryo transfer.

molecular biology The study of living processes at the molecular level.

molecular chaperone See: chaperone.

molecular cloning The biological amplification of a DNA sequence via the mitotic division of a host cell into which it has been transformed or transfected. See: cloning.

molecular genetics The study of the expression, regulation and inheritance of genes at the level of DNA and its transcription products.

molecular marker A genetic marker which is assayed at the DNA level.

molecular pharming See: biopharming.

molecule The stable union of two or more atoms; some organic molecules contain very large numbers of atoms.

monoclonal antibody (Abbreviation: mAb). An antibody, produced by a hybridoma, directed against a single antigenic determinant of an antigen.

monocot Abbreviation for monocotyledon.

monocotyledon (Abbreviation: monocot). A flowering plant whose embryo has one cotyledon. Examples are cereals (corn, wheat, rice etc.), banana, and lily.

monoculture The agricultural practice of cultivating a single crop over a whole farm or area.

monoecious A plant species that has separate male and female flowers on the same plant (e.g. maize).

monogastric animal A non-ruminant animal with a simple stomach.

monogenic Trait controlled by a single gene. Opposite: multigenic, polygenic.

monohybrid Heterozygous with respect to one gene.

monohybrid cross A cross between parents differing in only one trait or in which only one trait is being considered.

monokine Generic name for proteins that are released by monocytes to act on other cells involved in the immune response. A sub-class of cytokines.

monolayer A single layer of cells growing on a surface.

monolignols The building blocks of lignin that undergo polymerization.

monomer A small molecule (in the biological sciences typically individual amino acids, nucleotides or monosaccharides) that can combine with identical or similar others to form a larger, more complex molecule called a polymer.

monomorphic Absence of variation for a marker, gene, chromosome, or genetically determined trait in a population.

monophyletic A group of organisms that are assumed to have originated from the same ancestor.

monoploid See: haploid.

monosaccharide A simple sugar (e.g. glucose, fructose). See: disaccharide, polysaccharide.

monosomic A form of aneuploidy in which a diploid organism lacks one member of a homologous chromosome pair.

mono-unsaturates Oils containing mono-unsaturated fatty acids (i.e. where one -CH₂-CH₂- group in the hydrocarbon chain is replaced by -CH=CH-).

monozygotic twin One of a pair of twins derived from a single fertilized egg. Synonym: identical twin.

morphogen A substance that stimulates the development of form or structure in an organism.

morphogenesis The development, through growth and differentiation, of form and structure in an organism.

morphogenic response The effect on the developmental history of a plant or its parts exposed to a given set of growth conditions or to a change in the environment.

morphology Shape, form, external structure or arrangement.

mosaic An organism or part of an organism that is composed of cells with different origin.

mother plant See: donor plant.

motif A conserved sequence of nucleotides or amino acids that can be associated with some function of, respectively, a length of DNA or a protein.

movable genetic element See: transposon.

mRNA Abbreviation for messenger RNA. The RNA molecule resulting from transcription of a protein-encoding gene, following any splicing (1). The information encoded in the mRNA molecule is translated into a gene product by the ribosomes.

MRU Abbreviation for minimum recognition units. See: dAb.

mtDNA Abbreviation for mitochondrial DNA.

multi-copy Describing plasmids which replicate to produce many copies per host bacterial cell.

multigene family A set of genes (not necessarily mapping to the same genomic location) that are related in nucleotide sequence and/or that produce polypeptides with similar amino acid sequences. Sequence similarity does not always result in functional similarity.

multigenic Trait controlled by several genes, as opposed to monogenic. Synonym: polygenic.

multi-locus probe A DNA sequence that hybridizes to a number of different genomic sites.

multimer A protein made up of more than one polypeptide chain.

multiple alleles The existence of more than two alleles at a locus in a population.

multiple arbitrary amplicon profiling A collective term for a number of related polymerase chain reaction techniques, all of which use arbitrary primers, and which generate a number of distinct amplification products. See: random amplified polymorphic DNA.

multiple cloning site (Abbreviation: MCS). See: polylinker.

multiple drop array (Abbreviation: MDA). See: microdroplet array.

multiple ovulation and embryo transfer (Abbreviation: MOET). A technology by which a single female that usually produces only one or two offspring can produce a litter of offspring. Involves stimulation of a female to shed large numbers of ova; natural mating or artificial insemination; collection of fertilized ova (either surgically, or non-surgically through the cervix); and transfer (usually non-surgical, through the cervix) of these fertilized ova to recipient females.

multiplex 1. The simultaneous amplification of a number of amplicons in a single polymerase chain reaction, achieved by including more than one set of primers in the reaction mix. 2. The inheritance pattern of alleles in autopolyploids. See: quadruplex.

multivalent vaccine A vaccine designed to elicit an immune response either to more than one infectious agent or to several different antigenic determinants of a single agent.

mutable gene A gene which has an unusually high rate of mutation.

mutagen An agent or process capable of inducing mutations (e.g. irradiation, alkylating agents).

mutagenesis Induction of heritable change(s) in the genetic constitution of a cell through alterations to its DNA.

mutant An organism or an allele bearing a mutation. Usually applied when a characteristic change in phenotype can be recognized.

mutation Any change in the genome with respect to a defined wild type. Can occur at the level of ploidy, karyotype, or nucleotide sequence. Most of the latter mutations are silent (i.e. cannot be associated with any change in phenotype), either because the DNA sequence affected is in the non-coding part of the genome, or because the specific change does not alter the function of a coding sequence. See: back mutation, single nucleotide polymorphism.

mutation pressure A constant mutation rate that adds mutant genes to a population; repeated occurrences of mutations in a population.

mutualism See: symbiosis.

mycelium (pl.: mycelia) Threadlike filament making up the vegetative portion of thallus fungi.

mycoprotein Fungal protein.

mycorrhiza Fungi that form an association with, or have a symbiotic relationship with roots of more developed plants.

mycotoxin Toxic substance of fungal origin, e.g. aflatoxin.

myeloma A plasma cell cancer.

myo inositol See: inositol.

naked bud A bud not protected by bud scales.

narrow-host-range plasmid A plasmid that can replicate in one, or at most a few, different bacterial species.

narrow-sense heritability The proportion of the phenotypic variance that is due to variation in breeding values; the proportion of the phenotypic variance that is due to additive genetic variance.

native protein The naturally occurring form of a protein.

natural selection The differential survival and reproduction of organisms because of differences in characteristics that affect their ability to utilize environmental resources.

necrosis Death of tissue evidenced by discolouration, dehydration and loss of organization.

negative autogenous regulation Inhibition of the expression of a gene or set of co-ordinately regulated genes by the product of the gene or the product of one of the genes. Synonym: negative self-regulation.

negative control system A mechanism by which a regulatory protein is required to turn off gene expression.

negative selection Selection against individuals possessing a certain character. Opposite: positive selection.

negative self-regulation See: negative autogenous regulation.

nematode Slender, unsegmented worms, often parasitic. Also known as eelworm, especially when phytoparasitic.

neo-formation See: organogenesis.

neomycin phosphotransferase II (Abbreviation: npt-II). An enzyme which detoxifies the antibiotic neomycin, used as a marker gene to select for successfully transformed cells in plant transgenesis. See: neo^r.

neoplasm Localized cell multiplication, forming a tumour. Generally the result of genetic transformation. Neoplasmic cells differ in structure and function from the original cell type.

neo^r Neomycin-resistance gene. See: antibiotic resistance marker gene, neomycin phosphotransferase II, selectable marker.

neoteny The retention of juvenile body characters in the adult state, or the occurrence of adult characters in the juvenile state.

net photosynthesis Photosynthetic activity minus respiratory activity, measured by the net absorption of carbon dioxide.

neutral mutation A mutation that changes the nucleotide sequence of a gene, but has no observable effect on the fitness of the organism.

neutral theory The theory that much of evolution has been primarily due to random drift of neutral mutations.

neutrophil A type of leukocyte involved in the early inflammatory response.

