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D - H

D loop Abbreviation for displacement loop. Formed when a short stretch of RNA is paired with one strand of DNA. This displaces the original partner DNA; also the displacement of a region of one strand of duplex DNA by a single-stranded invader in the reaction catalyzed by recA.

dA - dT tailing See complementary homopolymeric tailing.

dAb (Full term: single domain antibody). Antibodies with only one (instead of two) protein chain derived from only one of the two domains of the normal antibody structure. Exploits the finding that for some antibodies, half of the molecule binds to its target antigen almost as well as the whole molecule. The major advantage of dAbs over other antibodies is that they can be cloned and expressed into bacteria, so that large numbers of antibodies can be generated and screened in parallel.

DAF See: DNA amplification fingerprinting.

Dalton (Abbreviation: Da). A unit of atomic mass roughly equivalent to the mass of a hydrogen atom. Used as to express molecular weight, which for biological macromolecules is usually in the range kilo- (kDa) to megaDaltons (MDa).

DAMD See: directed amplification of minisatellite DNA.

Darwinian cloning Selection of a clone from a large number of essentially random starting points, rather than isolating a natural gene or making a carefully designed artificial one. Molecules which are more similar to those needed are selected, mutated to generate new variants, and re-selected. The cycle proceeds until the required molecule is found. The advantage of the system is that the selection is from a vast number of possibilities.

dATP Abbreviation for deoxyadenosine 5'-triphosphate. dATP is required for DNA synthesis since it is a direct precursor molecule. See: adenosine, adenylic acid.

dCTP Abbreviation for deoxycytidine 5'-triphosphate. dCTP is required for DNA synthesis since it is a direct precursor molecule. See: cytidine, cytidylic acid.

ddNTP Abbreviation for di-deoxynucleotide.

death phase The final growth phase of cell culture, during which nutrients have been depleted and cell number decreases.

deceleration phase The phase of declining growth rate, following the linear phase and preceding the stationary phase in most batch-suspension cultures. See: growth phase.

de-differentiation The process, in response to wounding and in tissue cultures, by which plant cells can become unspecialized and start to proliferate by cell division to form a mass of undifferentiated cells (or callus) which, in response to appropriate stimuli, may later differentiate again to form either the same cell type or a different one.

defective virus A virus that, by itself, is unable to reproduce when infecting its host cell, but that can grow in the presence of another virus. This other virus provides the necessary molecular machinery that the first virus lacks.

deficiency Lack of adequate supply of nutritional, enzymatic, or environmental requirements, so that development, growth or physiological functions are affected.

defined 1. Fixed conditions of medium, environment and protocol for growth. 2. Precisely known and stated elements of a tissue culture medium.

degeneracy The specification of one amino acid by more than one codon. It arises from the inevitable redundancy resulting from the 64 possible codons encoding only 20 amino acids.

degeneration 1. Changes in cells, tissues or organs due to disease. 2. The reduction in size or complete loss of organs during evolution.

dehalogenation The removal of halogen atoms (fluorine, chlorine, bromine, iodine) from molecules, for example during biodegradation.

dehiscence The spontaneous and often violent opening of a fruit, seed pod or anther to release and disperse the seeds or pollen.

dehydrogenase An enzyme that catalyses the removal of hydrogen atoms in biological reactions.

dehydrogenation A chemical reaction in which hydrogen is removed from a compound.

de-ionized water Water from which most salts have been removed - with varying degrees of efficiency - by ion exchange.

deletion A mutation involving the removal of one or more base pairs in a DNA sequence. Large deletions are sometimes microscopically visible in karyotype analyses.

deliberate release In a biotechnology context, the intentional release of genetically modified organisms.

delta endotoxins See: cry proteins.

deme A group of organisms in the same taxon.

demineralize To remove the mineral content (salts, ions) from a substance, especially water. Removal methods include distillation, electrodialysis and ion exchange. See: de-ionized water.

denature To disrupt the normal in vivo conformation of a nucleic acid or (more usually) a protein by physical or chemical means, usually accompanied by the loss of activity. See: denatured DNA, denatured protein.

denatured DNA Double-stranded DNA that has been converted to single strands by breaking the hydrogen bonds linking complementary nucleotide pairs. Often reversible. Usually achieved by heating.

denatured protein Altering the in vivo conformation of a protein by heat or salt treatment, thereby destroying its biological activity. Unlike denatured DNA, denatured proteins are seldom able to be renatured.

denaturing gradient gel electrophoresis (Abbreviation: DGGE). An electrophoresis method for separating similar sized DNA fragments on the basis of their sequence, by applying across the gel a gradient of increasingly denaturing conditions (usually by increasing the concentration of a denaturing chemical, such as formamide or urea). As the double-stranded molecules denature into a partially and eventually a fully single-stranded state, their electrophoretic mobility changes.

dendrimer A polymer that repeatedly branches until stopped by the physical constraint of having formed a complete, hollow sphere. These structures possess sites on their exterior surface to which DNA fragments can be attached, and are thus useful as carriers of DNA for transgenesis.

denitrification A chemical process in which nitrates in the soil are reduced to molecular nitrogen, which is released to the atmosphere.

density gradient centrifugation High-speed centrifugation in which molecules are separated on the basis of their different densities using a concentration gradient of caesium chloride or sucrose. The density gradient may either be formed before centrifugation by mixing two solutions of different density (as in sucrose density gradients) or it can be formed by the process of centrifugation itself (as in CsCl and Cs2SO4 density gradients).

deoxyadenosine See: adenosine, dATP.

deoxycytidine See: cytidine, dCTP.

deoxyguanosine See: guanosine, dGTP.

deoxyribonuclease See: DNase.

deoxyribonucleic acid See: DNA.

deoxyribonucleoside See: nucleoside.

deoxyribonucleotide See: nucleotide.

deoxyribose (2-deoxyribose) See: ribose.

deoxythymidine Strictly correct but rarely used synonym for thymidine.

derepression The process of "turning on" the expression of a gene or set of genes whose expression has been repressed (turned off), usually by the displacement of a repressor from a promoter, since, when attached to the DNA, the repressor prevents transcription.

derivative 1. Resulting from or derived from. 2. Term used to identify a variant during meristematic cell division.

desiccant Any compound used to remove moisture or water.

desoxyribonucleic acid Obsolete spelling of deoxyribonucleic acid.

desulphurization See: biodesulphurization.

detergent Substance which lowers the surface tension of a solution, improving its cleaning properties.

determinate growth Growth determined and limited in time, with a bud or flower terminating the growth of the main axis. Once established, it is usually irreversible. Opposite: indeterminate growth.

determination Process by which undifferentiated cells in an embryo become committed to develop into specific cell types, such as neurons, fibroblasts or muscle cells.

determined Describing embryonic tissue at a stage when it can develop only as a certain kind of tissue.

development The sum total of events that contribute to the progressive elaboration of an organism. The two major aspects of development are growth and differentiation.

deviation 1. An alteration from the typical form, function or behaviour. Mutation or stress are the common reasons behind deviation. 2. A statistical term describing the difference between an actual observation and the mean of all observations.

dextrin An intermediate polysaccharide compound resulting from the hydrolysis of starch to maltose by amylase enzymes.

dG - dC tailing See: complementary homopolymeric tailing.

DGGE See: denaturing gradient gel electrophoresis.

dGTP Abbreviation for deoxyguanosine 5'-triphosphate. dGTP is required for DNA synthesis since it is a direct precursor molecule. See: guanosine, guanylic acid.

diagnostic procedure A test or assay used to determine the presence of a specific substance, organism or nucleic acid sequence alteration, etc.

diakinesis A stage of meiosis at the end of prophase I, in which the contraction of the chromosomes is almost at a maximum, pairing configurations are well defined, the nucleolus normally disappears and the nuclear envelope is disrupted.

dialysis A biochemical technique by which large molecules such as proteins in solution are separated from smaller species such as salts. The technique is based on the properties of certain membrane structures, which selectively only allow the passage of the smaller molecules. A frequently used method for the purification of proteins.

diazotroph An organism that can fix atmospheric nitrogen.

dicentric chromosome A chromosome having two active centromeres.

dichogamy The condition in which the male and the female reproductive organs of a flower (or certain hermaphroditic animals) mature at different times, thereby making self-fertilization improbable or impossible.

dicot See: dicotyledon.

dicotyledon (Abbreviation: dicot). A plant with two cotyledons. One of the two major classes of flowering plants (along with the monocotyledons). Examples include many crop plants (potato, pea, beans), ornamentals (rose, ivy) and timber trees (oak, beech, lime).

di-deoxynucleotide (Abbreviations: ddNTP,didN). A synthetic deoxynucleotide that lacks a 3'-hydroxyl group, and is thus unable to form the 3'?5' phosphodiester bond necessary for chain elongation. Used as strand terminators in the Sanger DNA sequencing reaction and in the treatment of some viral diseases.

didN See: di-deoxynucleotide.

differential centrifugation A method for separating sub-cellular particles according to their sedimentation coefficients, which are roughly proportional to their size. Cell extracts are subjected to a succession of centrifuge runs at progressively faster rotation speeds. Large particles, such as nuclei or mitochondria, will be precipitated at relatively slow speeds; higher G forces will be required to sediment small particles, such as ribosomes.

differential display A method to identify mRNAs which are present at different levels in different tissues, or in response to specific treatments. The mRNAs are converted to cDNA, and a defined proportion of these are amplified by the polymerase chain reaction, and separated by electrophoresis.

differentially permeable Referring to a membrane, through which different substances diffuse at different rates. Some substances may be unable to diffuse through such a membrane, usually because they are too large to fit through the pores of the membrane.

differentiation A process as a result of which unspecialized cells develop structures and functions characteristic of a particular type of cell, typically during the process of development from one cell to many cells, accompanied by a modification of the new cells for the performance of particular functions. The process is generally irreversible in vivo in higher organisms. In tissue culture, the term is used to describe the formation of different cell types.

diffusion The spontaneous movement of molecules from a region of higher concentration to a region of lower concentration.

digest To treat DNA molecules with one or more restriction endonucleases in order to cleave them into smaller fragments.

dihaploid An individual which arises from a doubled haploid.

dihybrid An individual that is heterozygous for two pairs of alleles; the progeny of a cross between homozygous parents differing at two loci.

dimer 1. A molecule formed by the covalent combination of two monomers, generally accompanied by elimination of water. 2. The reversible association of two similar (or nearly similar) molecules. The active form of many enzymes is as a dimer between two non-active monomeric subunits.

dimethyl sulphoxide (Abbreviation: DMSO). A highly hygroscopic liquid and powerful solvent with little odour, colour or toxicity when pure. It is employed in small quantities to dissolve organic substances in tissue culture media preparation and has uses as a cryoprotectant and in promoting the passage of chemicals through skin.

dimorphism The existence of two distinctly different types of individuals within a species. An obvious example is sexual dimorphism in mammals.

dinucleotide A nucleotide dimer.

dioecious A plant species in which male and female flowers form on different plants.

diplochromosome See: endoreduplication.

diploid The status of having two complete sets of chromosomes, most commonly one set of paternal origin and the other of maternal origin. Somatic tissues of higher plants and animals are ordinarily diploid in chromosome constitution, in contrast with the haploid gametes.

diplonema Stage in prophase I of meiosis following the pachytene stage, but preceding diakinesis, in which one pair of sister chromatids begin to separate from the other pair.

diplotene (adj.) See diplonema.

direct embryogenesis The formation in culture, on the surface of zygotic or somatic embryos or on explant tissues (leaf section, root tip, etc.), of embryoids without an intervening callus phase. Opposite: indirect embryogenesis.

direct organogenesis Formation of organs directly on the surface of cultured intact explants. The process does not involve callus formation. Opposite: indirect organogenesis.

direct repeat Two or more stretches of DNA within a single molecule which have the same nucleotide sequence in the same orientation. Direct repeats may be either adjacent to one another or far apart on the same molecule.

directed amplification of minisatellite DNA (Abbreviation: DAMD). A polymerase chain reaction technique used for obtaining molecular markers in the region of minisatellites. To target these regions, one of the primers is directed to a VNTR core sequence.

directed mutagenesis The generation of changes in the nucleotide sequence of a cloned gene by one of several procedures. Undertaken to explore the relationship between nucleotide sequence and gene function, and to modify gene products. Synonym: in vitro mutagenesis.

directional cloning The technique by which a vector and a DNA insert are both digested with two different restriction endonucleases to create non-complementary sticky ends at either end of both molecules, so favouring the insert to be ligated into the vector in a specific orientation, while also preventing the vector from re-circularizing.

disaccharide A dimer consisting of two covalently linked monosaccharides.

disarm The deletion from a plasmid or virus of genes that are pathogenic.

discontinuous variation Variation where individuals can be classified as belonging to one of a set of discrete, non-overlapping classes. Generated by simple genetic control of a trait (one or a small number of genes, each of large effect) and involving minimal non-genetic effect. Characters showing discontinuous variation are referred to as qualitative. Opposite: continuous variation.

discordant Members of a pair showing different, rather than similar, characteristics.

disease resistance The genetically determined ability to prevent the reproduction of a pathogen, thereby remaining healthy. Some resistances operate by pathogen exclusion, some by preventing pathogen spread, and some by tolerating pathogen toxin.

disease-free A plant or animal certified through specific tests as being free of specified pathogens. Should be interpreted to mean "free from any known disease" as "new" diseases may yet be discovered to be present.

disease-indexing Disease-indexed organisms have been assayed for the presence of known diseases according to standard testing procedures.

disinfection Attempted elimination by chemical means of internal micro-organisms (particularly pathogens) from a culture or sample; rarely attained. See: sterilize (1).

disinfestation The elimination or inhibition of the activity of surface-adhering micro-organisms and removal of insects.

disjunction Separation of homologous chromosomes during anaphase I of meiosis, or of sister chromatids during anaphase of mitosis and anaphase II of meiosis.

disomic (adj.) See disomy.

disomy The presence of a pair of a specific homologous chromosomes. This is the norm for diploids.

dispense The transfer of a measured volume of a solution.

disrupter gene Used to enforce the sterility of seed saved from a genetically engineered crop. See: genetic use restriction technology.

dissecting microscope A microscope with a magnifying power of about 50x, used as an aid in the manipulation of small objects, e.g. excision of embryos from young zygotes.

dissection Separation of a tissue by cutting into components, for analysis or observation.

distillation The process of heating a mixture to separate the more volatile from the less volatile parts, and then condensing fractions of the resulting vapour so as to produce a more nearly pure or refined substance.

disulphide bond See: disulphide bridge.

disulphide bridge A chemical bond between pairs of sulphur atoms that stabilizes the three-dimensional structure of proteins, and hence the protein's normal function. These form particularly readily between cysteine residues in the same or different peptide molecules. Synonym: disulphide bond.

ditype In fungi, a tetrad that contains two kinds of meiotic products (spores), e.g. 2AB and 2ab.

diurnal An event that occurs repetitively on a daily basis, generally during daylight hours.

dizygotic twins Two-egg twins, i.e. a pair of individuals that shared the same uterus at the same time, but which arose from separate and independent fertilization of two ova.

