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label  A compound or atom that is either attached to or incorporated into a macromolecule and is used to detect the presence of a compound, substance, or macromolecule in a sample. a.k.a. tag.

labelling  The process of replacing a stable atom in a compound with a radioactive isotope of the same element to enable it to be detected by autoradiography or other techniques. Increasingly, radioactive labelling is being replaced by fluorescent labelling. The method is used to trace the path of the labelled compound through a biological or chemical system.

lac repressor-lac promoter system  See IPTG.

lactose  Milk sugar, a disaccharide with one unit each of glucose and galactose.

lag phase  1. A state of apparent inactivity preceding a response; called also a latent phase.

lagging strand  The strand of DNA that is synthesized discontinuously during replication (because DNA synthesis can proceed only in the 5´ to 3´ direction).

lambda chain  One of two classes of antibody light chains. cf kappa chain.

lamella (L. diminutive of lamina, a thin blade; pl: lamellae)  A double-membrane structure, plate or vesicle that is formed by two membranes lying parallel to each other.

lamina (L. lamina, a thin plate)  Blade or expanded part of a leaf.

laminar air-flow cabinet; laminar air-flow hood  Cabinet for inoculation of cultures. The working area is kept sterile by a continuous, non-turbulent flow of sterilized air through a HEPA filter. See HEPA filter; prefilter.

laminarin  A storage polysaccharide of the brown algae.

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

landrace  In PGR: An early, cultivated form of a crop species, evolved from a wild population.

latent agent  Something, usually a virus, that is present in a host organism without producing any symptoms.

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

latent phase See lag phase.

lateral bud  A bud produced at the base of a leaf petiole. See axillary bud.

lateral meristem  A meristem giving rise to secondary plant tissues, such as the vascular and cork cambia. The term is sometimes used to refer to an axillary meristem.

lawn  In biotechnology: a uniform and uninterrupted layer of bacterial growth, in which individual colonies cannot be observed.

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

LCR  See ligase chain reaction.

LD50 (lethal dose50%)  The amount of a chemical required to kill 50% of the test population. The higher the LD50, the lower the presumed toxicity of the chemical.

leader peptide  See signal sequence.

leader sequence  A sequence of nucleotides at the 5´ end of an mRNA that is not translated into protein.

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

leaf blade  The usually flattened portion of the leaf.

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

leaf margin  The edge of a leaf.

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

leaf roll  Virus diseases characterized by symptomatic curling of the host's leaves.

leaf scar  Mark left on a stem after leaf abscission.

leaflet  Expanded leaflike part of a compound leaf.

lectin  Any of a group of proteins, derived from plants, that can bind to specific oligosaccharides on the surface of cells, causing cells to clump together.

leptonema (adj: leptotene)  Stage in meiosis immediately preceding synapsis, in which the chromosomes appear as single, fine, threadlike structures (but they are really double because DNA replication has already taken place).

lethal allele; lethal gene  A mutant form of a gene that eventually results in the death of an organism if expressed in the phenotype.

lethal gene  See lethal allele.

lethal mutation  See lethal allele.

library (gene library). A collection of cells, usually bacteria or yeast, that have been transformed with recombinant vectors carrying DNA inserts from a single species. See cDNA library; expression library; genomic library; bank; gene bank.

life cycle  The complete sequence of events undergone by organisms of a particular species, from the fusion of gametes in one generation to the same stage in the following generation.

ligand  A molecule that can bind to another molecule in or on cells.

ligase See DNA ligase.

ligase chain reaction (LCR)  A technique for determining the presence or absence of a specific nucleotide pair within a target gene.

ligate (to)  The process of joining two or more DNA fragments.

ligation  The joining of two linear nucleic acid molecules by the formation of phospho-diester bonds. In cloning experiments, a restriction fragment is often ligated to a linearized vector molecule using T4 DNA ligase.

lignification  (L. lignum, wood + facere, to make)  Impregnation of a cell wall with lignin.

lignin (L. lignum, wood) A group of high-molecular-weight amorphous materials, comprising polymers of phenylpropanoid compounds, giving strength to certain tissues. Wood is composed of lignified xylem cells (about 15 to 30% by weight).

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

lineage  Several individuals originating from a common descent, such as the production of a cell line from a single cell plated in vitro.

linear phase  When cell numbers constantly increases. The linear phase is located between the exponential growth and the deceleration phases. See growth phases.

linearized vector  See ligation.