NFT Abbreviation for nutrient film technique.

nick To break (or a break in) a phosphodiester bond in one of the strands of a double-stranded DNA molecule.

nick translation A procedure for labelling DNA by treating a fragment with DNAse to produce single-stranded nicks followed by excision of a nucleotide and repair of the gaps with radiolabelled nucleotide.

nicked circle During the extraction of plasmid DNA from a bacterial cell, one strand of the DNA often becomes nicked. This relaxes the torsional strain which normally ensures a supercoiled structure. Synonym: relaxed circle.

nif gene cluster Group of bacterial genes responsible for the biological fixation of atmospheric nitrogen.

nitrate The form of nitrogen that can be used directly by plants; a major component of inorganic fertilizers.

nitrification The natural process in which nitrogen in plant and animal wastes is oxidized, first to nitrites and then to nitrates, through the action of soil-borne microbes.

nitrocellulose A derivative of cellulose, which has the property of binding to many biological macromolecules, in particular DNA, RNA and protein. Filters made from nitrocellulose are commonly used in Southern and northern blotting experiments. Synonym: cellulose nitrate

nitrogen assimilation The incorporation of nitrogen into the cells of living organisms.

nitrogen fixation The conversion of atmospheric nitrogen gas to oxidized forms that can be assimilated by plants, particularly by blue-green algae and some genera of bacteria (e.g. Rhizobium spp.; Azotobacter spp.). An important source of nitrogen in unfertilized soils. See: nif gene cluster.

nitrogenous base The purines (adenine and guanine) and pyrimidines (cytosine, and thymine or uracil) that are present in DNA and RNA.

NO Abbreviation for nucleolar organizer.

nod box A DNA sequence that controls the transcriptional regulation of Rhizobium nodulation genes.

nodal culture The culture of a lateral bud and a section of adjacent stem tissue.

node Slightly swollen structure on the stem, where leaves and buds arise and where branches originate. Stems have nodes but roots do not.

nodular Term commonly used to describe the pebbly (rough) texture of a callus.

nodulation The formation of nodules on the roots of legumes following colonization by symbiotic nitrogen-fixing bacteria.

nodule Swollen globular structures formed on the roots of legumes, containing the nitrogen-fixing bacteria.

non repetitive DNA/RNA A nucleotide sequence which does not include a significant proportion of repetitive sequences of nucleotides.

non-additive genetic variation The proportion of the total genetic variation in a population that does not respond to simple mass selection and that causes specific pairwise crosses to depart from performance values predicted by the breeding values of the parents.

non-autonomous A term referring to biological units that cannot function by themselves; such units require the assistance of another unit, or "helper". Opposite: autonomous.

non-coding strand See: antisense DNA.

non-disjunction Failure of separation of homologous chromosomes or chromatids in mitosis or meiosis, resulting in too many chromosomes in some daughter cells and too few in others.

non-histone chromosomal protein In chromosomes, all of the proteins except the histones.

nonsense mutation A mutation which converts an amino-acid-specifying codon into a stop codon, e.g. a single base change from UAU to UAG generates the premature termination of the polypeptide chain at the position where a tyrosine was incorporated in the wild type.

non-target organism An organism which is affected by a treatment (e.g. pesticide application) for which it was not the intended recipient.

non-template strand The non-transcribed strand of DNA. Synonyms: sense strand, coding strand

non-virulent agent See: attenuated vaccine.

NOR Abbreviation for nucleolar organizer region.

northern blot A technique analogous to Southern blotting, but involving the transfer of RNA from an agarose gel to a membrane, prior to probing.

npt-II Abbreviation for neomycin phosphotransferase II.

nucellar embryo An embryo which has developed vegetatively from somatic tissue surrounding the embryo sac, rather than by fertilization of the egg cell.

nucellus Tissue forming the major part of the young ovule in which the embryo sac develops; Also known as megasporangium.

nuclear transfer A technology by which novel animals are generated by cloning a single diploid somatic cell. It involves inserting a single diploid cell from a culture of cells into an enucleated ovum. The resultant diploid ovum develops into an embryo that is placed in a recipient female, which gives birth to the cloned animal in the normal manner. Note that the term is somewhat of a misnomer, since it is a whole cell that is transferred, not just the nucleus.

nuclease A class of largely bacterial enzymes that degrade DNA or RNA molecules by catalysing the cleavage of the phosphodiester bonds that link adjacent nucleotides. For deoxyribonuclease (DNAse) the substrate is DNA, for ribonuclease (RNAse) the substrate is RNA, and for S1 nuclease, the substrate is single-stranded DNA or RNA. Endonucleases cleave at internal sites in the substrate molecule, while exonucleases progressively cleave from the end of the substrate molecule. Nucleases have varying degrees of base-sequence specificity, the most specific being the restriction endonucleases.

nucleic acid A macromolecule consisting of polymerized nucleotides. Two forms are found, DNA and RNA. Nucleic acids may be linear or circularized, and single- or double-stranded. See: helix.

nucleic acid probe See: DNA probe.

nuclein The term used by Friedrich Miescher to describe the nuclear material he discovered in 1869, which today is known as DNA.

nucleo-cytoplasmic ratio In a cell, the ratio of nuclear to cytoplasmic volume. This ratio is high in meristematic cells and low in differentiated cells.

nucleolar organizer (Abbreviation: NO). See: nucleolar organizer region.

nucleolar organizer region (Abbreviation: NOR). A chromosomal segment containing a large array of genes that encode ribosomal RNA; located at the secondary constriction of specific chromosomes.

nucleolus An RNA-rich nuclear organelle in the nucleus of eukaryotic cells, produced by a nucleolar organizer. It represents the storage place for ribosomes and ribosome precursors. The nucleolus consists primarily of ribosomal precursor RNA, ribosomal RNA, their associated proteins, and some, perhaps all, of the enzymatic equipment (RNA polymerase, RNA methylase, RNA cleavage enzymes) required for synthesis, conversion and assembly of ribosomes. Subsequently the ribosomes are transported to the cytoplasm.

nucleoplasm The non-staining or slightly chromophilic, liquid or semi-liquid, ground substance of the interphase nucleus and which fills the nuclear space around the chromosomes and the nucleoli. Little is known of the chemical composition of this ground substance, which is not easily defined. Sometimes called "karyoplasm" when it is gel-like, and "karyolymph" when it is a colloidal fluid.

nucleoprotein Conjugated protein composed of nucleic acid and protein; the material of which the chromosomes are made.

nucleoside A base (purine or pyrimidine) that is covalently linked to a 5-carbon (pentose) sugar. When the sugar is ribose, the nucleoside is a ribonucleoside; when it is 2-deoxyribose, the nucleoside is a deoxyribonucleoside. See: nucleotide.

nucleoside analogue A synthetic molecule that resembles a naturally occurring nucleoside.

nucleosome Spherical sub-units of eukaryotic chromatin that are composed of a core particle consisting of an octamer of histones (two molecules each of histones H_2a, H_2b, H₃ and H₄) and 146 bp of DNA.

nucleotide A nucleoside with one or more phosphate groups linked at the 3'- or 5'-hydroxyl of a pentose sugar. When the sugar is ribose, the nucleotide is a ribonucleotide; when it is 2-deoxyribose, the nucleotide is a deoxyribonucleotide. RNA and DNA are polymers of, respectively, ribonucleoside 5'-monophosphates and deoxyribonucleoside 5'-monophosphates. Nucleotides containing the bases adenine, guanine and cytosine (A, G, C) occur in both DNA and RNA; thymine (T) occurs only in DNA, and uracil (U) only in RNA. Ribonucleoside mono-, di-, and triphosphates for which a specific base is not assigned are abbreviated NMP, NDP, and NTP, while deoxyribonucleoside mono-, di-, and tri-phosphates are abbreviated dNMP, dNDP, and dNTP. Otherwise, the "N" is replaced by the base letter abbreviation. See: base pair.

nucleotide sequence See: sequence.

nucleus A dense protoplasmic membrane-bound region of a eukaryotic cell that contains the chromosomes, separated from the cytoplasm by a membrane; present in all eukaryotic cells except mature sieve-tube elements and red blood cells.

null allele An allele which produces no functional gene product.

null mutation See: amorph.

nullisomic (Adj.) See: nullisomy.

nullisomy An otherwise diploid cell or organism lacking both members of a homologous chromosome pair.

nurse culture Culturing cells from a suspension culture on a raft of filter paper above a callus tissue piece (nurse tissue). The filter paper serves to prevent tissue union but allows the flow of essential substances from the nurse to the isolated cells.

nutriceutical A conventional food product that has been modified (potentially by genetic engineering) to have improved nutritional characteristics and/or pharmaceutical properties.

nutrient cycle The passage of a nutrient or element through an ecosystem, including its assimilation and release by various organisms and its transformation into various organic or inorganic chemical forms.

nutrient deficiency Absence or insufficiency of an essential factor for normal growth and development.

nutrient film technique (Abbreviation: NFT). A hydroponic technique used to grow plants. NFT delivers a thin film of water or nutrient solution either continuously or through on-off cycles.

nutrient gradient A diffusion gradient of nutrients and gases that develops in tissues where only a portion of the tissue is in contact with the medium. Gradients are less likely to form in liquid media than in callus cultures.

nutrient medium (pl.: nutrient media) A solid, semi-solid or liquid formulation for in vitro cell or tissue growth.

ochre stop codon See: stop codon.

octoploid. An organism, or a tissue whose cells contain eight haploid sets of chromosomes.

oestrogen The generic term for a group of female sex hormones which control the development of sexual characteristics and control oestrus. AlteRNAtive spelling: estrogen.

oestrous (adj.) See: oestrus.

oestrous cycle The cycle of reproductive activity shown by most sexually mature non-pregnant female mammals. AlteRNAtive spelling: estrous.

oestrus In female mammals, the period of sexual excitement and acceptance of the male. Synonyms: rut, heat. AlteRNAtive spelling: estrus (adj.: estrous).

offset Young plant produced at the base of a mature plant.

offshoot Short, usually horizontal, stem produced near the crown of a plant.

offspring New individuals resulting from sexual or asexual reproduction. Synonym: progeny.