DMSO See: dimethyl sulphoxide.

DNA Abbreviation for deoxyribonucleic acid, former spelling desoxyribonucleic acid. A long chain polymer of deoxyribonucleotides. DNA constitutes the genetic material of most known organisms and organelles, and usually is in the form of a double helix, although some viral genomes consist of a single strand of DNA, and others of a single- or a double-stranded RNA. See: base pair, genetic code.

DNA amplification Many-fold multiplication of a particular DNA sequence either in vivo in a plasmid, phage or other vector; or in vitro using, most commonly, the polymerase chain reaction.

DNA amplification fingerprinting (Abbreviation: DAF). A arbitrarily primed polymerase chain reaction technique for obtaining molecular markers using very short (5-8 bp) primers.

DNA chip See: micro-array.

DNA cloning See: gene cloning.

DNA construct A chimeric DNA molecule, carrying all the genetic information necessary for its transgenic expression in a host cell.

DNA delivery system A generic term for any procedure that transports DNA into a recipient cell.

DNA diagnostics The use of DNA polymorphisms to detect the presence of a specific sequence, which could indicate the presence of a contaminant, of a pathogen, or of a particular allele at a target gene. Most commonly utilises the polymerase chain reaction.

DNA fingerprint A description of the genotype of an individual from the pattern of DNA fragments obtained from DNA fingerprinting. Synonym: DNA profile.

DNA fingerprinting The derivation of unique patterns of DNA fragments obtained using a number of marker techniques; historically these were RFLPs, but latterly they are generally polymerase chain reaction based. Synonym: genetic fingerprinting.

DNA helicase An enzyme that catalyses the unwinding of the complementary strands of a DNA double helix. Synonym: gyrase.

DNA hybridization The annealing of two single-stranded DNA molecules, possibly of different origin, to form a partial or complete double helix. The degree of hybridization varies with the extent of complementarity between the two molecules, and this is exploited to test for the presence of a specific nucleotide sequence in a DNA sample.

DNA ligase An enzyme that catalyses a reaction to link two separate DNA molecules via the formation of a phosphodiester bond between the 3'-hydroxyl end of one and the 5'-phosphate of the other. Its natural role lies in DNA repair and replication. An essential tool in recombinant DNA technology, as it enables the incorporation of foreign DNA into vectors.

DNA micro-array See: micro-array, somatic cell hybrid panel, radiation hybrid cell panel

DNA polymerase See: polymerase.

DNA polymorphism The existence of two or more alteRNAtive alleles at a DNA-based marker locus.

DNA primase An enzyme that catalyses the synthesis of the short strands of RNA that initiate the synthesis of DNA strands.

DNA probe See: probe.

DNA profile See: DNA fingerprint.

DNA repair A variety of mechanisms that repair errors (e.g. the incorporation of a non-complementary nucleotide) that occur naturally during DNA replication.

DNA replication The process whereby DNA copies itself, under the action of and control of DNA polymerase.

DNA sequencing Procedures for determining the nucleotide sequence of a DNA fragment. Two common methods available: 1. The Maxam Gilbert technique, which uses chemicals to cleave DNA into fragments at specific bases; or, most commonly, 2. the Sanger technique (also called the di-deoxy or chain-terminating method) which uses DNA polymerase to make new DNA chains, in the presence of di-deoxynucleotides (chain terminators) to stop the chain randomly as it grows. In both cases, the DNA fragments are separated according to length by polyacrylamide gel electrophoresis, enabling the sequence to be read directly from the gel. The procedure has become increasingly automated and large-scale in recent years.

DNA topo-isomerase An enzyme that catalyses the introduction or removal of supercoils in DNA. Synonym: topo-isomerase.

DNA transformation See: transformation.

DNA vaccine A vaccine generated by the injection of specific DNA fragments to stimulate an immune response.

DNAase See DNAse.

DNAse Abbreviation for deoxyribonuclease. Any enzyme that catalyses the cleavage of DNA phosphodiester bonds. DNAse I is a digestive endonuclease secreted by the pancreas, that degrades DNA into shorter fragments. Many other endonucleases and exonucleases are involved in DNA repair and replication. Synonym: DNAase. See: restriction endonuclease.

Dolly The first mammal (a sheep) to be created (via nuclear transfer) by the cloning of an adult cell (from the mammary tissue of a ewe). This showed that the process of differentiation into adult tissue is not, as previously thought, irreversible.

domain A portion of a protein or DNA molecule that has a discrete function or conformation. At the protein level, can be as small as a few amino acid residues or as large as half of the entire protein.

dominance The gene action exhibited by a dominant allele.

dominant 1. Of alleles, one whose effect with respect to a particular trait is the same in heterozygotes as in homozygotes. The opposite is recessive. 2. Of an individual animal, one that is allowed priority in access to food, mates, etc., by others of its species because of its success in previous aggressive encounters. 3. Of an animal or plant species, the most conspicuously abundant and characteristic in a particular location or environment.

dominant (-acting) oncogene A gene that stimulates cell proliferation and contributes to oncogenesis when present in a single copy.

dominant marker selection Selection of cells via a gene encoding a product that enables only the cells that carry the gene to grow under particular conditions. For example, plant and animal cells that express the introduced neor gene are resistant to neomycin and analogous antibiotics, while cells that do not carry neor are killed. See: positive selection.

dominant selectable marker A gene that allows the host cell to survive under conditions where it would otherwise die. Synonym: positive selectable marker.

donor junction site The junction between the 5' end of an exon and the 3' end of an intron. See: acceptor junction site.

donor plant See: ortet.

dormancy A period in the life of an animal (hibeRNAtion and aestivation) or plant during which growth slows or completely ceases. Evolved to allow survival of adverse environmental conditions. Annual plants survive the winter as dormant seeds, while many perennial plants survive as dormant tubers, rhizomes, or bulbs. Premature breaking of seed dormancy post harvest can be a major problem for maintaining nutritional and/or functional quality, while difficulties in breaking dormancy will lead to poor germination of the crop. See: quiescent.

dosage compensation A regulatory mechanism for sex-linked genes, to allow equivalent levels of gene expression from (in mammals) XY or XX genotypes, even though the gene copy number in XX is double that in XY. See: sex linkage, Barr body.

double crossing-over The formation of two chiasmata within a chromosome arm, leading to the generation of a double recombinant gamete with respect to genes located within the segment defined by the two genes concerned.

double fertilization A process, unique to flowering plants, in which two male nuclei, which have travelled down the pollen tube, separately fuse with different female nuclei in the embryo sac. The first male nucleus fuses with the egg cell to form the zygote; the second male nucleus fuses with the two polar nuclei to form a triploid nucleus that develops into the endosperm.

double helix Describes the coiling of the two strands of the double-stranded DNA molecule, resembling a spiral staircase in which the base pairs form the steps and the sugar-phosphate backbones form the rails on each side. One strand runs 3'?5', while the complementary one runs 5'?3'

double recessive An organism homozygous for a recessive allele at each of two loci.

double-stranded complementary DNA (Abbreviation: dscDNA). A double-stranded DNA molecule created from a cDNA template.

double-stranded DNA (Abbreviation: dsDNA). Two complementary strands of DNA annealed in the form of a double helix. Synonym: duplex DNA.

doubling time See: cell generation time.

down promoter mutation A mutation that decreases the frequency of initiation of transcription. This leads to a fall in the level of mRNA compared to the wild type state.

down-regulate To induce genetically a reduction in the level of a gene's expression.

downstream 1. With respect to DNA, the nucleotides that lie in the 3' direction from the point of reference, which is frequently the site at which transcription is initiated. This is generally designated +1, with downstream nucleotides numbered +2, +10 etc. 2. In chemical engineering, those phases of a manufacturing process that follow the biotransformation stage. Usually refers to the recovery and purification of the product of a fermentation process. See: downstream processing.

downstream processing A general term for biotechnological processes which follow the biology, i.e. fermentation of a micro-organism or growth of a plant. Particularly relevant to fermentation processes, which produce a large quantity of a dilute mixture of substances, products and micro-organisms. These must be separated, and the product concentrated, purified and converted into a useful form.

drift See: genetic drift.

Drosophila melanogaster The fruit fly, used for many years as a model for eukaryotic genetics. Of the nearly 300 disease-causing genes in the human genome, more than half have an analogous gene in the Drosophila genome.

drug See: therapeutic agent.

drug delivery Method by which a drug is delivered to its site of action. For traditional drugs this is another name for formulation. However, biotechnology has allowed the development of a range of new therapeutic-agent delivery systems, such as liposomes and other encapsulation techniques, and a range of mechanisms that target a therapeutic agent to a particular cell or tissue.

dry weight The weight of tissue obtained following sufficiently prolonged oven-drying at high temperature to remove all water. Freeze-drying may also be employed but generates a slightly different result because bound water is not removed. See: free water.

dscDNA See: double-stranded complementary DNA.

dsDNA See: double-stranded DNA.

dTTP Rarely used but strictly correct abbreviation for deoxythymidine 5'-triphosphate. Required for DNA synthesis since it is a direct precursor molecule. See: TTP.

dual culture A culture made of a plant tissue and one organism (such as a nematode) or an obligate parasite/micro-organism (such as a fungus). Dual culture techniques are used for a variety of purposes, including assessing host-parasite interactions and the production of axenic cultures.

duplex DNA See: double-stranded DNA.

duplication Multiple occurrence of: 1. A DNA sequence within a defined length of DNA; or 2. A specific segment in the same chromosome or genome.

E site See: exit site.

E. coli See: Escherichia coli.

EBV See: estimated breeding value.

EC See: Enzyme Commission number.

ecdysone A steroid hormone in insects stimulating the synthesis of proteins involved in moulting and metamorphosis.

eclosion 1. Emergence of an adult insect from the pupal stage. 2. Initial phase of germination of fungal spores.

ecological diversity See: biodiversity.

economic trait locus (Abbreviation: ETL). A locus influencing a trait that contributes to producer's income.

ecosystem The complex of a living community and its environment, functioning as an ecological unit in nature. See: abiotic; biotic factors.

ecotype A population or a strain of an organism that is adapted to a particular habitat.

ectopic Anomalous situation or relation, particularly with respect to pregnancy, where the foetus is implanted outside the uterus.

edible vaccine Edible antigen-containing material, that activates the immune system via gut-associated lymphoid tissues. A preferred route for vaccine administration, particularly in areas where the technological infrastructure needed for maintenance of vaccines is absent. The vaccine is synthesized in vivo in the edible parts of transgenic plants (e.g. grains, tubers, fruits, etc.) or eggs.

editing See: splicing (1).

EDTA See: ethylenediamine tetraacetic acid.

EDV Abbreviation for essential derivation of varieties.

effector cells Cells of the immune system that are responsible for the production of cell-mediated cytotoxicity.

effector molecule A molecule that influences the behaviour of a regulatory molecule, such as a repressor protein, thereby influencing gene expression.

egg 1. The fertilized zygote in egg-laying animals. 2. The mature female reproductive cell in animals and plants.

EGS See: external guide sequence.