LINEs (long interspersed nuclear elements)  Families of long (average length = 6 500 bp), moderately repetitive (about 10,000 copies). LINEs are cDNA copies of functional genes present in the same genome; also known as processed pseudo-genes.

linkage  The tendency of non-allelic genes to be inherited together more than would be expected if they were assorting independently. Linkage exists between two loci when they are located sufficiently close on the same chromosome that some gametes are produced without crossing-over occurring between the two loci.

linkage disequilibrium  See gametic (phase) disequilibrium.

linkage equilibrium  See gametic phase equilibrium.

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

linked genes; linked markers  Genes or markers that show linkage q.v..

linker  A synthetic double-stranded oligonucleotide that carries the sequence for one or more restriction endonuclease sites.

lipases  Enzymes which break down lipids into their component fatty acid and "head group" moieties. The lipases used in biotechnology are almost invariably digestive lipases, meant to break down the fats in food. They can be used to break down complex fats into their components, which are then used to make other materials.

lipid  (Gr. lipos, fat)  Any of a group of fats or fat-like compounds insoluble in water and soluble in fat solvents.

lipofection  Delivery into eukaryotic cells of DNA, RNA or other compounds that have been encapsulated in an artificial phospholipid vesicle.

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

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

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

liquid media  Media without a solidifying agent.

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

liquid nitrogen  Nitrogen gas condensed to a liquid with a boiling point of about -196°C. Very commonly the medium in which containers of genetic material are stored. See cryopreservation.

litmus  A pH indicator paper (range 4.5-8.3) impregnated with an extracted lichen pigment. It turns red in acidic and blue in alkaline solutions. However, the use of litmus paper as an indicator is not a precise method of pH measurement.

live vaccine  A living, non-virulent form of a micro-organism or virus that is used to elicit an antibody response that will protect the inoculated organism against infection by a virulent form of the micro-organism or virus. Also a living, non-virulent micro-organism or virus that express a foreign antigenic protein and is used to inoculate humans or animals. The latter organisms are also called a live recombinant vaccines.

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

lod score  The logarithm of the odds of linkage between two loci. Calculated from pedigree data, as the log (to base 10) of the ratio of the probability of the observed pedigree assuming linkage with a specified recombination fraction q, to the probability of the observed pedigree assuming no linkage, i.e., recombination fraction = 0.5. A lod score (also called a z value) is thus calculated as z = log10 {Prob(data|q)/Prob(data|0.5)}. A lod score of +3.00 (which is odds of 1000:1) or greater is regarded as acceptable evidence for linkage; -2.00 (which is the log of 1:100) or less indicates that no linkage exists.

log phase  See logarithmic phase.

logarithmic phase (log(arithmic) or exponential growth phase)  The steepest slope of the growth curve; the phase of vigorous growth, during which cell number doubles every 20-30 minutes. See growth phases.

long-day plant  Plant requiring short nights before flowering is initiated.

long interspersed nuclear elements  See LINEs.

long template  A DNA strand that is synthesized during the polymerase chain reaction and has a primer sequence at one end but is extended beyond the site that is complementary to the second primer at the other end.

long terminal repeat (LTR)  A string of bases that occurs at each end of the "genome" of a retrovirus that has become integrated into the host genome. Involved in the integration process.

loop bioreactors  Fermenters in which the fermenting material is cycled between a bulk tank and a smaller tank or loop of pipes. The circulation helps to mix the materials and to ensure that gas injected into the fermenter is well distributed in the liquid. The reactors are also very useful for photosynthetic fermentations, where they allow the photosynthesizing organisms to be passed along a large number of small pipes, where the light can get to them easily, rather than inside a single volume, where only the organisms near the edges get much light.

LPS  See lipopolysaccharide.

LTR  See long terminal repeat.

lux (SI symbol: lx)  The unit of measurement for illuminance (i.e., the amount of illumination) impinging upon a surface. 1 lx is the illuminance impinging upon a surface of 1 m2, each point of which is at a distance of 1 m away from a uniform point source of light of 1 cd (candela). It supersedes the foot-candle. See foot-candle; photon.

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

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

lymphocyte  A general class of white blood cells that are important components of the immune system of vertebrates.

lymphokine  Generic name for proteins that are released by lymphocytes to act on other cells involved in the immune response. The term includes interleukins and interferons.

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

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

lyophilize  Rapid freezing followed by a dehydration under high vacuum. The process is lyophilization. cf  freeze-drying.

lysis   (Gr. lysis, a losing)  The destruction or breakage of cells either by viruses or by chemical or physical treatment.

lysogen  A bacterial cell whose chromosome contains integrated viral DNA.

lysogenic  A type or phase of the virus life cycle during which the virus integrates into the host chromosome of the infected cell, remaining essentially dormant for some period of time. See lysogen.

lysogenic bacteria  Bacteria harbouring temperate (non-virulent, lysogenic) bacteriophages.

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

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

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

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

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