Okazaki fragment Replicated DNA fragments formed on the lagging strand in DNA synthesis from a double-stranded template. The fragments are subsequently joined together by DNA ligase. See: primosome.

OLA Abbreviation for oligonucleotide ligation assay.

oligomer A molecule formed by the covalent joining of a small (undefined) number of monomers. See: polymer.

oligonucleotide A nucleotide oligomer. Often synthesized for use as primers for in vitro DNA synthesis. See: polymerase chain reaction.

oligonucleotide ligation assay (Abbreviation: OLA). A diagnostic technique for determining the presence or absence of a single nucleotide polymorphism within a target DNA sequence, often indicating whether the gene is wild type (normal) or mutant (usually defective).

oligonucleotide-directed mutagenesis See: site-specific mutagenesis.

oligonucleotide-directed site-specific mutagenesis See: site-specific mutagenesis.

oligosaccharide Carbohydrate consisting of several linked monosaccharide units.

oncogene A gene that causes cells to grow in an uncontrolled (i.e. tumourous) manner. Oncogenes are mutant forms of normal functional genes (called proto-oncogenes) that have a role in regulating cell proliferation. See: cellular oncogene; dominant oncogene; immortalizing oncogene; recessive oncogene, p53 gene.

oncogenesis The progression of cytological, genetic and cellular changes that culminate in the development of a tumour.

onco-mouse A mouse that has been genetically modified to incorporate an oncogene, which acts as an animal model for studies of human cancer.

ontogeny Developmental life history of an organism.

oocyte The egg mother cell; it undergoes two meiotic divisions (oogenesis) to form the egg cell. The primary oocyte refers to the state before completion of the first meiotic division; the secondary oocyte after completion of the first meiotic division.

oogenesis The formation and growth of the egg in an animal ovary.

oogonium 1. A germ cell of the female animal, that gives rise to oocytes by mitotic division. 2. The female sex organ of algae and fungi.

oosphere The non-motile female gamete in plants and some algae.

oospore A spore developing from a zygote of certain algae and fungi, following the fusion of heterogametes.

opal stop codon See: stop codon.

open continuous culture A continuous culture system, in which inflow of fresh medium is balanced by outflow of a corresponding volume of spent medium plus cells. In the steady state, the rate of cell wash-out equals the rate of formation of new cells in the system. See: continuous culture; batch culture; closed continuous culture.

open pollination Pollination by wind, insects or other natural mechanisms.

open reading frame (Abbreviation: ORF). A sequence of nucleotides in a DNA molecule that has the potential to encode a peptide or protein: comprisesa start triplet (ATG), followed by a series of triplets (each of which encodes an amino acid), and ending with a stop codon (TAA, TAG or TGA). The term is generally applied to sequences of DNA fragments, for which no function has yet been determined. The number of ORFs provides an estimate of the number of genes transcribed from the DNA sequence.

operational definition An operation or procedure that can be carried out to define or delimit something.

operator The region of DNA that is upstream from a gene or genes and to which one or more regulatory proteins (repressor or activator) bind to control the expression of the gene(s).

operon A functionally integrated genetic unit for the control of gene expression in bacteria. It consists of one or more genes that encode one or more polypeptide(s) and the adjacent site (promoter and operator) that controls their expression by regulating the transcription of the structural genes.

opine The condensation product of an amino acid with either a keto-acid or a sugar, produced by the plant host as a result of Agrobacterium infection, and used exclusively by the Agrobacterium as a carbon source for growth and reproduction within the plant.

OPU Abbreviation for ovum pickup.

ORF Abbreviation for open reading frame.

organ A tissue or group of tissues that constitute a morphologically and functionally distinct part of an organism.

organ culture The aseptic culture of complete living organs of animals and plants outside the body in a suitable culture medium. Animal organs must be small enough to allow the nutrients in the culture medium to penetrate all the cells.

organellar gene Genes located on organelles outside the nucleus.

organelle A membrane-bounded specialized region within a cell, such as the mitochondrion or dictyosome, that carries out a specialized function in the life of a cell.

organic complex A chemically undefined compound added to nutrient media to stimulate growth, e.g. coconut milk; malt extract; yeast extract; casein hydrolysate.

organic evolution See: evolution.

organism An individual living system, such as animal, plant or micro-organism, that is capable of reproduction, growth and maintenance.

organized growth The development under tissue culture conditions of organized explants (meristem tips or shoot tips, floral buds or organ primordia). Opposite: unorganized growth.

organized tissue Composed of normally differentiated cells.

organogenesis The initiation of adventitious or de novo shoots or roots from callus, meristem or suspension cultures. See: micropropagation; regeneration.

organoid An organ-like structure produced in culture.

organoleptic Having an effect on one of the organs of sense, such as taste or smell.

origin of replication The nucleotide position on a DNA sequence from which DNA synthesis (replication) is initiated.

orphan gene A gene or DNA sequence whose function is not known.

orphan receptor A receptor for which a cellular function or ligand has yet to be identified.

ortet The plant from which a clone is obtained. Synonym: donor plant.

orthologous Homologous genes/gene products that have evolved divergently between species; many rice genes have orthologues in other cereal genomes, because of the common ancestry of cereal species. See: paralogous.

osmosis Diffusion of water across a semi-permeable membrane from zones of low solute concentration to zones of higher solute concentration.

osmotic potential Change in the energy state of solvent brought about by dissolving a substance in the solvent - water in the biological sciences. The potential of aqueous solutions is always negative compared to pure water. Solvent flows from higher to lower osmotic potential solutions by diffusion or osmosis.

osmoticum A chemical agent (e.g. polyethylene glycol, mannitol, glucose or sucrose) employed to maintain the osmotic potential of a nutrient medium equivalent to that of the cultured cells, i.e. the medium and the cells are isotonic. Because of this osmotic equilibrium, cells are not damaged in vitro.

outbreeding A mating system characterized by the inter-breeding of genetically unrelated or dissimilar individuals. Since genetic diversity tends to be enhanced, and vigour or fitness of individuals increased by this process, it is often used to counter the detrimental effects of continuous inbreeding. Synonym: exogamy.

outflow The volume of growing cells that is removed from a bioreactor during a continuous fermentation process.

ovary 1. Enlarged basal portion of the pistil of a plant flower that contains the ovules. 2. The reproduction organ in female animals in which eggs are produced.

overdominance Where heterozygote performance is superior to that of either parental genotype.

overhang See: extension.

overlapping reading frame Start codons in different reading frames generate different polypeptides from the same DNA sequence.

ovulation The release of mammalian egg(s) from the ovary.

ovule The part of the seed plant's reproductive organs that comprises the nucellus, the embryo sac and the integuments.

ovum (pl.: ova) Egg.

ovum pickup (Abbreviation: OPU). The non-surgical collection of ova from a female.

oxidative phosphorylation The enzymatic addition of a phosphate to ADP to make ATP, coupled to electron transport from a substrate to molecular oxygen. A critical reaction for the generation of cellular energy.

oxygen-electrode-based sensor Sensor in which an oxygen electrode, which measures the amount of oxygen in a solution, is coated with a biological material such as an enzyme which generates or absorbs oxygen when the appropriate substrate is present. When the biological reaction takes place, the amount of oxygen close to the electrode changes and the signal from the electrode changes, thus signalling the concentration of substrate.

p Denotes the shorter of the two chromosome arms, e.g. human 14p is the short arm of human chromosome 14.