EIA See: enzyme immunoassay, ELISA.

elastin A fibrous protein that is the major constituent of the yellow elastic fibres of animal connective tissue.

electro-blotting The electrophoretic transfer of DNA, RNA or protein from a gel, in which they have been separated, to a support matrix, such as nitrocellulose. A transfer technique employed in Southern and northern blotting.

electrochemical sensor Biosensors, such as an enzyme electrode, in which a biological process is harnessed to an electrical sensor system. Other types couple a biological event to an electrical one via a range of mechanisms, including the reduction of oxygen or pH change.

electron microscope (Abbreviation: EM). A microscope that uses an electron beam focussed by magnetic 'lenses'. See: scanning electron microscope.

electrophoresis A ubiquitous molecular biology technique, with many variants, used to resolve complex mixtures of macromolecules into their components. Its principle is to subject samples to an electric field applied across a porous matrix. Molecules will migrate under these conditions at a rate dependent on their net electric charge and/or their molecular weight. See: agarose gel electrophoresis, polyacrylamide gel electrophoresis, denaturing gradient gel electrophoresis, capillary electrophoresis, sodium dodecyl sulphate polyacrylamide gel electrophoresis, thermal gel gradient electrophoresis pulsed-field gel electrophoresis, and iso-electric focusing gel.

electroporation The induction of transient pores in bacterial cells or protoplasts by the application of a pulse of electricity. These pores allow the entry of exogenous DNA into the cell. Widely used for the transformation of bacteria.

ELISA Abbreviation for enzyme-linked immunosorbent assay. An immunoassay, i.e. an antibody-based technique for the diagnosis of the presence and quantity of specific molecules in a mixed sample. It combines the specificity of an immunoglobulin with the detectability of an enzyme-generated coloured product. In one form, the primary antibody (specific to the test protein) is adsorbed onto a solid substrate, and a known amount of the sample is added; all the antigen in the sample is bound by the antibody. A second antibody (conjugated with an enzyme) specific for a second site on the test protein is added; and the enzyme generates a colour change in the presence of a substrate reagent.

elite tree A phenotypically superior tree in a tree breeding programme.

elongation factors Soluble proteins required for the elongation of polypeptide chains on ribosomes.

embryo An immature organism in the early stages of development. In mammals, develops in the first months in the uterus. In plants, it is the structure that develops in the megagametophyte, as result of the fertilization of an egg cell, or occasionally without fertilization. Somatic embryos can often be induced in in vitro plant cell cultures.

embryo cloning The creation of identical copies of an embryo by embryo splitting or by nuclear transfer from undifferentiated embryonic cells.

embryo culture The culture of embryos on nutrient media.

embryo multiplication and transfer (Abbreviation: EMT). The cloning of animal embryos and their subsequent transfer to recipients via artificial inembryonation. The cloned embryos can be derived from embryonic or adult tissue.

embryo rescue A sequence of tissue culture techniques utilized to enable a fertilized immature embryo resulting from an interspecific cross to continue growth and development, until it can be regenerated into an adult plant.

embryo sac The mature female gametophyte in angiosperms. Generally a seven-celled structure - two synergids, one egg cell, three antipodal cells (each with a single haploid nucleus) and one endosperm mother cell with two haploid nuclei.

embryo sexing The determination of the sex of an embryo prior to birth. Typically achieved by the polymerase chain reaction-mediated amplification of DNA extracted from a sample of embryonic tissue. Dependent on the availability of reliable markers for the differential sex chromosome.

embryo splitting The splitting of young embryos into several sections, each of which develops into an animal. A form of animal cloning, i.e. of producing animals that are genetically identical. In practice, the number of animals that can be produced from a single embryo is less than 10.

embryo storage Cryogenic preservation of animal embryos, allowing inembryonation or other manipulations long after embryo formation.

embryo technology Generic name for any modification of mammalian embryos. It encompasses embryo cloning, embryo splitting, embryo storage, and in vitro fertilization.

embryo transfer (Abbreviation: ET). See: embryo multiplication and transfer, multiple ovulation and embryo transfer.

embryogenesis 1. (General) Development of an embryo. 2. (In plants) In vitro formation of plants from plant tissues, through a pathway closely resembling normal embryogeny from the zygote. Somatic cell embryogenesis is an alteRNAtive technique. The generation of embryos has two stages: initiation and maturation. Initiation needs a high level of the group of plant hormones called auxins; maturation needs a lower level.

embryoid Plant biotechnology term no longer commonly used. An embryo-like body developing in vitro, forming a complete, self-contained plantlet with no vascular connection with the callus.

embryonic stem cells (Abbreviation: ES cells). Cells of the early embryo that can give rise to all differentiated cells, including germ line cells.

emission wavelength The specific wavelength of light emitted by a fluorescent molecule, such as a labelled probe, upon absorption of light at the (higher) excitation wavelength.

EMT See: embryo multiplication and transfer.

encapsidation The process by which the nucleic acid of a virus is enclosed in a capsid.

encapsulating agents Anything which forms a shell around an enzyme or bacterium, common agents being polysaccharides such as alginate or agar. The agents are inert and allow nutrients and oxygen to diffuse readily into and out of the sphere, and are easy to convert from gel (solid) to sol (liquid) or solution form by altering the temperature or the concentration of ions.

encapsulation Any method packaging an enzyme or bacterium and maintaining its normal functions. Used to immobilize cells in a bioreactor.

encode The gene product specified by a particular nucleic acid sequence. See: genetic code.

endangered species A plant or animal species in immediate danger of extinction because its population number has reached a critical level, or its habitat has been drastically reduced.

endemic Describing an organism, often a disease or pest, that is always present in a stated area.

end-labelling The introduction of a readily-visualized tag at the end of a DNA or RNA molecule. A commonly used method is to introduce a 32P atom onto the end of a DNA molecule by means of the enzyme T4 polynucleotide kinase.

endocrine gland Any gland in an animal that manufactures hormones and secretes them directly into the bloodstream to act at distant sites in the body, known as target organs or cells.

endocrine interference Interference with the normal balance of hormones.

endocytosis The process by which materials enter a cell without passing through the cell membrane. The membrane folds around material outside the cell, resulting in the formation of a sac-like vesicle inside which the material is entrapped. This vesicle is then pinched off from the cell surface so that it lies within the cell. See: phagocytosis, pinocytosis.

endoderm The internal layer of cells of the gastrula, which develops into the alimentary canal (gut) and digestive glands of the adult.

endodermis The layer of living cells, with various characteristically thickened walls and no intercellular spaces, which surrounds the vascular tissue of certain plants and occurs in nearly all roots and certain stems and leaves. It separates the cortical cells from cells of the pericycle.

endogamy See: inbreeding.

endogenous Derived from within; from the same cell type or organism. Opposite: exogenous.

endomitosis Duplication of chromosomes without division of the nucleus, resulting in a doubling (or more) in the chromosome number within a cell.

endonuclease An enzyme that cleaves a phosphodiester bond within a DNA strand, forming two smaller strands. See: exonuclease, restriction endonuclease.

endophyte An organism that lives inside a plant.

endoplasmic reticulum (Abbreviation: ER). A cytoplasmic net of membranes, adjacent to the nucleus, visible under the electron microscope. The sites of protein synthesis.

endopolyploidy The net result of endomitoses. The somatic chromosome number has doubled (or more), forming a polyploid cell line. If these differentiate into a germ line, then the gametic number will have also increased proportionately, giving rise to homogeneously polyploid individuals, termed endopolyploids.

endoprotease An enzyme that cleaves internal peptide bonds within a polypeptide molecule. Site of cleavage is usually specific to certain amino acid residues.

endoreduplication Chromosome reproduction during interphase. Four-chromatid chromosomes (diplochromosomes) are seen during this phase.

endosperm The nutritive tissue that develops in the seed of most angiosperms, containing varying proportions of carbohydrate (usually starch), protein and lipid. In most diploid plants, the endosperm is triploid.

endosperm mother cell One of the seven cells of the mature plant embryo sac, containing the two polar nuclei and, which, following fertilization, gives rise to the primary endosperm cell from which the endosperm develops.

endotoxin A component of the cell wall of gram-negative bacteria that elicits, in mammals, an inflammatory response and fever.

end-product inhibition The inhibition of an enzyme by a metabolite. Typically, the enzyme is the first enzyme in a biosynthetic pathway, and the metabolite the product of the last step in the pathway. See: feedback inhibition.

enhancer 1. A substance or object that increases a chemical activity or a physiological process. 2. A eukaryotic DNA sequence (also found in some eukaryotic viruses) which increases the transcription of a gene. Located up to several kbp, usually (but not exclusively) upstream of the gene in question. In some cases can activate transcription of a gene with no (known) promoter. Synonyms: enhancer element; enhancer sequence. 3. A major or modifier gene that increases the rate of a physiological process.

enhancer element See: enhancer.

enhancer sequence See: enhancer.

enolpyruvyl-shikimate-3-phosphate synthase (Abbreviation EPSP synthase or EPSPS). An enzyme produced by virtually all plants, which is essential for normal metabolism, and for the biosynthesis of aromatic amino acids. Glyphosate- and sulfosate-containing herbicides act by inhibiting EPSP synthase activity, but because strain CP4 of Agrobacterium sp. is unaffected by glyphosate, the introduction of the CP4 EPSPS gene into crop plants generates a tolerance of glyphosate-containing herbicides.

enterotoxin A bacterial protein that, following release into the intestine, causes cramps, diarrhoea and nausea.

enucleated ovum Egg cell from which the nucleus has been removed, usually as a preparatory step for nuclear transfer.

enzyme A protein which, even in very low concentration, catalyses specific chemical reactions but is not used up in the reaction. Enzymes are classified into six major groups (1-6), according to the type of reaction they catalyse: 1. oxidoreductases; 2. transferases; 3. hydrolases; 4. lyases; 5. isomerases; 6. ligases. Generally enzymes are named by the addition of the suffix -ase to the name of their substrate, and are classified by a standard numerical system: the Enzyme Commission (EC) number.

enzyme bioreactor A reactor in which a chemical conversion reaction is catalysed by an enzyme.

Enzyme Commission number (Abbreviation: EC number). Systematic label which identifies an enzyme in the technical literature. Consists of four numbers separated by dots: the first classifies the enzyme into one of the six broad enzyme groups (see: enzymes); each group is subdivided into sub-groups, each sub-group into sub-sub-groups, and the last number is specific for the enzyme, e.g. EC is deoxyribonuclease I.

enzyme electrode A type of biosensor, in which an enzyme is immobilized onto the surface of an electrode. When the enzyme catalyses its reaction, electrons are transferred from the reactant to the electrode, and so a current is generated. There are two types of enzyme electrodes: 1. Ampometric (measuring current flow) where the electrode is kept as near zero voltage as possible. When the enzyme catalyses its reaction, electrons move into the electrode, and so a current flows; 2. Potentiometric (measuring changes in electrical potential) when the electrode is held at a voltage which counteracts the voltage determined by the enzyme's tendency to push electrons into it. Usually enzymes transfer their electrons inefficiently to the electrode, so a mediator compound is coated onto the electrode to help the transfer.

enzyme immunoassay A range of immunoassay techniques employing enzymes, which includes ELISA.

enzyme kinetics The quantitative characteristics of enzyme reactions.

enzyme stabilization Maintaining the active conformation of an enzyme. This can be achieved in vitro by providing the appropriate chemical environment and cofactors. In some cases the criticality of these factors can be reduced by binding an antibody to the enzyme, in such a way that the active site of the enzyme is left unblocked.

enzyme-linked immunosorbent assay See: ELISA.

EPD See: expected progeny difference.

epicotyl The upper portion of the axis of a plant embryo or seedling, above the cotyledons.

epidermis 1. The outmost layer of cells of the body of an animal. In invertebrates the epidermis is normally only one cell thick and is covered by an impermeable cuticle. In vertebrates the epidermis is the thinner of the two layers of skin. 2. The outermost layer of cells covering a plant. It is overlaid by a cuticle and its functions are principally to protect the plant from injury and to reduce water loss. Some epidermal cells are modified to form guard cells or hairs of various types. In woody plants the functions of the shoot epidermis are taken over by the periderm tissues and in mature roots the epidermis is sloughed off and replaced by the hypodermis.

epigenesis Describes the developmental process whereby each successive stage of normal development is built up on the foundations created by the preceding stages of development; an embryo is built up from a zygote, a seedling from an embryo, and so on.

epigenetic variation Non-hereditary and reversible variation; often the result of a change in gene expression due to methylation of DNA.

epinasty A process by which the growth of branches or petioles is abnormally pointing downward. This phenomenon is caused by the more rapid growth of the upper side. Epinasty may result from either nutritional deficiencies or irregularities at the plant growth regulator level. Not to be confused with wilting, as epinastic tissues are turgid.

epiphyte A plant that grows upon another plant, but is neither parasitic on it nor rooted in the ground.

episome A genetic extrachromosomal element (e.g. the F factor in Escherichia coli) which replicates within a cell independently of the chromosome and is able to integrate into the host chromosome. The step of integration may be governed by a variety of factors and so the term episome has lost favour and been superseded by the wider term plasmid.

epistasis Interaction between genes at different loci, e.g. one gene suppresses the effect of another gene that is situated at a different locus. Dominance is associated with members of allelic pairs, whereas epistasis describes an interaction among products of non-alleles.

epitope Synonym for antigenic determinant.

epizootic A disease simultaneously affecting a large number of animals.

EPSP synthase Abbreviation for enolpyruvyl-shikimate 3-phosphate synthase.

EPSPS Abbreviation for enolpyruvyl-shikimate 3-phosphate synthase.

equational division A chromosome division in which the two chromatids of each duplicated chromosome separate longitudinally, prior to being incorporated into two daughter nuclei. Seen at the mitotic-type second division of meiosis; also in somatic mitosis and the non-reductional division of meiosis. The number of chromosomes is the same at the end of the division as at the beginning.

equilibrium density gradient centrifugation A procedure used to separate macromolecules based on their density (mass per unit volume).

ER See: endoplasmic reticulum.

Erlenmeyer flask A conical flat-bottomed laboratory flask with a narrow neck, widely used for culturing micro-organisms.

ES cells See: embryonic stem cells.

Escherichia coli A commensal bacterium inhabiting the colon of many animal species, including human. E. coli is widely used as a model of cell biochemical function, and as a host for cloning DNA. In environmental studies, its presence is a key indicator of water pollution due to human sewage effluent. Some strains, notably E. coli 0157:H7, are significant pathogens.

essential amino acid An amino acid required for normal metabolism, but which cannot be synthesized by an organism. It therefore has to be supplied via feed or food.

essential derivation of varieties (Abbreviation: EDV). Genotypes very similar to an originating cultivar, obtained, for example, by the selection of a mutant or a variant individual from plants of the initial variety, or by backcrossing or transformation.

essential element Any of a number of elements required by living organisms to ensure normal growth, development and maintenance.

essential nutrient Any substance required by living organisms to ensure normal growth, development and maintenance.

essential requirement In plant cell tissue culture, comprises inorganic salts, including all of the elements necessary for plant metabolism; organic factors (amino acids, vitamins); usually also endogenous plant growth regulators (auxins, cytokinins and often gibberellins); as well as a carbon source (sucrose or glucose).

EST See: expressed sequence tag.

established culture 1. An aseptic viable explant (See: micropropagation). 2. A suspension culture subjected to several passages with a constant cell number per unit time.

estimated breeding value (Abbreviation: EBV). Twice the expected progeny difference. The difference is doubled because breeding value is a reflection of all the genes of an individual, in contrast to progeny difference, which is a reflection of a sample half of an individual's genes. The predicted performance of the offspring of the mating between any two parents is the average of their EBVs (averaged because each parent makes an equal contribution to each offspring).

estrogen See: oestrogen.

ET Abbreviation for embryo transfer. See: multiple ovulation and embryo transfer.

ethanol Commonly used to disinfect plant tissues, glassware utensils and working surfaces in tissue culture manipulations; to precipitate aqueous solutions of nucleic acids; and to dissolve water-insoluble components of culture media. Synonym: ethyl alcohol.

ethephon A synthetic compound commonly used as a source of ethylene, a gaseous plant growth regulator.

ethidium bromide A fluorescent dye which can intercalate between base pairs of double-stranded DNA, and hence is much used to stain DNA in gels. The dye fluoresces when exposed to UV light. It is a known to be a strong mutagen and is also possibly both a carcinogen and a teratogen.

ethyl alcohol See: ethanol.

ethylene A gaseous plant growth regulator acting on various aspects of vegetative growth, fruit ripening and abscission of plant parts. Synonym: ethene.

ethylenediamine tetraacetic acid (Abbreviation: EDTA). A chelating compound. Used to keep nutrients, such as iron, bound in a soluble form that leaves them still available to the plant cells in vitro. Also a potent inhibitor of DNase activity and therefore used as an additive for long-term storage of dissolved DNA.

etiolation An abnormal increase in stem elongation, accompanied by poor (if any) leaf development. Physiological etiolation is caused by a lack of chlorophyll, and is typical of plants growing under low light intensity or in complete darkness. It can also be induced by some fungal pathogens.

ETL See: economic trait locus.

eucaryote See: eukaryote.

eucaryotic (adj.) See: eukaryote.

euchromatin Chromosomal material that is stained less intensely by certain dyes. Thought to be the chromosomal domains which are gene-rich, since the DNA in these regions remains less contracted than those rich in repetitive DNA - the heterochromatin.

eugenics The application of the principles of genetics to the 'improvement' of humankind. Wholly discredited as a scientific approach since the Nazi period.

eukaryote One of the two major evolutionary clades, characterized by having the nucleus enclosed by a membrane, and possessing chromosomes that undergo mitosis and meiosis. Eukaryotic organisms include animals, plants, fungi and some algae. See: prokaryote.

euploid An organism or cell having a chromosome number that is an exact multiple of the haploid number. Terms used to identify different levels in an euploid series are diploid (2x), triploid (3x), tetraploid (4x) etc. Opposite: aneuploid.

evapotranspiration The net water loss (in vapour form) per unit area of land, both directly from the land surface, and indirectly through transpiring leaves.

evolution The process by which the present diversity of plant and animal life has arisen, and which continues to drive changes in form and mode of existence of all living organisms.

ex-situ conservation The conservation of components of biological diversity outside their natural habitats.

ex vitro Organisms removed from tissue culture and transplanted; generally plants to soil or potting mixture.

ex vivo gene therapy The delivery of a gene or genes to the isolated cells of an individual, with the intention of alleviating a genetic disorder. After culturing, the transformed cells are re-introduced into the individual by transfusion, infusion or injection.

excinuclease The endonuclease-containing protein complex that excises a segment of damaged DNA during excision repair.

excision 1. The natural or in vitro enzymatic removal of a DNA segment from a chromosome or cloning vector. 2. The cutting out and preparation of a tissue, organ, etc., for culture. 3. The removal of adventitious shoots from callus tissue.

excision repair DNA repair processes that involve the removal of a damaged or incorrect segment of one strand of double-stranded DNA and its replacement by the synthesis of a new segment using the complementary strand of DNA as template.

excitation wavelength The specific wavelength of light required to stimulate a fluorescent molecule, such as a labelled probe, to emit light at the (lower) emission wavelength.

excrete To transport material out of a cell or organism.

exit site (Abbreviation: E site). The ribosome binding site that contains the free tRNA prior to its release.

exo III See: exonuclease III.

exocrine gland An animal gland that secretes through a duct.

exodeoxyribonuclease III See: exonuclease III.

exogamy See: outbreeding.

exogenous Produced outside of; originating from, or due to, external causes. Opposite: endogenous.

exogenous DNA DNA that has been derived from one organism, and is to be introduced into a cell a different species. Also referred to as foreign DNA or heterologous DNA.

exon A segment of a eukaryotic gene that is transcribed as part of the primary transcript and is retained, after processing, with other exons to form a functional mRNA molecule. Many eukaryotic genes are composed of a mosaic of exons and introns.

exon amplification A procedure that is used to amplify exons.

exonuclease An enzyme that digests DNA or RNA, beginning at the end of a strand. It therefore requires a free end in order to begin the degradation. 5'-exonucleases require a free 5' end and degrade the molecule in the 5'?3' direction. 3'-exonucleases require a free 3' end and degrade in the opposite direction.

exonuclease III (Abbreviation: exo III). An Escherichia coli enzyme that removes nucleotides from the 3' hydroxyl ends of double-stranded DNA. Synonym: exodeoxyribonuclease III.

exopolysaccharide A polysaccharide that is secreted by a micro-organism into the surrounding environment.

exotoxin A toxin released by a bacterium into the medium in which it grows.

expected progeny difference (Abbreviation: EPD). The predicted performance of the future offspring of an individual for a particular trait, calculated from measurement(s) of the individual's own performance and/or the performance of one or more of its relatives, for the trait in question and/or for one or more correlated traits. Typically, the prediction is expressed as a deviation from a well-defined base population, assuming the individual in question is mated to a sample of individuals whose average genetic merit equals that of the base population. The predicted performance of the offspring of the mating between any two individuals is the sum of their EPDs.

explant A portion of a plant aseptically excised and prepared for culture in a nutrient medium.

explant donor The plant from which an explant has been taken.

explantation The removal of cells, tissues or organs of animals and plants for observation of their growth and development in appropriate culture media.

explosion method A technique for the genetic transformation of cells, in which the transgene is driven into the target (plant) cells by the sudden vaporization (effected by the application of a pulse of high voltage) of a water droplet containing the DNA and gold particles.

exponential phase See: logarithmic phase.

export The removal of a compound from a cell by active transport.

express To transcribe and translate a gene.

expressed sequence tag (Abbreviation: EST). Partially sequenced cDNA clone. Because the read length of a standard DNA sequencing reaction is shorter than the majority of cDNA clones, full length sequence can only be obtained by further manipulations. For the purposes of (1) assigning putative function to a cDNA and (2) designing PCR primers to extract the genomic DNA equivalent to the cDNA, full length sequence is usually unnecessary. By restricting sequencing to a single run, large numbers of cDNAs can be characterized at the EST level.

expression library A cDNA library that has been inserted into a bacterial host cell engineered to express transgenes. See: library.

expression system Combination of host and vector which provides a genetic context for making a cloned gene functional, i.e. produce peptide, in the host cell.

expression vector A cloning vector that has been constructed in such a way that, after insertion of a DNA molecule, its coding sequence is properly transcribed and the mRNA is translated. The cloned gene is put under the control of a promoter sequence for the initiation of transcription, and often also has a transcription termination sequence at its end.

expressivity Degree of expression of a trait controlled by a particular gene. The gene may show different degrees of expression in different individuals. See: variable expressivity.

extension The short single-stranded stretch of nucleotides remaining on a double-stranded DNA molecule, following treatment with a restriction endonuclease which makes a staggered cut. The presence of these unpaired regions make the molecule more easily ligatable, and are thus important in gene cloning. Synonyms: protruding end; sticky end; overhang; cohesive end.

external guide sequence (Abbreviation: EGS). See: guide sequence.

extrachromosomal In eukaryotes, non-nuclear DNA, present in cytoplasm organelles such as mitochondria and chloroplasts. In prokaryotes, non-chromosomal DNA, i.e. plasmids.

extrachromosomal inheritance See: cytoplasmic inheritance.

extranuclear genes Genes residing elsewhere than in the nucleus (e.g. in mitochondria, chloroplasts or plastids).

exude Slowly discharge liquid material (such as tannins or oxidized polyphenols from plant material) through pores or cuts, or by diffusion into the medium.

F factor Abbreviation for fertility factor. A bacterial plasmid that confers the ability to function as a genetic donor in conjugation. See: Hfr.

F1 Abbreviation for filial generation 1. The initial hybrid generation resulting from a cross between two parents. See Fn.

F2 The second hybrid generation, produced either by intercrossing two F1 individuals, or by self-fertilizing an F1 individual. See Fn.

Fab A product of hydrolysis of an IgG antibody, consisting of the variable region with some of the constant region of a heavy chain, and an entire light chain. Contains a single antigen-binding site.

FACS See: fluorescence-activated cell sorting.

factorial mating A mating scheme in which each male parent is mated with each female parent. Made possible in animals by means of in vitro embryo production. Such a mating scheme substantially reduces the rate of inbreeding in a selection programme.

facultative anaerobe An organism that will grow under either aerobic or anaerobic conditions.

FAD See: flavin adenine dinucleotide.

false fruit See: pseudocarp.

false negative A negative assay result that should have been positive.

false positive A positive assay result that should have been negative.

farm animal genetic resources Those animal species that are used, or may be used, for the production of food and agriculture, and the populations within each of them. Within each species, these populations can be classified as wild and feral populations, landraces and primary populations, standardized breeds, selected lines, and any conserved genetic material.

farmers' privilege Rights to hold germplasm, covered by plant variety protection, as a seed source for subsequent seasons. Considered as optional for governments to include in their legislation. Synonym: farmer-saved seed.

farmers' rights Rights first recognized by Resolution 5 of the 1989 FAO Conference as "rights arising from the past, present and future contributions of farmers in the conservation, improvement and the making available of plant genetic resources"; this item became an attachment to the 'International Undertaking on Plant Genetic Resources'. The binding 'International Treaty of Plant Genetic Resources for Food and Agriculture' that resulted from the renegotiations of the Undertaking makes provision for the Farmers' Rights in Article 9.

fascicle See: vascular bundle.

Fc A product of hydrolysis of an IgG antibody, consisting of parts of the constant regions of two heavy chains held together by a disulphide bridge, but excluding the antigen-binding regions, and also excluding the light chains.

fed-batch fermentation Culture of cells or micro-organisms where nutrients are added periodically to the bioreactor.

feedback inhibition The process by which the accumulated end product of a biochemical pathway stops synthesis of that product. The effect is that a late metabolite of a synthetic pathway regulates the synthesis of an earlier step of the pathway. See: end-product inhibition.

fermentation The anaerobic breakdown of complex organic substances, especially carbohydrates, by micro-organisms, yielding energy. Often misused to describe large-scale aerobic cell culture in specialized vessels (fermenters, bioreactors) for secondary product synthesis.

fermentation substrates Materials used as food for growing micro-organisms. The fermentation substrates and the trace materials needed, together with chemicals added to make the fermentation easier, form the culture medium.

fermenter See: bioreactor.

fertile Capable of breeding and reproduction.

fertility factor See: F factor.

fertilization The union of two gametes from opposite sexes to form a zygote. Typically, each gamete contains a haploid set of chromosomes. Hence the zygotic nucleus contains a diploid set of chromosomes. Several categories can be distinguished: 1. Self-fertilization (selfing): fusion of male and female gametes from the same individual. 2. Cross-fertilization (crossing): fusion of male and female gametes from different individuals. 3. Double fertilization; restricted to flowering plants, in which the fusion of one male gamete with the ovum occurs at about the same time as the second male gamete nucleus fuses with the female polar nuclei (or secondary nucleus) to form the endosperm.

fertilizer Any substance that is added to soil in order to increase its productivity. Fertilizers can be of biological origin (e.g. composts), or they can be synthetic (artificial fertilizer).

fetus See: foetus.