P element A Drosophila transposon.

P₁, P₂ Generational symbols for the two parents of a given individual.

p53 gene A human tumour-suppressor transcription factor gene, damage or mutation to which is believed to be responsible for up to 60% of all human cancer tumours. If, in spite of the presence of p53 protein, a cell begins to divide uncontrollably following damage to its DNA, the p53 gene acts to prevent tumours by triggering apoptosis.

pachynema A mid-prophase stage in the first meiotic division, between zygonema and diplonema. Chromosomes appear as long, paired threads. Occasionally all four chromatids can be recognized.

pachytene (adj.) See: pachynema.

packaging cell line A cell line designed to produce viral particles that do not contain nucleic acid. After transfection of these cells with a full-size viral genome, fully infective viral particles are assembled and released.

packed cell volume (Abbreviation: PCV). The proportion of a cell culture volume that is occupied by cells. Cell volume is determined by sedimenting using low speed centrifugation.

PAGE Abbreviation for polyacrylamide gel electrophoresis.

pairing The pairing of homologous chromosomes during the prophase of the first meiotic division. Pairing is the first prerequisite before crossing over and recombination can occur. Synonym: synapsis.

pair-rule gene A gene that influences the formation of body segments in Drosophila.

palaeontology The study of the fossil record of past geological periods and of the phylogenetic relationships between extinct and contemporary plant and animal species.

palindrome A segment of double-stranded DNA, in which the order of bases, read 5'?3' in one strand, is the same as that in the complementary antiparallel strand, also read 5'?3'. If the sequence is written in the normal convention, on two lines with paired bases shown one above the other, the base order on one strand runs in the opposite direction to that on the complementary strand. They are often found at the ends of transposable elements, and recognition sites for type II restriction endonucleases are also palindromes. Synonym: inverted repeat.

palisade parenchyma Elongated cells found just beneath the upper epidermis of leaves, typically rich in chloroplasts.

pAMP Ampicillin-resistant plasmid.

panicle An inflorescence, the main axis of which is branched; the branches bear loose racemose flower clusters. Rice is a prominent crop plant with a panicle inflorescence.

panicle culture Aseptic culture of immature panicle explants to induce microspore germination and development.

panmictic population A population in which mating occurs at random.

panmixis Random mating in a population.

paper raft technique See: nurse culture.

PAR Abbreviation for photosynthetically active radiation.

par gene One of a class of genes required for faithful plasmid segregation at cell division. Initially, par loci were identified on plasmids, but have also been found on bacterial chromosomes.

paracentric inversion A chromosomal rearrangement in which a segment of chromosome, excluding the centromere, is rotated.

paraffin [wax] A translucent, white, solid hydrocarbon with a low melting point. One use is as an embedding medium to support tissue for sectioning for light microscopy observation.

Parafilmä A stretchable film based on paraffin wax; used to seal tubes and Petri dishes. Parafilmä is a proprietary name which is incorrectly applied colloquially to similar products.

parahormone A substance with hormone-like properties that is not a secretory product (e.g. ethylene; carbon dioxide).

parallel evolution The development of different organisms along similar evolutionary paths due to similar selection pressures acting on them.

paralogous Homologous genes/gene products that have duplicated and evolved divergently within a species. e.g. beta- and gamma-globulin genes

parameter A value or measurement that varies with circumstances, and is used as a reference to quantify a situation or a process.

parasexual cycle A sexual cycle involving changes in chromosome number but differing in time and place from the usual sexual cycle; occurring in those fungi in which the normal cycle is suppressed or apparently absent.

parasexual hybridization See: somatic hybridization.

parasite An organism deriving its food from the living body of another organism.

parasitism The close association of two or more dissimilar organisms, where the association is harmful to the host, but beneficial to the parasite.

parasporal crystal Tightly packaged insect pro-toxin molecules that are produced by strains of Bacillus thuringiensis during the formation of resting spores.

paratope Synonym for antibody binding site.

parenchyma 1. A plant tissue consisting of spherical, undifferentiated cells, frequently with air spaces between them. 2. Loose connective tissue formed by large cells.

parenchymatous (adj.) See: parenchyma.

parthenocarpy The development of fruit without fertilization.

parthenogenesis Production of an embryo from an unfertilized egg.

partial digest Incomplete reaction of a restriction enzyme with DNA, such that only a proportion of the target sites are cleaved. Partial digests are often performed to give an overlapping collection of DNA fragments for use in the construction of a gene bank. Synonym: incomplete digest. Opposite: complete digest.

particle radiation High energy nuclear emissions, used as physical agents of mutagenesis. Three major types typically used: alpha () (positively charged), beta () (negatively charged), or neutrons (uncharged).

parts per million (Abbreviation: ppm). Unit of concentration: 1ppm = 1mg dissolved matter per litre of solution.

parturition The process of giving birth.

passage The transfer or transplantation of cells from one culture medium to another.

passage number The number of times cells in culture have been sub-cultured.

passage time Time interval between successive sub-cultures.

passive immunity 1. Natural acquisition of antibodies by the foetus or neonate (newborn) from the mother. 2. The artificial introduction of specific antibodies by the injection of serum from an immune animal. In both cases, temporary protection is conferred on the recipient. See: immunization, immunoprophylaxis.

pat gene A gene obtained from Streptomyces sp. encoding resistance to glufosinate-ammonium containing herbicides. (These inhibit plant synthesis of glutamine). Used widely as a means of transgenically inducing herbicide resistance in crop plants. Synonym: bar gene.

patent A legal permission to hold exclusive right - for a defined period of time - to manufacture, use or sell an invention.

paternal Pertaining to the father.

pathogen A disease-causing organism (generally microbial: bacteria, fungi, viruses; but can extend to other organisms: e.g. nematodes etc.). Synonym: infectious agent. See: latent agent.

pathogenesis related protein (Abbreviation: PR protein). One of a group of proteins that are characteristically highly expressed as part of a plant's response to pathogen attack. Many of these proteins are similarly expressed following infection with a broad range of pathogens, indicating their role in the hypersensitive response.

pathogen-free Uncontaminated with pathogen.

pathotoxin Substance secreted by certain pathogens, in order to attack the host tissue. Some pathotoxins are also toxic to non-hosts, especially animals or humans.

pathovar Strain of a plant-attacking bacterium or fungus that can be differentiated from others by their interaction with specific host cultivars.

PBR Abbreviation for plant breeders' rights.

pBR322 One of the first plasmids used for cloning DNA in E. coli.

PCR Abbreviation for polymerase chain reaction.

PCR-RFLP AlteRNAtive term for cleaved amplified polymorphic sequence.

PCV Abbreviation for packed cell volume.

pectin A group of naturally occurring complex polysaccharides, containing galacturonic acid, found in plant cell walls, where their function is to cement cells together. Used as a thickening agent in solid culture media and as a food additive.

pectinase Enzyme catalysing the hydrolysis of pectin. Used in conjunction with cellulase to solubilize plant cell walls.

pedicel Stalk or stem of the individual flowers of an inflorescence.

pedigree The ancestry of an individual.

peduncle Stalk or stem of a flower that is born singly; the main stem of an inflorescence.