Feulgen staining A histochemical stain by which the distribution of DNA in the chromosomes of dividing cell nuclei can be observed.

FIA Abbreviation for fluorescence immunoassay.

fibril A microscopic to sub-microscopic cellulose thread that is part of the cellulose matrix of plant cell walls.

fibroblasts Irregularly shaped, branching cells distributed throughout vertebrate connective tissue. A cell type which is readily cultured in vitro.

fibrous root Root system in which both primary and lateral roots have approximately equal diameters. Opposite: tap root.

field gene bank See: gene bank (2).

filial generation See: F1, F2, Fn.

filter bioreactor A cell culture system, in which cells are grown on a fine mesh of an inert material, which allows the culture medium to flow past it but retains the cells. This is similar in idea to membrane and hollow fibre reactors, but can be much easier to set up, being similar to conventional tower bioreactors, but with the mesh replacing the central reactor space. Synonym: mesh bioreactor.

filter sterilization Process of removing microbial contaminants from a liquid by passing through a filter with pores too small to allow the passage of micro-organisms and spores.

filtration 1. Separation of solids from liquids by using a porous material that only allows passage of the liquid or of solids smaller than the pore size of the filter. The material passing the filter forms the filtrate. 2. Removal of cell aggregates to obtain a filtrate of single cells that can be utilized as plating inocula.

fingerprinting See: DNA fingerprinting.

FISH See: fluorescence in situ hybridization.

fission Asexual reproduction involving the division of a single-celled individual into two daughter single-celled individuals of approximately equal size.

fitness The survival value and the reproductive capability of an individual, compared to that of competitor individuals of the same or other species within a population or an environment.

fixation The situation in which only one allele for a given gene/locus is present in a population. This can occur as a result of direct selection where the allele delivers a greater level of fitness; because of indirect selection, where the locus is linked to a gene that is subject to direct selection; or because of genetic drift.

FLAG See affinity tag.

flaming A technique for sterilizing instruments, to remove live micro-organism contaminants. The instrument is dipped in alcohol, and the alcohol remaining on the instrument is ignited, thereby heat-sterilizing the surface.

flanking region The DNA sequences extending either side of a specific sequence.

flavin adenine dinucleotide (Abbreviation: FAD). A co-enzyme important in various biochemical reactions. It comprises a phosphorylated vitamin B2 (riboflavin) molecule linked to AMP, and functions as a hydrogen acceptor in dehydrogenation reactions. The reduced form is oxidized back to FAD by the electron transport chain, generating two molecules of ATP per molecule of reduced FADH.

flocculant A chemical agent that causes small particles to aggregate (flocculate).

floccule A micro-organism aggregate or colloidal particle floating in or on a liquid. The cloudy appearance of micro-organism contaminated liquid media illustrates the flocculation phenomenon.

flow cytometry Automated measurements on large numbers of individual cells or other small biological materials, made as the cells flow one by one in a fluid stream past optical and/or electronic sensors. A similar approach may be used for sorting cells - see fluorescence-activated cell sorting.

fluorescence immunoassay (Abbreviation: FIA). An immunoassay based on the use of fluorescence-labelled antibody.

fluorescence in situ hybridization (Abbreviation: FISH). Hybridization of cloned, fluorescently labelled DNA or RNA, to intact biological materials, notably chromosome spreads and thin tissue sections. The technique allows the visualization of the physical location of nucleic acid sequences homologous to the probe, and is used for the placement of genes on chromosomes and for the spatial and temporal pattern of gene expression of specific mRNA molecules.

fluorescence-activated cell sorting (Abbreviation: FACS). A flow cytometry method in which targets (cells, individual chromosomes etc.) are labelled with a fluorescent dye, which is excited by a laser beam. Differences in the fluorescence signal emitted are used as a criterion for sorting the material. A specific application is in sperm sexing.

fluorescent probe A probe which is labelled with a fluorescent dye, so that the signal emitted can be captured by photometric methods.

flush end See: blunt end.

flush-end cut See: blunt-end cut.

F1, F2, Fn Subsequent hybrid generations, counting from the F1. Thus, for example, F4 describes the progeny of the F3, which is the progeny of the F2 generation, where all progeny are derived from intercrossing or self-fertilization.

foetus Pre-natal stage of a viviparous animal, between the embryonic stage and birth. AlteRNAtive spelling: fetus. See: embryo.

fog Fine particles of liquid suspended in the air, such as of water in a fog chamber used for acclimatizing recent ex vitro transplants. See: mist propagation.

fold-back The structure of a double-stranded DNA molecule formed when a molecule containing an inverted repeat sequence is denatured and then allowed to re-anneal at low DNA concentrations. Under these conditions, the repeated sequence self-anneals to form a double-stranded region within each of the separated strands of the original molecule.

folded genome The condensed state of the chromosomal DNA of a bacterium. The DNA is segregated into domains, and each domain is independently negatively supercoiled.

follicle An enclosing cluster of cells that protects and nourishes a cell or structure within. Thus a follicle in the ovary contains a developing egg cell, while a hair follicle envelops the root of hair.

follicle stimulating hormone (Abbreviation: FSH). A hormone, secreted by the anterior pituitary gland in mammals, that stimulates the ripening of the specialized structures in the ovary (Graafian follicles) that produce ova in female mammals; and in males, the formation of sperm in the testis. FSH is a major constituent of fertility drugs.

food processing enzyme Enzyme used to control food texture, flavour, appearance, or nutritional value. Amylases break down complex polysaccharides to simpler sugars; proteases tenderize meat proteins. A prominent target of food biotechnology is to develop novel food enzymes which can improve the quality of processed foods.

forced cloning The insertion of foreign DNA into a cloning vector in a predetermined orientation.

foreign DNA Exogenous DNA that is incorporated into a host genome.

formulation See: medium formulation.

forskolin A medicinal, diterpenoid, compound exclusive to plant roots and used in the preparation of drugs for the treatment of cardiomyopathy, glaucoma and certain cancers.

fortify To add strengthening components or beneficial ingredients to a nutrient medium.

forward mutation A mutation from the wild type to the mutant type. Opposite: reverse mutation.

fouling The coating or plugging (by materials or micro-organisms) of equipment, thus preventing it from functioning properly.

founder animal An organism that carries a transgene in its germ line and can be used in matings to establish a pure-breeding transgenic line, or one that acts as a breeding stock for transgenic animals.

founder principle The possibility that a new, isolated population, initiated by a small number of individuals taken from a parent population, may be genetically different from the parent population, because the founding individuals might not be typical of the parent population. See: genetic drift.

four-base cutter A type II restriction endonuclease with a four-nucleotide recognition site. Because any particular sequence of four bases occurs more frequently by chance than one of six bases, four-base cutters cleave more frequently than six-base cutters, and therefore generate, on average, smaller restriction fragments. Synonyms: four-base-pair-cutter, four-cutter.

fractionation The separation in components of a complex mixture of molecules.

fragment Partial structure. See: restriction fragment.

frameshift mutation A mutation that changes the reading frame of a DNA, either by the insertion or the deletion of nucleotides. Because of the triplet nature of codons, this occurs if the number of nucleotides involved is not a multiple of three.

free water The cellular water released into the intercellular spaces when tissue is frozen and thawed. Opposite: bound water.

free-living conditions Natural or greenhouse conditions experienced by plantlets upon transfer from in vitro conditions to soil. Prior to transfer, nutrients were supplied by the culture medium, but following transfer, plantlets must take up nutrients from soil and synthesize their own food supply.

freeze preservation See: cryobiological preservation.

freeze-dry The removal of water as vapour from frozen material under vacuum. Used to measure water content and to preserve samples, particularly spores. Unlike oven-drying, bound water remains associated with the specimen. Synonym: lyophilize.

fresh weight The weight, including the water content, of a specimen. Synonym: wet weight.

friable A term commonly used to describe a crumb-like callus. In this state, the callus is easily dissected and readily dispersed into single cells or clumps of cells in solution.

FSH See: follicle stimulating hormone.

functional food A foodstuff that provides a health benefit beyond basic nutrition, demonstrating specific health or medical benefits, including the prevention and treatment of disease.

functional gene cloning See: candidate-gene strategy.

functional genomics The field of research, that aims to determine patterns of gene expression and interaction in the genome, based on the knowledge of extensive or complete genomic sequence of an organism.

fungicide A chemical agent toxic to fungi.

fungus (pl.: fungi) Multinucleate single-celled or multicellular heterotrophic micro-organisms, including yeasts, moulds, and mushrooms. They live as parasites, symbionts, or saprophytes. Lacking any vascular tissues (unlike plants), their cell walls are made of chitin or other non-cellulose compounds.

Fusarium spp. A group of fungal pathogens of many economic crop species, particularly cereals, where severe infestation leads to losses in both grain yield and quality. The latter can be a particularly serious problem as many of these fungi produce mycotoxins, some of which are dangerous to both livestock and human health (See: aflatoxin). Specific strains are also employed on an industrial scale to produce protein for human consumption.

fusion biopharmaceuticals Fusion proteins with pharmaceutical properties. Their advantages are: 1. Synergistic activities in one molecule, i.e. when the molecule binds to its target, it can perform more than one function simultaneously; 2. An adverse effect or poor stability of one part of the molecule may be offset by the properties of the other; and 3. One part of the molecule can act as a targeting mechanism for the active protein. See: immunotoxin, fusion toxin.

fusion gene See: chimeric gene.

fusion protein A polypeptide translated from a chimeric gene. The different genes are joined so that their coding sequences are in the same reading frame, and the resulting construct is transcribed and translated as a single gene, producing a single protein. These are used for a number of purposes, including: 1. To add an affinity tag to a protein; 2. To produce a protein with the combined characteristics of two natural proteins; 3. To produce a protein where two different activities are physically linked. See: fusion biopharmaceuticals.

fusion toxin A fusion protein that consists of a toxic protein domain plus a cell receptor binding domain. The latter delivers the toxin directly to the target cell, thus sparing other healthy tissues from the effect of the toxin.

fusogenic agent Any chemical or virus, etc., that causes cells to fuse together.

G Abbreviation for guanine.

G cap The 5'-terminal methylated guanine nucleoside that is present on many eukaryotic mRNAs. It is joined to the mRNA, via a 5'?5' phosphodiester bond, after transcription. See: cap site.

G protein Proteins found on the inner surface of the plasma membrane, which bind to the guanine nucleotides, GTP and GDP. They transmit signals from outside the membrane, via trans-membrane (G-protein-coupled) receptors, to adenylate cyclase, which catalyses the formation of the second messenger, cyclic AMP, inside the cell

galactomannan A gum in which the structural chain is made up of D-mannose units with 1?4 linkages. The ratio of galactose to mannose is 1:2.

gall A tumorous growth in plants. See: crown gall.

gamete A mature reproductive cell which is capable of fusing with a cell of similar origin but of opposite sex to form a zygote from which a new organism can develop. Gametes normally have a haploid chromosome content. In animals, a gamete is a sperm or egg; in plants, it is pollen, spermatic nucleus, or ovum.

gamete and embryo storage Storage of ova, sperm or fertilized embryos outside their original source. Almost invariably this means cryopreservation.

gametic (phase) disequilibrium In relation to any two loci, the occurrence of haplotypes (gametes) at a frequency other than that predicted from the product of the respective allele frequencies. Opposite: gametic (phase) equilibrium.

gametic (phase) equilibrium In relation to any two loci, the occurrence of haplotypes (gametes) at a frequency equal to the product of the frequency of the two relevant alleles. For example, A and B are in gametic equilibrium if the frequency of AiBi gametes equals the product of the frequencies of alleles Ai and Bi. Opposite: gametic (phase) disequilibrium.

gametoclone A plant regenerated from a tissue culture originating from gametic tissue.

gametogenesis The process of the formation of gametes.

gametophyte The phase of the plant life cycle that carries the gamete producing organs. In flowering plants, the pollen grain is the male gametophyte and the embryo sac is the female gametophyte.

gametophytic incompatibility A phenomenon in plants, in which a pollen grain is genetically incapable of fertilizing a particular egg, because both gametes carry the identical allele at an incompatibility locus (usually denoted S). This is a mechanism for forcing crossfertilization.

gap A missing section on one of the strands of double-stranded DNA. The DNA will therefore have a single-stranded region.

gapped DNA A double-stranded DNA molecule with one or more internal single-stranded regions.

gas transfer The rate at which gases are transferred from gas into solution, an important parameter in fermentation systems because it controls the rate at which the organism can metabolize. Efficient gas transfer can be achieved in several ways, including the use of small bubbles, from which gas dissolves faster than from larger ones, due to their larger surface area per unit of volume; or spreading the liquid out, for example in a thin sheet, or in a thin permeable tube, as in hollow fibre bioreactor.

gastrula An early animal embryo consisting of two layers of cells; an embryological stage following the blastula.

GC island A segment of double-stranded DNA that is rich in GC base pairs. This type of sequence is characteristic of eukaryotic genomic regions with a high gene content.

GDP Abbreviation for guanosine 5'-diphosphate.

gel A jelly-like solid, used widely as a matrix for the electrophoresis of macromolecules, for encapsulation, and to solidify media for cell cultures.

gel electrophoresis See: electrophoresis.

gel filtration A method of protein or DNA purification, where differences in size are used to separate the components of a complex mixture.

gelatin A glutinous, proteinaceous gelling and solidifying agent. Gelatin is produced by the partial hydrolysis (via boiling) of collagen, found in the connective tissues of many farm animals. Used to gel or solidify nutrient solutions for tissue culture, and as a food additive.

gelatinization The swelling of starch when added to hot water. Hydrolysis causes the molecule to lose structure, and technically gelatinization is not complete until there is no structure left at all.

GelriteTM The brand name of a Pseudomonas-derived refined polysaccharide used as a gelling agent and agar substitute.

GEM Abbreviation for genetically engineered micro-organism. See: genetically modified organism.

gene The unit of heredity transmitted from generation to generation during sexual or asexual reproduction. More generally, the term is used in relation to the transmission and inheritance of particular identifiable traits. The simplest gene consists a segment of nucleic acid that encodes an individual protein or RNA.

gene (resources) conservation The conservation of species, populations, individuals or parts of individuals, by in situ or ex situ methods, to provide a diversity of genetic materials for present and future generations.

gene addition The addition of a functional copy of a gene to the genome of an organism.

gene amplification The selective production of multiple copies of one gene without a proportional increase in others.

gene bank 1. The physical location where collections of genetic material in the form of seeds, tissues or reproductive cells of plants or animals are stored. 2. Field gene bank: A facility established for the ex situ storage and maintenance, using horticultural techniques, of individual plants. Used for species whose seeds are recalcitrant, or for clonally propagated species of agricultural importance, e.g. apple varieties. 3. A collection of cloned DNA fragments from a single genome. Ideally the bank should contain cloned representatives of all the DNA sequences in the genome. 4. See: library.

gene cloning The synthesis of multiple copies of a chosen DNA sequence using a bacterial cell or another organism as a host. The gene of interest is inserted into a vector, and the resulting recombinant DNA molecule is amplified in an appropriate host cell. Synonym: DNA cloning.

gene construct See: construct.

gene conversion A process, often associated with recombination, during which one allele is replicated at the expense of another, leading to non-Mendelian segregation ratios.

gene expression The process by which a gene produces mRNA and protein, and hence exerts its effect on the phenotype of an organism.

gene flow The spread of genes from one breeding population to another (usually) related population by migration, thereby generating changes in allele frequency.

gene frequency See: allele frequency.

gene gun See: biolistics.

gene imprinting The differential expression of a single gene according to its parental origin.

gene insertion The incorporation of one or more copies of a gene into a chromosome.

gene interaction The modification of the action of one gene by another, non-allelic gene.

gene knockout See: knockout.

gene library See: library.

gene linkage See: linkage.

gene machine See: transposon tagging.

gene mapping See: mapping.

gene modification Chemical change to a gene's DNA sequence.

gene pool 1. The sum of all genetic information in a breeding population at a given time. 2. In plant genetic resources, use is made of the terms 'primary', 'secondary' and 'tertiary' gene pools. In general, members of the primary gene pool are inter-fertile; those of the secondary can be crossed with those in the primary gene pool under special circumstances; but to introgress variation from the tertiary gene pool, special techniques are required to achieve crossing.

gene probe See: probe.

gene recombination See: recombination.

gene regulation The process of controlling the synthesis or suppression of gene products in specific cells or tissues.

gene replacement The incorporation of a transgene into a chromosome at its normal location by homologous recombination, thus replacing the copy of the gene originally present at the locus.

gene sequencing See: DNA sequencing.

gene shears See: ribozyme.

gene silencing See: silencing.

gene splicing See: splicing (1).

gene stacking See: stacked genes.

gene therapy The proposed treatment of an inherited disease by the transformation of an affected individual with a wild-type copy of the defective gene causing the disorder. In germ-line (or heritable) gene therapy, reproductive cells are transformed; in somatic-cell (or non-inheritable) gene therapy, cells other than reproductive ones are modified.

gene tracking Following the inheritance of a particular gene from generation to generation.

gene transfer See: transformation.

gene translocation The movement of a gene from one chromosomal location to another.

genera Plural form of genus.

generally regarded as safe (Abbreviation: GRAS). Designation given to foods, drugs, and other materials with a long-term history of not causing illness to humans, even though formal toxicity testing may not been conducted. Certain host organisms for recombinant DNA experimentation have recently been given this status.

generation time See: cell generation time.

generative See: germ line.

generative nucleus In many flowering plants, shed pollen is two-celled (in others it is three-celled or has a variable number). Before pollen is shed, the male gametophyte divides mitotically to give a generative and a vegetative nucleus. The former is the progenitor of the sperm cells.

genet The individual(s) descended vegetatively from a single sexually produced zygote, including all entities derived from it. All these individuals are genetically identical to one another (barring mutation).

genetic assimilation Eventual extinction of a natural species as massive gene flow occurs from a related species.

genetic code The correspondence between the set of 64 possible nucleotide triplets and the amino acids and stop codons that they specify. See annex 3.

genetic complementation When two DNA molecules that are in the same cell together produce a function that neither DNA molecule can supply on its own.

genetic disease A disease caused by an abnormality in the genetic material, which could be at the level of DNA sequence at a locus, or at the level of karyotype. Usually refers to inherited diseases, although somatic mutations can also cause disease without being inherited.

genetic distance A measure of the genetic similarity between any pair of populations. This is measured on the basis of variation in a combination of phenotypic traits, allele frequencies or DNA sequences. For example, the genetic distance between two populations having the same allele frequencies at a particular locus, and based solely on that locus, is zero.

genetic distancing The collection of the data on phenotypic traits, marker allele frequencies or DNA sequences for two or more populations, and estimation of the genetic distances between each pair of populations.

genetic diversity The heritable variation within and among populations which is created, enhanced or maintained by evolutionary or selective forces.

genetic drift Change in allele frequency from one generation to another within a population, due to the sampling of finite numbers of genes that is inevitable in all finite-sized populations. The smaller the population, the greater is the genetic drift, with the result that some alleles are lost, and genetic diversity is reduced. Thus minimization of genetic drift is an important consideration for conservation of genetic resources.

genetic engineering Modifying genotype, and hence phenotype, by transgenesis.

genetic equilibrium The maintenance of a steady state with respect to allele frequencies in a group of interbreeding organisms.

genetic erosion The loss over time of allelic diversity, particularly in farmed organisms, caused by either natural or man-made processes. See: genetic drift.

genetic fingerprinting See: DNA fingerprinting.

genetic gain The increase in productivity achieved following a change in gene frequency effected by selection.

genetic heterogeneity Occurs where the genetic determination of a given phenotype differs between individuals.

genetic immunization Delivery to a host organism of a cloned gene that encodes an antigen. After the cloned gene is expressed, it elicits an antibody response that protects the organism from infection by the relevant pathogen.

genetic information Information contained in a nucleotide base sequence in chromosomal DNA or RNA.

genetic linkage See: linkage.

genetic map The linear array of genes on a chromosome, based on recombination frequencies (linkage map) or physical location (physical or chromosomal map). See: linkage map.

genetic mapping See: mapping.

genetic marker A DNA sequence used to identify a particular location (locus) on a particular chromosome. See: marker gene.

genetic pollution Uncontrolled spread of genetic information (frequently referring to transgenes) into the genomes of organisms in which such genes are not present in nature.

genetic polymorphism See: polymorphism.

genetic relatedness A quantitative estimate of the proportion of genes, ®, shared between the genomes of any two individuals, groups or populations, e.g. r = 0.5 for full siblings and parent offspring pairs.

genetic resources genetic material of actual or potential value.

genetic selection The process of selecting genes, cells, clones, etc., within populations or between populations or species. Genetic selection usually results in differential survival rates of the various genotypes, reflecting many variables, including the selection pressure and genetic variability present in populations.

genetic transformation See: transformation.

genetic use restriction technology (Abbreviation: GURT). A proposed technology applying transgenesis to genetically compromise the fertility or the performance of saved seed of a cultivar or of second generation animals. The intention is to protect the market for the seed producer or to prevent undesired escape of genes. Two types of GURTs have been patented: variety-level GURT (V-GURT), which produces sterile progeny, and trait-specific GURT (T-GURT), in which only the added value transgenic trait is genetically protected. See: terminator gene, disrupter gene.

genetic variation Differences between individuals attributable to differences in genotype.

genetically engineered organism (Abbreviation: GEO). Occasional alteRNAtive term for genetically modified organism.

genetically modified organism (Abbreviation: GMO). An organism that has been transformed by the insertion of one or more transgenes.

genetics The science of heredity.

genome 1. The entire complement of genetic material (genes plus non-coding sequences) present in each cell of an organism, virus or organelle. 2. The complete set of chromosomes (hence of genes) inherited as a unit from one parent.

genomic library A clone library specifically constructed from restriction fragments of the genomic DNA of an organism.

genomics The research strategy that uses molecular characterization and cloning of whole genomes to understand the structure, function and evolution of genes and to answer fundamental biological questions. See: bio-informatics, functional genomics and proteomics.

genotype 1. The genetic constitution of an organism. 2. The allelic constitution at a particular locus, e.g. Aa or aa. 3. The sum effect of all loci that contribute to the expression of a trait.

genus (pl.: genera) A group of closely related species, whose perceived relationship is typically based on physical resemblance, now often supplemented with DNA sequence data.

GEO Abbreviation for genetically engineered organism. See: genetically modified organism.

geotropism A growth curvature induced by gravity. Synonym: gravitropism.

germ 1.The botanical term for a plant embryo. 2. Colloquial: a disease-causing micro-organism.

germ cell A member of a cell lineage (the germ line) leading to the production of gametes. In mammals, germ cells are found in the germinal epithelium of the ovaries and testes. Synonym: germ line cell. Opposite: somatic cell.

germ cell gene therapy The repair or replacement of a defective gene within the gamete-forming tissues, resulting in a heritable change in an organism's genetic constitution.

germ layer The layers of cells in an animal embryo at the gastrula stage, from which the various organs of the animal's body will be derived.

germ line A lineage of cells which, during the development of an organism, are set aside as potential gamete-forming tissues. The location, nature and time of formation of potential gamete-forming tissues are species specific, and may vary greatly from one species to another. See: somatic

germ line cell See: germ cell.

germ line gene therapy The delivery of a gene or genes to a fertilized egg or an early embryonic cell. The transferred gene(s) is present in all or some of the nuclei of the cells of the mature individual, including possibly the reproductive cells, and alters the phenotype of the individual that develops.

germicide Any chemical agent used to control or kill any pathogenic and non-pathogenic micro-organisms.

germinal epithelium 1. A layer of epithelial cells on the surface of the ovary that are continuous with the mesothelium. 2. The layer of epithelial cells lining the seminiferous tubules of the testis, which gives rise to spermatogonia. See: spermatogenesis.

germination 1. The initial stages in the growth of a seed to form a seedling. 2. The growth of spores (fungal or algal) and pollen grains.

germplasm 1. An individual, group of individuals or a clone representing a genotype, variety, species or culture, held in an in situ or ex situ collection. 2. Original meaning, now no longer in use: the genetic material that forms the physical basis of inheritance and which is transmitted from one generation to the next by means of the germ cells

gestation The period between conception (fertilization of the egg) to parturition (birth) spent in utero by the foetus of viviparous animals.

GFP Abbreviation for green fluorescent protein.

GH Abbreviation for growth hormone.

gibberellins A class of plant growth regulators which are active in the elongation, enhancement of flower, fruit and leaf size, germination, vernalization and other physiological processes.

gland A specialized group of cells or a single cell in animals or plants that secretes a specific substance. The two types of animal glands are: endocrine, which secrete directly into the blood vessels; and exocrine, which secrete through a duct or network of ducts into a body cavity or onto the body surface.

glaucous A surface with a waxy, white coating. In most cases, this waxy covering can be rubbed off.

globulins Common class of proteins in blood, eggs and milk, and seeds. Characterized by their slight solubility in water but are freely soluble in dilute salt solutions. Gamma- globulins are defined further by their electrophoretic behaviour, and include the immunoglobulins.