PEG Abbreviation for polyethylene glycol.

penetrance The proportion of individuals in a population that express the phenotype expected from their genotype with respect to a specific gene. Measures the extent to which phenotype predicts genotype.

peptidase An enzyme that catalyzes the hydrolysis of a peptide bond.

peptide A sequence of amino acids linked by peptide bonds; a breakdown or build-up unit in protein metabolism. Typically used to describe low molecular weight species. See: polypeptide.

peptide bond The chemical bond holding amino acid residues together in peptides and proteins. The (CO-NH) bond is formed by the condensation, with loss of a water molecule, between the carboxyl (-COOH) group of one amino acid and the amino (-NH₂) of the next amino acid.

peptide expression library A collection of peptide molecules, produced by recombinant cells, in which the amino acid sequences are varied.

peptide nucleic acid (Abbreviation: PNA). A synthetic oligonucleotide analogue, in which the sugar backbone is replaced by a peptide chain, upon which the nucleoside residues are strung. Probes made from PNA appear to have greater specificity than those made from DNA.

peptide vaccine A short chain of amino acids that can induce antibodies against a specific infectious agent.

peptidyl transferase An enzyme bound tightly to the large sub-unit of the ribosome, that catalyses the formation of peptide bonds between amino acids during translation.

peptidyl-tRNA binding site (Abbreviation: P-site). The site on a ribosome that hosts the tRNA to which the next amino acid for the growing polypeptide chain is attached.

perennial A plant that flowers continuously for several years.

pericentric inversion A chromosomal rearrangement in which a segment of chromosome, including the centromere, is rotated.

periclinal The orientation of cell wall or plane of cell division parallel to the reference surface.

periclinal chimera 1. Genotypically or cytoplasmically different tissues arranged in concentric layers. 2. A chimera in which one or more layers of tissue derived from one graft member enclose the central tissue derived from the other member of the graft.

pericycle Region of the plant bounded externally by the endodermis and internally by the phloem. Most roots originate from the pericycle.

periplasm The space between the cell (cytoplasmic) membrane of a bacterium or fungus and the outer membrane or cell wall. Synonym: periplasmic space.

permanent wilting point (Abbreviation: PWP). The moisture content of a soil below which plants wilt to such an extent that they fail to recover even when fully watered.

permeable Referring to a membrane, cell or cell system through which small molecules can diffuse.

persistence Ability of an organism to remain in a particular setting for a period of time after it is introduced.

persistent Chemicals with a long inactivation or degradation time, such as some pesticides. Persistent substances can become dangerously concentrated in the tissues of organisms at the top end of a food chain.

PERV Abbreviation for porcine endogenous retrovirus.

pesticide A toxic chemical product that kills harmful organisms (e.g. insecticides, fungicide, weedicides, rodenticides).

petal One of the parts of the flower that make up the corolla.

petiole Stalk of leaf. See: pedicel; peduncle.

Petri dish Flat round glass or plastic dish with a matching lid, used for small-scale culturing of organisms, germinating seeds etc. Also referred to as plates, hence to plate a culture.

PFGE Abbreviation for pulsed-field gel electrophoresis.

PG Abbreviation for polygalacturonase.

pH Logarithmic measure of acidity/alkalinity of a solution. A pH of 7 is neutral (e.g. pure water), whereas below 7 is acid and above 7 is alkaline.

phage Abbreviation for bacteriophage.

phagemids Cloning vectors that contain components derived from both phage and plasmid DNA.

phagocytes Immune system cells that ingest and destroy viruses, bacteria, fungi and other foreign substances or cells.

phagocytosis The process by which foreign particles invading the body are engulfed and broken down by phagocytes.

pharmaceutical agent See: therapeutic agent.

pharmacokinetics The quantitative measurement of how drugs move around the body, and the processes which control their absorption, distribution, metabolism, and excretion.

phase change The developmental change from one maturation state to another.

phase state The coupling or repulsion of two linked genes.

PHB Abbreviation for polyhydroxybutyrate.

pH-electrode-based sensor Sensor in which a standard pH electrode is coated with a biological material. Many biological processes raise or lower pH, and the changes can be detected by the pH electrode.

phenocopy An environmentally induced, non hereditary variation in an organism, resembling a genetically determined trait.

phenolic oxidation Common aspect of the wound response in plants. Phenolic oxidation is often indicated by blackening of tissue and it may be a precursor to growth inhibition or, in severe cases, to tissue necrosis and death.

phenolics Compounds with hydroxyl group(s) attached to the benzene ring, forming esters, ethers and salts. Phenolic substances produced from newly explanted tissues are liable to oxidise, and as a result form coloured compounds visible in nutrient media.

phenotype The visible appearance of an individual (with respect to one or more traits) which reflects the reaction of a given genotype with a given environment.

pheromone A hormone-like substance that is secreted by an organism into the environment as a specific signal to another organism, usually of the same species.

phloem Specialized vascular plant tissue for the transport of assimilates (generally sugars) from the point of synthesis (in the leaf) to other parts of the plant. It consists of sieve tubes, companion cells, phloem parenchyma and fibres.

phosphatase An class of enzymes that catalyze the hydrolysis of esters of phosphoric acid, removing a phosphate group from an organic compound.

phosphodiester (phospho-diester) bond A bond in which a phosphate group joins adjacent carbons through ester linkages. A condensation reaction between adjacent nucleotides results in a phosphodiester bond between 3' and 5' carbons in DNA and RNA.

phospholipase A2 An enzyme which degrades type A2 phospholipids.

phospholipid A class of lipid molecules in which glycerol is linked to a phosphate group and two fatty acyl groups. Contains both polar and non-polar regions. A major component of biological membranes. See: inositol lipid.

phosphorolysis The cleavage of a bond by orthophosphate; analogous to hydrolysis referring to cleavage by water.

phosphorylation The addition of a phosphate group to a compound.

photoautotroph See: autotroph, heterotroph.

photo-bioreactor Bioreactor dependent on sunlight, which is taken up by its content of plant material, usually algae.

photoheterotroph See: heterotroph.

photoperiod The length of daylight or period of daily illumination provided for growth.

photoperiodism The photoperiod required by a plant to switch from the vegetative to the reproductive stage.

photophosphorylation The formation of ATP from ADP and inorganic phosphate using light energy harvested by photosynthesis.

photoreactivation A light dependent DNA repair process.

photosynthate The carbohydrates and other compounds produced in photosynthesis.

photosynthesis A chemical process by which green plants synthesize organic compounds from carbon dioxide and water in the presence of sunlight.

photosynthetic Able to use sunlight energy to convert atmospheric carbon dioxide into organic compounds. Nearly all plants, most algae and some bacteria are photosynthetic.

photosynthetic efficiency Efficiency of converting light energy into organic compounds.

photosynthetic photon flux (Abbreviation: PPF). A measure of the intensity of light utilized by plants for photosynthetic activity.

photosynthetically active radiation (Abbreviation: PAR). The part of the radiant energy that is capturable by natural photosynthetic systems (approximately equivalent to the natural light spectrum of wavelengths 400-700nm).

phototropism The tendency of plants to direct shoot growth towards the source of light.

phylogeny The deduced evolutionary history of related organisms.

physical map An indication of the separation, in bp, between pairs of linked loci. See: mapping.

phyto- (Prefix) To do with plants.

phytochemical Molecules characteristically found in plants.

phytochrome A pigment, found in the cytoplasm of green plants, which can exist in two forms Pr (biologically inactive) and Pfr (biologically active). Pfr is converted into Pr by exposure to light of wavelength 730 nm. Involved in the timing of many plant processes, e.g. dormancy, leaf formation, flowering and germination.

phytohormone A substance that stimulates growth or other processes in plants. Major species are auxins, abscisic acid, cytokinins, gibberellins and ethylene.

phytokinin See: cytokinin.

phytoparasite Parasite on plants.

phytoparasitic (adj.) See: phytoparasite.

phytopathogen A plant pathogen.

phytoremediation The use of plants actively to remove contaminants or pollutants from either soils (e.g. polluted fields) or water resources (e.g. polluted lakes). An example is the exploitation of the Brazil water hyacinth (Eichhornia crassipes) to accumulate in its tissues toxic metals such as lead, arsenic, cadmium, mercury, nickel, and copper.

phytosanitary Plant health, including quarantine.

phytostat Apparatus designed for the semi-continuous chemostatic culture of plant cells.

phytosterol One of a group of biologically active phytochemicals present in the seeds of certain plants. Evidence suggests that human consumption of certain phytosterols, such as â-sitosterol, can help to lower total serum cholesterol and low-density lipoproteins levels, thereby reducing the risk of coronary heart disease.

pigment Compounds that are coloured by the light they absorb. Light absorption is exploited by plants both as a means of energy capture (see: photosynthesis) and as a signalling mechanism (see: phytochrome).

pinocytosis The engulfing of a minute droplet of liquid by a living cell.

pipette Widely used device for accurate dispensing of small volumes of liquids.

pistil Central organ of the flower, typically consisting of ovary, style and stigma. Usually referred to as the female part of a perfect flower.

plant breeders' rights (Abbreviation: PBR). Legal protection of a new plant variety granted to the breeder or his successor in title. The effect of PBR is that prior authorization is required before the material can be used for commercial purposes.

plant cell culture In vitro growth of plant cells.

plant cell immobilization Entrapment of plant cells in gel matrices so that they are protected from physical damage. The cells are suspended in liquified droplets which are then allowed to harden. Commonly used matrices are alginates, agar or polyacrylamide.

plant genetic resources (Abbreviation: PGR). The reproductive or vegetative propagating material of: 1. cultivated varieties (cultivars) in current use and newly developed varieties; 2. obsolete cultivars; 3. primitive cultivars (landraces); 4. wild and weed species, near relatives of cultivated varieties; and 5. special genetic stocks (including elite and current breeder's lines and mutants).

plant growth regulator An organic compound, either natural or synthetic, and other than a nutrient, that modifies or controls one or more specific physiological processes within a plant.

plant hormone See: plant growth regulator.

plant variety protection (Abbreviation: PVP). Synonym for plant breeders' rights.

plant variety rights See: plant breeders' rights.

plantibody An antibody expressed transgenically in an engineered plant.

plantlet A small rooted shoot regenerated from cell culture following embryogenesis or organogenesis. Plantlets can normally develop into normal plants when transplanted to soil.

plaque A clear spot on an otherwise opaque lawn of bacteria, where cells have been lysed by phage infection.

plasma The fluid portion of the blood in which is suspended the white and red blood cells. Contains 8-9% solids, of which 85% is composed of the proteins fibrinogen, albumin, and globulin. The essential function of plasma is the maintenance of blood pressure and the transport of nutrients and waste.

plasma cells Antibody-producing white blood cells derived from B lymphocytes.

plasma membrane See: plasmalemma.

plasmalemma The lipid bilayer and associated proteins and other molecules that surrounding the protoplast, within the cell wall. Synonyms: cell membrane; plasma membrane.

plasmid An circular self-replicating non-chromosomal DNA molecule found in many bacteria, capable of transfer between bacterial cells of the same species, and occasionally of different species. Antibiotic resistance genes are frequently located on plasmids. Plasmids are particularly important as vectors for genetic engineering.

plasmodesma (pl.: plasmodesmata) Fine protoplasmic thread that connects adjacent plant cells by passing through the plant cell wall. Exploited by viruses as a conduit for cell to cell movement.

plasmolysis Shrinkage of protoplasm caused by removal of water from a cell through osmosis when surrounded by a hypertonic solution.

plastid A general term for a number of plant cell organelles which carry non-nuclear DNA. Includes the pigment-carrying bodies: 1. chloroplasts in leaves, 2. chromoplasts in flowers, and 3. the starch-synthesizing amyloplasts in seeds.

plastoquinone One of a group of compounds involved in the transport of electrons as part of the process of photosynthesis.

plate 1. Verb: to distribute a thin film of micro-organisms or plant cells onto a solid medium. 2. Noun: refers to the two segments of a Petri dish or similar.

platform shaker See: shaker.

plating efficiency The percentage of inoculated cells which give rise to cell colonies when seeded into culture vessels.

pleiotropic (adj.) See: pleiotropy.

pleiotropy The simultaneous effect of a given gene on more than one apparently unrelated trait.

ploidy The number of complete sets of chromosomes per cell, e.g. one set: haploid, two sets: diploid, etc.

plumule The first bud of an embryo, or that portion of the young shoot above the cotyledons.

pluripotent See: totipotent.

plus tree See: elite tree.

PNA Abbreviation for peptide nucleic acid.

pneumatic reactor See: airlift fermenter.

point mutation A change in DNA sequence at a specific locus. The smallest change involves the substitution, deletion or insertion of a single nucleotide. See: single nucleotide polymorphism.

polar bodies In female animals, the products of a meiotic division that do not develop into a functional ovum. The first polar body comprises one of the two products of the first meiotic divisions, which may fail to divide at the second division. The second polar body comprises one of the products of the second division.

polar mutation A mutation that influences the functioning of genes that are downstream from the site of mutagenesis but are in the same transcription unit.

polar nuclei Two centrally located nuclei in the embryo sac that unite with a second sperm cell in a triple fusion. In some plant species (particularly the monocotyledons), the product of this fusion develops into the endosperm.

polar transport A directed movement within plants of compounds (usually endogenous plant growth regulators) mostly in one direction; polar transport overcomes the tendency for diffusion in all directions.

polarity The observed differentiation of an organism, tissue or cell into parts having opposed or contrasted properties or form.

pole cells A group of cells in the posterior of Drosophila embryos that are precursors to the adult germ line.

pollen Mature microspores of seed plants.

pollen culture The in vitro culture and germination of pollen grains to generate haploid plants. See: anther culture; microspore culture.

pollen grain The mature microspore, produced in the pollen sac of angiosperms or the microsporangium of gymnosperms. Unicellular, with variable shape and size, and an elaborately structured wall.

pollination Transfer of pollen from anther to stigma in the process of fertilization in angiosperms; transfer of pollen from male to female cone in the process of fertilization in gymnosperms.

poly-(A) polymerase Enzyme that catalyses the addition of adenine residues to the 3' end of mRNA molecules, forming the characteristic poly-(A) tail.

poly-(A) tail See: polyadenylation.

polyacrylamide gel Inert electrophoresis matrix, formed by the polymerization of acrylamide monomer in the presence of the cross-linker N,N'-methylene-bis-acrylamide. Gels are usually supported between two glass plates, which need to be removed for post-electrophoresis manipulations. Sometimes referred to incorrectly as acrylamide gels.

polyacrylamide gel electrophoresis (Abbreviation: PAGE). Ubiquitous method for separating nucleic acids and proteins on the basis of their molecular size. The method relies on the migration through an inert matrix (polyacrylamide gel) of electrically charged molecules as a result of the imposition of an electric field.

polyadenylation Post-transcriptional addition of multiple adenine residues to the 3' end of eukaryotic mRNA. Also called poly-(A) tailing. The adenine-rich 3' terminal segment is called a poly (A) tail.

polycistronic A single mRNA that contains the information necessary for the production of more than one polypeptide. Particularly characteristic of prokaryotic mRNAs.

polyclonal antibody A serum sample that contains a mixture of distinct immunoglobulin molecules, each recognizing a different antigenic determinant of a given antigen.

polycloning site See: polylinker.

polyembryony The production of more than one embryo from a single egg cell (in animals) or from a range of embryogenic cell types (in plants). These embryos are genetically identical to one another.

polyethylene glycol (Abbreviation: PEG). A polymer having the general formula HOCH₂(CH₂OCH₂)_nCH₂OH and available in a range of molecular weights. Thus PEG 1000 is a polyethylene glycol of average molecular weight 1000. PEG 4000 and 6000 are commonly used to promote cell or protoplast fusion, and to facilitate DNA uptake in the transformation of organisms such as yeast. Also used to concentrate solutions by withdrawing water from them via osmosis.

polygalacturonase (Abbreviation: PG). An enzyme which catalyses the breakdown of pectin. A tomato engineered to contain an antisense-PG gene succeeded in delaying the onset of softening, by inhibiting the expression of PG. This allowed the fruit to be picked at a riper stage than is conventionally possible, and represented the first commercialized genetically engineered crop plant.

polygene One of a number of genes, each of small effect, which together act to determine the phenotype of a quantitative trait. The result is continuous variation in the trait and a seemingly non-Mendelian mode of inheritance. See: quantitative trait locus, continuous variation.

polygenic Character controlled by many genes of small effect. See: polygene.

polyhydroxybutyrate (Abbreviation: PHB). A biopolymer, with physical properties similar to polystyrene, originally discovered in the bacterium Alcaligenes eutropus. The gene coding for this compound has since been transformed into both other bacteria and into some crop plants in order to produce a source of renewable raw material for the plastics industry. It is rapidly degraded by soil micro-organisms.

polylinker A synthetic segment of DNA, designed to include a number of different restriction endonuclease sites. When ligated to a DNA fragment that is to be cloned, this enables a wide choice of restriction endonucleases to be used for the cloning process. Synonym: multiple cloning site (MCS).

polymer A macromolecule synthesized by the chemical joining of many identical or similar monomers. For example, amino acids, monosaccharides and nucleotides give rise to proteins, polysaccharides and nucleic acids respectively. Water is eliminated between the monomers as they link to form chains. The individual monomer units condensed within a chain are often referred to as residues, a term which is also employed for the bases incorporated in polynucleotides.

polymerase An enzyme that catalyses the formation of polymers from monomers. A DNA polymerase synthesizes DNA from deoxynucleoside triphosphates using a complementary DNA strand and a primer. An RNA polymerase synthesizes RNA from ribonucleoside triphosphates and a complementary DNA strand.