GLP Abbreviation for good laboratory practice.

glucocorticoid A steroid hormone that regulates gene expression in higher animals.

glucose invertase An enzyme that catalyses the hydrolysis of sucrose into its component monosaccharides, glucose and fructose.

glucose isomerase An enzyme that catalyses the interconversion of the two sugars, glucose and fructose. As fructose is a lower energy compound compared with glucose, a mixture of glucose and fructose with the enzyme will end up almost entirely as fructose.

glucosinolates A class of molecules produced in the seeds and green tissue of a range of plants, in particular brassicas. Their natural role is thought to be involved in plant-insect interactions. Their importance in plant breeding is largely because of their negative influence on taste and their positive effect on the prevention of cancers of the alimentary tract.

glucuronidase See: beta-glucuronidase.

gluten A mixture of two seed storage protein classes, gliadin and glutenin, found in the endosperm of cereal (particularly wheat) grain. High levels of gluten impart elasticity to dough, and thus the composition of wheat glutens largely determines whether a specific flour is suitable for biscuit or bread making. Sensitivity of the lining of the intestine to gluten in some humans results in coeliac disease, a condition that requires a gluten-free diet.

glycoalkaloids A group of modified alkaloids, including solanine and tomatine, having a range of toxic effects in humans and other species. They are of particular significance in food plants from the Solanaceae.

glycoform One of several structures possible for a given glycoprotein, determined by the type and position of attachment of the component oligosaccharide(s). Certain glycoforms may exhibit different biological activities from one another because the oligosaccharide units mediate interactions with other cell components.

glycolysis The sequence of reactions that converts glucose into pyruvate, with the concomitant production of ATP.

glycoprotein A protein molecule modified by the addition of one or several oligosaccharide groups.

glycoprotein remodelling The use of restriction endoglycosidases to enzymatically remove oligosaccharide branches from glycoprotein molecules. Removal of one or more oligosaccharide branches can lessen or abolish the antigenicity of the glycoprotein, so allowing its injection for pharmaceutical purposes without incurring an unwanted immune response. See: glycoform.

glycosylation The covalent addition of sugar or sugar-related molecules to other classes of molecule, including proteins or nucleic acids.

glyphosate An active ingredient in some herbicides, killing plants by inhibiting the activity of plant enolpyruvyl-shikimate 3-phosphate synthase.

glyphosate oxidase An enzyme which catalyses the break-down of glyphosate, discovered in a strain of Pseudomonas bacteria which were found to produce unusually large amounts of the enzyme. The gene responsible has been incorporated into a variety of crop plants to enable them to tolerate applications of glyphosate-containing herbicides. It has also been used in conjunction with the CP4 EPSPS gene.

glyphosate oxidoreductase An enzyme from the micro-organism Ochrobactrum anthropi, which catalyses the break-down of glyphosate. If the encoding gene (called goxv247) is inserted and properly expressed in a plant, these plants become tolerant of the application of glyphosate- and/or sulfosate-containing herbicides. An alteRNAtive to CP4 EPSPS or glyphosate oxidase encoded glyphosate tolerance.

GM food Abbreviation for genetically modified food. Food that contains above a certain legal minimum content of raw material obtained from genetically modified organisms.

GMO Abbreviation for genetically modified organism.

GMP Abbreviation for 1. guanosine 5'-monophosphate. Synonym: guanylic acid. 2. good manufacturing practice.

gobar See: biogas.

golden rice A biotechnology-derived rice, which contains large amounts of beta carotene (a precursor of vitamin A) in its seeds. Achieved by inserting two genes from daffodil and one from the bacterium Erwinia uredovora.

Golgi apparatus An assembly of vesicles and folded membranes within the cytoplasm of plant and animal cells that stores and transports secretory products (such as enzymes and hormones) and plays a role in formation of a cell wall (when this is present).

gonad One of the (usually paired) animal organs that produce reproductive cells (gametes). The most important gonads are the male testis, which produces spermatozoa, and the female ovary, which produces ova (egg cells). The gonads also produce hormones that control secondary sexual characteristics.

good laboratory practice (Abbreviation: GLP). Written codes of practice designed to reduce to a minimum the chance of procedural or instrument problems which could adversely affect a research project or other laboratory work.

good manufacturing practice (Abbreviation: GMP). Codes of practice designed to reduce to a minimum the chance of procedural or instrument/manufacturing plant problems which could adversely affect a manufactured product.

G-protein coupled receptor See: G protein.

graft 1. Verb. To place a detached branch or bud (scion) in close cambial contact with a rooted stem (rootstock) in such a manner that scion and rootstock unite to form a single plant. 2. Noun. Colloquial synonym for scion. See: grafting, graft chimera, graft hybrid.

graft chimera A plant which is a mosaic of two sorts of tissue differing in genetic constitution and assumed to have arisen as the result of a nuclear fusion following grafting. See: graft hybrid.

graft hybrid An individual formed from graft (2) and stock showing the characteristics of both progenitors. See: graft chimera.

graft inoculation test A test based on the use of a suspected viral carrier which is grafted to an indicator plant. If symptoms appear in the indicator plant, the viral assay is positive.

graft union The point at which a scion from one plant is joined to a rootstock from another plant.

grafting The process of making a graft (1).

graft-versus-host disease The rejection of transplanted organs by the recipient's immune system, due to attack of the recipient's T lymphocytes on the transplanted organ caused by differences in major histocompatibility complex proteins.

Gram staining A technique to distinguish between two major bacterial groups, based on whether or not their cell wall retains the Gram stain. Gram-positive bacteria are stained dark purple, while Gram-negative bacteria are only faintly coloured. Stain retention is determined by the structure of the cell wall.

granum (pl.: grana) Structure within the chloroplasts, appear as green granules with the light microscope and as a series of parallel lamellae with the electron microscope. They contain the chlorophyll and carotenoid pigments directly involved in photosynthesis.

GRAS Abbreviation for generally regarded as safe.

gratuitous inducer A substance that can induce transcription of a gene or genes, but is not a substrate for the induced enzyme(s).

gravitropism See: geotropism.

green fluorescent protein (Abbreviation: GFP). A protein derived from a species of jelly fish, that fluoresces when exposed to ultra violet light. Its encoding gene has been isolated and is replacing GUS as a reporter gene in plant transgenesis, since it can be assayed non-destructively in real time.

green revolution Name given to the dramatic increase in crop productivity during the third quarter of the 20th century, as a result of integrated advances in genetics and plant breeding, agronomy, and pest and disease control.

Gro-luxä A wide-spectrum fluorescent lamp suitable for artificial light for plant growth.

growth cabinet An enclosed space in which environmental conditions can be controlled. The degree of control over temperature, light and humidity is a function of the quality of the cabinet.

growth curve See: growth phase.

growth factor Any of various chemicals, particularly polypeptides, that have a variety of important roles in the stimulation of new cell growth and cell maintenance. They bind to the cell surface on receptors. Specific growth factors can cause new cell proliferation.

growth hormone (Abbreviation: GH). A group of hormones, secreted by the mammalian pituitary gland, that stimulates protein synthesis and growth of the long bones in the legs and arms. They also promote the breakdown and use of fats as an energy source, rather than glucose. Synonym: somatotropin.

growth inhibitor Any substance inhibiting the growth of an organism. The inhibitory effect can range from mild inhibition (growth retardation) to severe inhibition or death (toxic reaction). The concentration of the inhibitor, the length of exposure to it, and the relative susceptibility of the organisms exposed to the inhibitor, are all important factors which determine the extent of the inhibitory effect.

growth phase Each of the characteristic periods in the growth curve of a bacterial culture, as indicated by the shape of a graph of viable cell number versus time, namely: lag phase; logarithmic phase; stationary phase; death phase.

growth rate Change in an organism's mass per unit of time.

growth regulator A synthetic or natural compound that at low concentrations elicits and controls growth responses in a manner similar to hormones.

growth retardant A chemical that selectively interferes with normal hormonal promotion of growth and other physiological processes, but without appreciable toxic effects.

growth ring Rings visible in a cross-section of a woody stem, such as a tree trunk. Each ring represents the xylem formed in one year as a result of fluctuating activity of the vascular cambium.

growth substance Any organic substance, other than a nutrient, that is synthesized by plants and regulates growth and development. They are usually made in a particular region, such as the shoot tip, and transported to other regions, where they take effect.

GTP Abbreviation for guanosine 5'-triphosphate, a nucleotide which is important as a ligand for G proteins and as a direct precursor molecule for RNA synthesis. See: guanylic acid.

guanine (Abbreviation: G). One of the bases found in DNA and RNA. See: guanosine.

guanosine The (ribo)nucleoside resulting from the combination of the base guanine (G) and a D-ribose sugar. The corresponding deoxyribonucleoside is called deoxyguanosine. See: GTP, dGTP, guanylic acid.

guanosine triphosphate (guanosine 5-triphosphate) Abbreviation: GTP. See: guanylic acid.

guanylic acid Synonym for guanosine monophosphate (abbreviation: GMP), a (ribo)nucleotide containing the nucleoside guanosine. The corresponding deoxyribonucleotide is called deoxyguanylic acid.

guard cell Specialized epidermal cells found in pairs around a stoma. Their function is to control the opening and closing of the stoma through changes in turgor.

guide RNA An RNA molecule that contain sequences that function as a template during RNA editing. See: guide sequence.

guide sequence An RNA molecule (or a part of it) which hybridizes with eukaryotic mRNA and aids in the splicing of intron sequences. Guide sequences may be either external (EGS) or internal (IGS) to the RNA being processed and may hybridize with either intron or exon sequences close to the splice junction. See: split gene.

GURT Abbreviation for genetic use restriction technology.

GUS Abbreviation for beta-glucuronidase.

gus gene An E. coli gene that encodes for production of beta-glucuronidase (GUS). Because this activity is absent in plants, the gene is commonly utilized as a reporter gene to detect the occurrence of transformation events.

gymnosperm A class of plant (e.g. conifers) whose ovules and the seeds into which they develop are borne unprotected, rather than enclosed in ovaries, as are those of the flowering plants, the (angiosperms).

gynandromorph An individual in which one part of the body is female and another part is male; a sex mosaic.

gynogenesis Female parthenogenesis: after fertilization of the ovum, the male nucleus is eliminated and the haploid (gynogenetic) individual possesses the maternal genome only.

gyrase See: DNA helicase.

h Prefix used to designate the human form of an enzyme. For example, hGH is human growth hormone.

habituation The phenomenon whereby, after a number of sub-cultures, cells can grow without the addition to the tissue culture medium of previously obligatory factors. Such cells are then autonomous.

HAC Abbreviation for human artificial chromosome.

haemoglobin Protein containing iron, located in erythrocytes of vertebrates; important for the transportation of oxygen to the cells of the body.

haemolymph The mixture of blood and other fluids in the body cavity of an invertebrate.

haemophilia A sex-linked hereditary bleeding disorder in which it takes a long time for the blood to clot and abnormal bleeding occurs. This disease affects mostly males.

hairpin loop A region in one strand of a polynucleotide which, due to an inverted repeat in the sequence, may under appropriate conditions fold back on itself and form a limited segment of double-stranded DNA with a loop at one end.

hairy root culture A culture consisting of highly branched roots of a plant. A plant tissue is treated with the bacterium Agrobacterium rhizogenes containing the Ri plasmid, which causes the explant to grow highly branched roots from the sites of infection. Transgenes engineered into the plasmid can be expressed in these cultures.

hairy root disease A disease of broad-leaved plants, where a proliferation of root-like tissue is formed from the stem. Hairy root disease is a tumorous state similar to crown gall, and is induced by the bacterium Agrobacterium rhizogenes, when containing an Ri plasmid.

halophyte A plant species adapted to soils containing a concentration of salt that is toxic to most plant species. See: salt tolerance.

hanging droplet technique See: microdroplet array.

haploid A cell or organism containing one of each of the pairs of homologous chromosomes found in the normal diploid cell.

haplotype A specific allelic constitution at a number of loci within a defined linkage block.

haplozygous See: hemizygous.

hapten A small molecule, which by itself is not an antigen, but which as a part of a larger structure when linked to a carrier protein, can serve as an antigenic determinant.

haptoglobin A serum protein that interacts with haemoglobin during recycling of the iron molecule of haemoglobin. Synonym: alpha globulin.

hardening off Adapting glasshouse or controlled environment grown plants to outdoor conditions by reducing availability of water, lowering the temperature, increasing light intensity, or reducing the nutrient supply. The hardening-off process conditions plants for survival when transplanted outdoors.

Hardy-Weinberg equilibrium The frequencies of genotypes at a locus resulting from random mating at that locus; for two alleles, A1 and A2, with respective frequencies in a population of p and q, the Hardy-Weinberg equilibrium frequencies are p2 A1A1; 2pq A1A2; q2 A2A2. Departure from these frequencies is an indication that random mating is not occurring.

harvesting 1. The process involved in gathering ripened crops. 2. The collection of cells from cell cultures or of organs from donors for the purpose of transplantation.

heat shock protein (Abbreviation: HSP). A class of protein chaperones which are typically over-expressed as a response to heat stress. Two such proteins - HSP 90 and HSP 70 - have a role in ensuring that crucial proteins are folded into the correct conformation. Synonym: stress protein.

heat therapy See: thermotherapy.

helix A structure with a spiral shape. The normal state of double-stranded DNA is in the form of a double helix.

helminth A class of parasitic worms, especially those which are internal parasites of man and animals.

helper cell T cells that assist in stimulating B and T lymphocytes to develop into antibody-producing plasma cells and killer T cells, respectively.

helper plasmid A plasmid that provides a function or functions to another plasmid in the same cell.

helper T cell See: helper cell.

helper T lymphocyte See: helper cell.

helper virus A virus that provides a function or functions to another virus in the same cell.

hemicellulase An enzyme that catalyses the degradation of hemicellulose.

hemicellulose Any cellulose-like carbohydrate, but excluding cellulose itself. Together with pectin and lignin, hemicelluloses form the cell wall matrix.

hemizygous The condition in which genes are present only once in the genotype and not in pairs. Occurs for all genes in haploids, for all genes located in the differential segments of the sex chromosomes in diploids, and in various aneuploids and deletion mutant heterozygotes. Synonym: haplozygous.

hemoglobin See: haemoglobin.

hemolymph See: haemolymph.

hemophilia See: haemophilia.