polymerase chain reaction (Abbreviation: PCR). A widespread molecular biology procedure that allows the production of multiple copies (amplification) of a specific DNA sequence, provided that the base pair sequence of each end of the target is known. It involves multiple cycles of DNA denaturation, primer annealing, and strand extension, and requires a thermostable DNA polymerase, deoxyribonucleotides, and specific oligonucleotides (primers).

polymerization The chemical fusion of a number of identical or similar monomers to form a polymer. Common biological polymers are starch (polymerized monosaccharides), DNA (deoxyribonucleotides) and proteins (amino acids).

polymery The phenomenon whereby a number of genes at different loci (which may be polygenes) can act together to produce a single effect.

polymorphism 1. The occurrence of allelic variation at a locus. Polymorphism in nucleotide sequences has provided powerful diagnostic tools. See: DNA diagnostics, microsatellites, restriction fragment length polymorphism. 2. The occurrence of two or more forms in a population. See: balanced polymorphism, chromosomal polymorphism.

polynucleotide A linear polymer composed of covalently linked nucleotides. Each link is formed by a single phosphodiester bond. The term is used to describe DNA and RNA.

polypeptide A linear polymer composed of covalently linked amino acids. Each link is formed by a single peptide bond. Sometimes used as a synonym for protein, but also describes non-natural and low-molecular-weight polymers.

polyploid Organism, tissue or cells having more than two complete sets of chromosomes. Many crop plants are polyploid, including bread wheat (hexaploid, 6x), cotton and alfalfa (tetraploid, 4x), and banana (triploid, 3x).

polysaccharide A linear or branched polymer (e.g. starch, cellulose, etc.) composed of covalently linked monosaccharides, including cellulose, pectin and starch. Synonym: carbohydrate.

polysaccharide capsule See: capsule.

polysome A multi-ribosomal structure representing a linear array of ribosomes held together by mRNA.

polyspermy The entry of several sperm nuclei into the egg during fertilization, although only one actually fuses with the egg nucleus.

polytene chromosome Giant chromosomes produced by interphase replication without division, and consisting of many identical chromatids arranged side by side.

polyunsaturates Oils in which some of the carbon-carbon bonds are not fully hydrogenated - i.e. of the form -CH=CH-, rather than -CH₂-CH₂-.

polyvalent vaccine A recombinant organism into which antigenic determinants have been cloned from a number of pathogens, for use as a vaccine.

polyvinylpyrrolidone (Abbreviation: PVP). An occasional constituent of plant tissue culture isolation media. PVP is of variable molecular weight and of general formula (C₆H₉NO)_n. Its antioxidant properties are used to prevent oxidative browning of excised plant tissues. Less frequently used as an osmoticum in culture media.

population A defined group of interbreeding organisms.

population density Number of cells or individuals per unit. The unit could be an area, or a volume of medium.

population genetics The branch of genetics that deals with frequencies of alleles and genotypes in breeding populations.

porcine endogenous retrovirus (Abbreviation: PERV). The provirus of a porcine retrovirus. The possibility that PERVs could be activated after xenotransplantation of pig organs into humans has raised concern that xenotransplantation may result in the transfer of novel infections to the human population.

position effect The influence of the location of a gene (particularly a transgene) on its expression and hence its effect on phenotype.

positional candidate gene A gene known to be located in the same region as a DNA marker that has been shown to be linked to a single-locus trait or to a QTL, and whose deduced function suggests that it could be the source of genetic variation in the trait in question.

positional cloning A strategy for gene cloning that relies on the identification of closely linked markers to the target trait, and then uses chromosome walking to identify, isolate and characterize the gene(s) responsible for the trait. The strategy is particularly appropriate when the biochemical basis of the target trait is unclear, thus precluding the use of a candidate gene approach.

positive control system A mechanism in which a regulatory protein(s) is required to turn on gene expression.

positive selectable marker See: dominant selectable marker.

positive selection A method by which cells that carry a DNA insert integrated at a specific chromosomal location can be selected, since this integration confers a predictable phenotype.

post-replication repair A recombination-dependent mechanism for repairing damaged DNA.

post-translational modification The addition of specific chemical residues to a protein after it has been translated. Common residues are phosphate groups (phosphorylation) and sugars (glycosylation).

potentiometric See: enzyme electrode.

PPF Abbreviation for photosynthetic photon flux.

ppm Abbreviation for parts per million.

PR protein Abbreviation for pathogenesis related protein.

precautionary principle The approach whereby any possible risk associated with the introduction of a new technology is avoided, until a full understanding of its impact on health, environment etc. is available. Particularly applied to the release of genetically modified organisms, since unlike many technologies, these cannot be recalled if problems arise.

precocious germination Early germination of a seed or embryo, prior to the full maturation of the embryo.

pre-filter A coarse filter used to screen out large particles from a fluid or gas, before it is passed through a much finer filter.

pre-mRNA See: primary transcript.

pressure potential The pressure generated within a cell, being the net difference between the cell's osmotic potential and the water potential of the external environment.

pre-transplant A stage in micropropagation - the rooting and hardening process prior to transfer to soil.

preventive immunization Infection with an antigen to elicit an antibody response that will protect the organism against future infections. Synonym: vaccination.

Pribnow box Consensus sequence near the mRNA start-point of prokaryotic genes. See: TATA box.

primary First in order of time or development.

primary antibody In an ELISA or other immunological assay, the antibody that binds to the target molecule.

primary cell A cell or cell line taken directly from a living organism, which is not immortalized.

primary cell wall The cell wall layer formed during cell expansion. Plant cells possessing only primary walls may divide or undergo differentiation.

primary culture A culture started from cells, tissues or organs taken directly from organisms. A primary culture may be regarded as such until it is sub-cultured for the first time. It then becomes a cell line.

primary germ layers See: germ layer.

primary growth 1. Apical meristem-derived growth; the tissues of a young plant. 2. Explant growth during the initial culture period.

primary immune response The immune response that occurs during the first encounter of a mammal with a given antigen.

primary meristem Meristem of the shoot or root tip giving rise to the primary plant body.

primary structure The linear sequence of residues making up a polymer such as a nucleic acid, polysaccharide or protein. See: secondary structure, tertiary structure and quaternary structure.

primary tissue A tissue that has differentiated from a primary meristem.

primary transcript The RNA molecule produced by transcription prior to any post-transcriptional modifications; also called a pre-mRNA in eukaryotes.

primer A short oligonucleotide annealed to a template of single-stranded DNA, providing a doubled stranded structure from which DNA polymerase will synthesize a new DNA strand to produce a duplex molecule.

primer walking A method for sequencing long (>1 kbp) cloned pieces of DNA. The initial sequencing reaction reveals the sequence of the first few hundred nucleotides of the cloned DNA. Using this, a new primer of about 20 nucleotides is synthesized, which is complementary to a sequence near the end of sequenced DNA, and used to sequence the next few hundred nucleotides of the cloned DNA. This procedure is repeated until the complete nucleotide sequence of the cloned DNA is determined.

primordium A group of cells which gives rise to an organ.

primosome A protein-replication complex that catalyses the initiation of synthesis of Okazaki fragments during discontinuous replication of DNA. It involves DNA primase and DNA helicase activities.

prion See: proteinaceous infectious particle.

probability The frequency of occurrence of an event.

proband The individual in a family in whom an inherited trait is first identified.

probe A labelled DNA or RNA sequence used to detect the presence of a complementary sequence by hybridization with a nucleic acid sample.

procambium A primary meristem that gives rise to primary vascular tissues and, in most woody plants, to the vascular cambium.

procaryote See: prokaryote, prokaryotic.

procaryotic (adj.) See: procaryote.

processed pseudo-gene A copy of a functional gene which has no promoter, no introns and which, consequently, is not itself transcribed.

production environment All input-output relationships, over time, at a particular location. The relationships include biological, climatic, economic, social, cultural and political factors, which combine to determine the productive potential of a particular enterprise. Production environments are classified as high-, medium- and low-input

production traits Characteristics of animals, such as the quantity or quality of the milk, meat, fibre, eggs, work, etc., they (or their progeny) produce, which contribute directly to the value of the animals for the farmer, and that are identifiable or measurable at the individual level.

productivity The amount of economically significant product generated within a given period of time from a specified quantity of resource.

pro-embryo A group of cells arising from the division of the fertilized egg cell or somatic embryo before those cells which are to become the embryo are recognizable.

progeny Synonym of offspring.

progeny testing With respect to discrete loci, the inference of the allelic state of an individual from the pattern of segregation among its offspring. For a quantitative trait, the use of progeny performance to estimate the breeding value of an individual.

progesterone A hormone produced primarily by the corpus luteum, but also by the placenta. Its function in mammals is to prepare the inner lining of the uterus for the implantation of a fertilized egg. Also made by non-placental animals, including fish.

programmed cell death See: apoptosis, p53 gene.

prokaryote A member of the large group of organisms, including bacteria and blue-green algae, in which the chromosome is not enclosed within a nucleus, but instead exists as a linear or circular strand. Prokaryotes do not undergo meiosis and do not have functional organelles such as mitochondria and chloroplasts. See: eukaryote.

prolactin A hormone, produced by the anterior pituitary gland, that stimulates and controls lactation in mammals.

proliferation Increase by frequent and repeated reproduction; growth by cell division.

pro-meristem The embryonic meristem that is the source of organ initials or foundation cells.

promoter 1. A short DNA sequence, usually upstream of (5' to) the relevant coding sequence, to which RNA polymerase binds before initiating transcription. This binding aligns the RNA polymerase so that transcription will initiate at a specific site. The nucleotide sequence of the promoter determines the nature of the enzyme that attaches to it and the rate of RNA synthesis. 2. A chemical substance that enhances the transformation of benign cells into cancerous cells. See: constitutive promoter.

promoter sequence See: promoter (1).

pro-nuclear micro-injection An early, low success-rate method to achieve transgenesis in animals, involving the micro-injection of many gene copies into one of the two pro-nuclei of a fertilized egg. Now being replaced by micro-injection into a culture of cloned embryos produced by nuclear transfer, which can be tested for expression of the transgene before transfer to recipient females.

pro-nucleus Either one of the two haploid gamete nuclei, just prior to their fusion in the fertilized egg.

proofreading The scanning of newly-synthesized DNA for structural defects, such as mis-matched base pairs. A functional activity of most DNA polymerases.

propagation The duplication of a whole plant from a range of vegetative materials; adapted for in vitro culture as micropropagation.

propagule Any structure capable of giving rise to a new plant by asexual or sexual reproduction, including bulbils, leafbuds, etc.

pro-phage The genome of a bacteriophage integrated into the chromosome of a lysogenic bacterial cell, and replicated along with its host chromosome.

prophase The first stage of nuclear division. The stage during which chromosome pairing occurs in the first division of meiosis (see: leptonema, zygonema, pachynema, diplonema, diakinesis). In mitosis and the second division of meiosis, the chromosomes shorten and thicken as a result of coiling.

protamine A class of small basic proteins that replace the histones in the chromosomes of some sperm cells.

protease An enzyme that catalyses the hydrolysis of proteins, cleaving the peptide bonds that link amino acids in protein molecules. Synonym: peptidase.

protein A macromolecule composed of one or more polypeptides, each comprising a chain of amino acids linked by peptide bonds.

protein crystallization The production of a pure preparation of a protein. In this form, the three-dimensional structure of the molecule can be determined.

protein drug See: therapeutic agent.

protein engineering Generating proteins with modified structures that confer novel properties such as higher catalytic specificity or thermal stability.

protein kinase An enzyme that catalyses the addition of a phosphate group(s) to a protein molecule at the sites of serine, threonine or tyrosine residues.

protein metabolic step One step in the chain of reactions that take place in an organism and dictate the composition of that organism.

protein sequencing The process of determining the amino acid sequence of a protein. Usually achieved following initially partial hydrolysis of the protein into smaller peptides by enzymatic digestion.

protein synthesis The creation of proteins from their constituent amino acids, in accordance with the encoding gene DNA sequence.

proteinaceous infectious particle Believed to be the agent responsible for the class of diseases called spongiform encephalopathies, including scrapie in sheep, bovine spongiform encephalopathy (BSE; mad cow disease) in cattle and CJD in humans. It is an abnormal form of a brain protein, and has no detectable nucleic acid content. Synonym: prion.

proteolysis Enzymatic degradation of a protein.

proteolytic Having the ability to degrade protein molecules.

proteome The complete complement of proteins made by a given species in all its tissues and growth stages.

proteomics An approach that seeks to identify and characterize complete sets of protein, and protein-protein interactions in a given species. See: proteome, genomics.

protoclone Regenerated plant derived from protoplast culture or a single colony derived from protoplasts in culture.

protocol The step-by-step experiments proposed to describe or solve a scientific problem, or the defined steps of a specific procedure.

protocorm A tuberous structure, formed following the germination of orchid seeds, from which develops a complete plant. It develops from an unorganized embryo in the seed, comprising only a few hundred cells. In culture, vegetative explants of several orchid species form round, smooth protocorms which can be multiplied indefinitely or induced to regenerate into a whole plant.

protoderm A primary meristem tissue that gives rise to epidermis.

protogyny The condition in which the female reproductive organs (carpels) of a flower mature before the male ones (stamens), thereby ensuring that self-fertilization does not occur.

protomeristem See: pro-meristem.

proto-oncogene A normal gene that can be mutated to an oncogene. Synonym: cellular oncogene.

protoplasm The essential, complex living substance of cells, upon which all vital functions of nutrition, secretion, growth and reproduction depend.

protoplast A bacterial or plant cell for which the cell wall has been removed either chemically or enzymatically, leaving its cytoplasm enveloped by a peripheral membrane. Protoplasts are spherical and smaller than the elongate, angular shaped and often vacuolated cells from which they have been released.

protoplast culture The culturing in vitro of plant protoplasts. Where protoplasts can be regenerated into whole plants, they represent an attractive target for genetic manipulation.

protoplast fusion The induced or spontaneous coalescence of two or more protoplasts of the same or different species origin. Where fused protoplasts can be regenerated into whole plants, the opportunity exists for the creation of novel genomic combinations. See: cybrid.

prototroph A nutritionally independent cell. Opposite: auxotroph.

pro-toxin A latent, non-active precursor form of a toxin.

protozoan (pl.: protozoa) A microscopic, single-cell organism.

protruding end See: extension.

provenance The geographical and/or genetic origin of an individual.

provirus A double stranded DNA copy of the single RNA strand of a retrovirus, which has been integrated into a host genome.

pseudo-affinity chromatography A chromatographic technique in which a ligand is immobilized selectively to retain enzymes or other proteins.

pseudo-autosomal region A section at one end of the X and Y chromosomes for which there is sufficient homology that there is synapsis between them during meiosis.

pseudocarp A fruit that incorporates, in addition to the ovary wall, other parts of the flower, such as the receptacle (e.g. strawberry). Synonym: false fruit.

pseudogene An incomplete or mutated copy of a gene which is not transcribed because it lacks a continuous open reading frame. Those that lack introns are called processed pseudogenes and are most likely cDNA copies synthesized from mRNA by reverse transcriptase

Pseudomonas spp. A widely distributed Gram-negative bacterial genus. Many of the soil forms produce a pigment that fluoresces under ultraviolet light, hence the descriptive term fluorescent Pseudomonas.

P-site Abbreviation for peptidyl-tRNA binding site.

psychrophile A micro-organism that can grow at temperatures below 30 °C and as low as 0 °C. See: mesophile, thermophile.

PUC A widely used plasmid, containing as a marker a galactosidase gene.

pulsed-field gel electrophoresis (Abbreviation: PFGE). A procedure used to separate very large (50 kbp to several Mbp) DNA molecules by alteRNAting the direction of electric current in a pulsed manner across a gel.

punctuated equilibrium The occurrence of speciation events in bursts, separated by long intervals of species stability.

pure line A strain in which all members are genetically nearly identical and are indistinguishable by phenotype. Usually created by repeated generations of self-fertilization or close inbreeding.

purification tag See: affinity tag.

purine A double-ring, nitrogen-containing base present in nucleic acids. Adenine (A) and guanine (G) are the two purines normally present in DNA and RNA molecules.

PVP 1. Abbreviation for polyvinylpyrrolidone. 2. Abbreviation for plant variety protection.

PVR Abbreviation for plant variety rights.

PWP Abbreviation for permanent wilting point.

pyrethrins Active constituents of pyrethrum (Tanacetum cinerariifolium) flowers, used as insecticides.

pyrimidine A single-ring, nitrogen-containing base present in nucleic acids. Cytosine (C) and thymine (T) are present in DNA, whereas uracil (U) replaces T in RNA. Thymine is a synonym for 5-methyluracil.

pyrogen Bacterial substance that causes fever in mammals.

pyrophosphate A phosphate ion dimer; may be released on hydrolysis of ATP.