HEPA filter Abbreviation for high efficiency particulate air filter. A filter capable of excluding particles larger than 0.3ìm. HEPA filters are used in laminar air flow cabinets to ensure that the air is pathogen-free. See: pre-filter.

herbicide A substance that is toxic to plants; the active ingredient in agrochemicals intended to kill specific unwanted plants, especially weeds.

herbicide resistance The ability of a plant to remain unaffected by the application of a herbicide.

heredity Resemblance among individuals related by descent; transmission of traits from parents to offspring.

heritability The degree to which a given trait is controlled by inheritance, as opposed to being controlled by non-genetic factors. See: broad-sense heritability; narrow-sense heritability.

hermaphrodite 1. An animal that has both male and female reproductive organs, or a mixture of male and female attributes. 2. A plant whose flowers contain both stamen and carpels. Synonym: intersex.

heteroallele A gene having mutations at two or more different sites.

heterochromatin Regions of chromosomes that remain contracted during interphase and therefore stain more intensely in cytological preparations. These regions have a high content of repetitive DNA, and a low content of genes; thus they are for the most part genetically inactive. Opposite: euchromatin.

heteroduplex A double-stranded DNA molecule or DNA-RNA hybrid, where each strand is of a different origin. Where the two DNAs differ significantly in sequence, single-stranded regions will be revealed when the heteroduplex is observed under the electron microscope. A map of homologous and non-homologous regions of the two molecules may thereby be constructed (heteroduplex mapping). Synonym: hybrid DNA (DNA/RNA). See: heteroduplex analysis.

heteroduplex analysis The use of the electrophoretic mobility of heteroduplex DNA to estimate the degree of non-homology between the sequences of the two strands. Gel mobility tends to be reduced as the amount of sequence divergence increases, because the effective size of a fully complementary pair of strands is smaller than that of a partially complementary structure.

heterogametic Producing unlike gametes with regard to the sex chromosomes. In mammals, the XY male is heterogametic, and the XX female is homogametic.

heterogeneity See: genetic heterogeneity.

heterogeneous nuclear RNA (Abbreviation: hnRNA). Large RNA molecules, which are found in the nucleus of a eukaryotic cell and the precursors of mRNA and other RNA molecules.

heterokaryon A cell with two or more different nuclei as a result of cell fusion. See: synkaryon.

heterologous From a different source.

heterologous probe A DNA probe that is derived from one species and used to screen for a similar DNA sequence from another species.

heterologous protein See: recombinant protein.

heteroplasmy A condition in which two genetically different organelles are present in the same cell. The equivalent to heterozygous in the context of nuclear genes. Opposite: homoplasmy.

heteroploid Cells with nuclei containing chromosome numbers other than diploid.

heteropyknosis The property of certain chromosomes, or of their parts, to remain more dense during the cell cycle and to stain more intensely than other chromosomes or parts.

heterosis See: hybrid vigour.

heterotroph Organism non capable of self-nourishment utilizing carbon dioxide or carbonates as the sole source of carbon and obtaining energy from radiant energy or from the oxidation of inorganic elements, or compounds such as iron, sulphur, hydrogen, ammonium and nitrites. Opposite: autotroph.

heterotrophic (adj.) See: heterotroph.

heterozygous (adj.) See: heterozygote.

heterozygote An individual with non-identical alleles for a particular gene or genes. The condition is termed "heterozygous". Opposite: homozygote.

Hfr High-frequency recombination strain of Escherichia coli; in these strains, the F factor (plasmid) is integrated into the bacterial chromosome.

hGH Abbreviation for human growth hormone.

high efficiency particulate air filter See: HEPA filter.

high throughput screening Automated systems designed to process large numbers of assays, especially in the context of genotyping.

histocompatibility The degree to which tissue from one organism is tolerated by the immune system of another organism.

histocompatibility complex See: major histocompatibility complex.

histoglobulin The peptides present on the surface of nucleate cells, responsible for the differences between genetically non-identical individuals that cause rejection of tissue grafts between such individuals. Products of the major histocompatibility complex genes.

histology Science that deals with the microscopic structure of animal and plant tissues.

histone Group of water-soluble proteins rich in basic amino acids, closely associated with DNA in plant and animal chromatin. Histones are involved in the coiling of DNA in chromosomes and in the regulation of gene activity.

HLA Abbreviation for human-leukocyte-antigen system. See: major histocompatibility antigens.

hnRNA Abbreviation for heterogeneous nuclear RNA.

Hogness box Synonym for TATA box.

hollow fibre A tube of porous material, with an internal diameter of a fraction of a millimetre, making its ratio of surface area to internal volume very large. Employed as filters or in bioreactors as a method of retaining cells while allowing the easy removal of spent medium and/or products.

holoenzyme See: apoenzyme.

holometabolous An insect that undergoes complete metamorphosis to the adult from a morphologically distinct larval stage.

homeobox A highly conserved 180 bp DNA sequence that controls body part-, organ- or tissue-specific gene expression, most particularly involved in segmentation in animals (e.g. development of antennae or legs of Drosophila melanogaster), but also in a variety of other eukaryotes. It encodes a DNA-binding region, the homeodomain, which acts as a transcription factor.

homeodomain See: homeobox.

homeotic genes Genes that act in concert to determine fundamental patterns of development.

homeotic mutation A mutation that causes a body part to develop in an inappropriate position in an organism, such as the mutation in Drosophila melanogaster that causes legs to develop on the head in place of antennae.

homoallele One of a number of otherwise identical alleles which differ at the same site in their sequence. Homoalleles are inherited as strict alteRNAtives; but heteroalleles, in principle, could through recombination create a genotype which contains a 'double' variant.

homodimer A protein comprising two identical polypeptide chains, or a dimer of identical residues.

homoduplex DNA A double-stranded fully complementary DNA molecule.

homoeologous Referring to chromosomes which are descended from a common progenitor, but which have evolved to be no longer fully homologous. Homoeologous chromosomes have similar gene content to one another, but are structurally altered in subtle ways to inhibit, and sometimes completely prevent their pairing with one another at meiosis.

homogametic Producing similar gametes with regard to the sex chromosomes. In mammals, the female is homogametic (XX), and the male is heterogametic (XY).

homogenotization An allele replacement technique, in which a bacterial cell is transformed with a plasmid containing an altered copy of the target sequence, and a double recombination event effects the substitution of the wild type allele by the altered one. An antibiotic resistance gene is usually fused to the altered copy in the plasmid, so that double recombinants can be selected.

homokaryon A cell with two or more identical nuclei as a result of fusion. Opposite: heterokaryon.

homologous 1. General definition: from the same source, or having the same evolutionary function or structure. 2. Of chromosomes: identical with respect to gene content and linear ordering. Homologous chromosomes pair and recombine with one another at meiosis. 3. Of DNA/proteins: identical, or nearly identical nucleotide/amino acid sequence.

homologous recombination The exchange of DNA fragments between the two non-sister chromatids of the same chromosome in the course of meiosis.

homology 1. The degree of identity between individuals, or characters. 2. The degree of identity of sequence (nucleotide or amino acid) between a number of DNA or polypeptide molecules.

homomultimer A protein consisting of a number of identical subunits.

homoplasmy The condition in which all copies of an organelle in a cell are genetically identical. Opposite: heteroplasmy.

homopolymer A polymer (nucleic acid, polypeptide, etc.) which contains only one kind of residue (e.g. the polynucleotide GGGGGGGGG...).

homopolymeric tailing See: tailing.

homoozygous (adj.) See: homozygote.

homozygote An individual that has two copies of the same allele for a given gene on its two homologous chromosomes. The condition is termed "homozygous". Opposite: heterozygote.

hormone A specific chemical, produced in one part of a plant or animal body, and transported to another part where, at low concentrations, it promotes, inhibits or quantitatively modifies a biological process.

host An organism that contains another organism or a cloning vector.

host-specific toxin A metabolite, produced by a pathogen, and which is responsible for the adverse effects of the pathogen. The toxin has a host specificity equivalent to that of the pathogen. Utilized for in vitro selection experiments to screen for tolerance or resistance to the pathogen.

hot spot See: recombinational hot spot.

HSA See: serum albumin.

HSP See: heat shock protein.

human artificial chromosome (Abbreviation: HAC). Analogous to yeast artificial chromosome, a construct comprising a human centromere and telomeres, which would allow for the cloning of very large fragments of DNA, and their transfer into human cells for the purpose of gene therapy. Not yet established as a working technology, although some partial success has been reported.

human growth hormone (Abbreviation: hGH). See: growth hormone.

human-leukocyte-antigen system (Abbreviation: HLA). See: major histocompatibility antigens.

humoral immune response See: antibody-mediated immune response.

Hup+ Abbreviation for hydrogen-uptake positive.

hybrid 1. The offspring of two genetically unlike parents. 2. Of DNA molecules, see: heteroduplex.

hybrid arrested translation A method used to identify what protein(s) are encoded by a particular cloned DNA sequence. A total mRNA preparation, which contains many different mRNAs, is hybridized with cloned DNA, so that those mRNA molecules homologous to the cloned DNA will anneal to give DNA/RNA heteroduplexes. The non-annealed mRNA molecules can be translated in vitro and this is then compared to translation products from the untreated mRNAs. See: hybrid released translation.

hybrid cell See: synkaryon.

hybrid dysgenesis Infertility and an increased incidence of chromosome mutations thought to be caused by the activation of transposons.

hybrid released translation A method used to identify the gene product of a cloned gene. The cloned DNA is immobilized and hybridized with a mixed mRNA sample, so that only mRNA sequences homologous to the cloned DNA will be retained. These mRNA molecules are subsequently removed and translated in vitro. See: hybrid arrested translation.

hybrid seed 1. Seed produced by crossing genetically dissimilar parents. 2. In plant breeding, used colloquially for seed produced by specific crosses of selected pure lines, such that the F1 crop is genetically uniform and displays hybrid vigour. As the F1 plants are heterozygous with respect to many genes, the crop does not breed true and so new seed must be purchased each season.

hybrid selection The process of choosing individuals possessing desired characteristics from among a hybrid population.

hybrid vigour The extent to which a hybrid individual outperforms both its parents with respect to one or many traits. The genetic basis of hybrid vigour is not well understood, but the phenomenon is widespread, particularly in inbreeding plant species. Synonym: heterosis.

hybridization 1. The process of forming a hybrid by cross pollination of plants or by mating animals of different types. 2. The production of offspring of genetically different parents, normally from sexual reproduction, but also asexually by the fusion of protoplasts or by transformation. 3. The pairing of two DNA strands, often from different sources, by hydrogen bonding between complementary nucleotides.

hybridoma A synthetic hybrid cell, derived by fusing a B lymphocyte with a tumour cell. The former secretes a single antibody, while the latter confers the property of growing indefinitely in tissue culture. The underlying technology behind the monoclonal antibody.

hydrogen-uptake positive (Abbreviation: Hup+). A term describing a micro-organism that is capable of assimilating (or taking up) hydrogen gas.

hydrolysis A chemical reaction in which water is added across a covalent bond, often cleaving the molecule into two. Occurs for example when polynucleotides, polypeptides, and polysaccharides are broken into their component monomers. Thus sucrose can be hydrolysed to glucose and fructose; and proteins to individual amino acids.

hydrophobic interaction An interaction between a hydrophobic ('water-hating') part of a molecule and an aqueous environment. Particularly significant in establishing the conformation of molecules in solution, and thus their biological activity. Many enzymes have a structure where the polypeptide chain is folded to form a hydrophobic core and a hydrophilic ('water-loving') surface.

hydroponics The growing of plants without soil. Plants are fed with an aerated solution of nutrients, and the roots are either supported within an inert matrix, or are freely floating in the nutrient solution.

hygromycin An antibiotic used as selective agent in bacterial and transgenic plant cell cultures.

hyperploid The situation in which a particular chromosome or chromosome segment is present in more than the normal number. Opposite: hypoploid.

hypersensitive response 1. A specific reaction of a plant to attack by a pathogen. The plant cells surrounding the point of infection rapidly die and dry out, so that pathogen spread within the plant is prevented. Often associated with the interaction of race-specific R genes with a matching pathogen avirulence. 2. The abnormal response of an animal to the presence of a particular antigen.

hypersensitive site Regions in the DNA that are highly susceptible to digestion with restriction endonucleases.

hypertonic A solution with an osmotic potential greater than that of living cells. Treatment with such solutions leads to the plasmolysis of cells. Opposite: hypotonic.

hypervariable region The parts of both the heavy and light chains of an antibody molecule that enable it to bind to a specific site on an antigen.

hypervariable segment A region of a protein that varies considerably between strains or individuals.

hypocotyl The portion of an embryo or seedling below the cotyledons. A transitional area between stem and root.

hypomorph A mutation that reduces, but does not completely abolish gene expression.

hypoplastic Defective and reduced growth or development (e.g. dwarfing and stunting in plants) resulting from an abnormal condition, for example disease or nutritional stress.

hypoploid The situation in which a particular chromosome or chromosome segment is present in less than the normal number. Opposite: hyperploid.

hypothalamic peptides Peptides generated in the vertebrate forebrain and concerned with regulating the body's physiological state.

hypotonic Osmotic potential less than that of living cells. Cells placed in a hypotonic solution will absorb water and display swelling and turgidity. Opposite: hypertonic.

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