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- E -

EBV  See estimated breeding value.

EC  See Enzyme Commission.

ecdysone  A steroid hormone in insects that stimulates moulting and metamorphosis. It acts on specific genes, stimulating the synthesis of proteins involved in these bodily changes.

eclosion  1. Emergence of an adult insect from the pupal stage.

    2. Beginning of germination of fungal spores.

E. coli  See Escherichia coli.

ecological diversity  See biodiversity.

ecology  The study of the interactions between organisms and their natural environment, both living and non-living.

economic trait locus (ETL)  A locus influencing a trait that contributes to income. The plural form (economic trait loci) is also abbreviated as ETL.

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

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

EDTA  See ethylene-diamine tetra-acetic acid.

effector cells  Cells of the immune system that are responsible for 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 ovum (zygote) in egg-laying animals after it emerges from the body.

    2. The mature female reproductive cell in animals and plants.

    See also ovum.

EGS  External guide sequence.  See guide sequence.

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

electro-blotting  The electrophoretic transfer of macromolecules (DNA, RNA or protein) from a gel, in which they have been separated, to a support matrix, such as a nitrocellulose sheet. A transfer usually used in techniques such as Southern and northern blotting.

electrochemical sensor  Type of biosensor in which a biological process is harnessed to an electrical sensor system, such as an enzyme electrode. Other types couple a biological event to an electrical one via a range of mechanisms, such as those based on oxygen and pH. See enzyme electrode.

electromagnetic radiation  Electromagnetic waves, including ultraviolet (UV), X-rays, and gamma radiation (g rays). Electromagnetic radiation is used to produce mutant cells or organisms, or, in the case of UV, disinfestation and sterilization, in tissue culture.

electromagnetic spectrum  The range of wavelengths or frequencies over which electromagnetic radiation extends.

electron microscope  A microscope that uses an electron beam focused by magnetic "lenses." See scanning electron microscope; transmission electron microscope.

electrophoresis  A technique that separates charged molecules - such as DNA, RNA or protein - on the basis of relative migration in an appropriate matrix (such as agarose gel or polyacrylamide gel) subjected to an electric field. See agarose gel electrophoresis; polyacrylamide gel electrophoresis (PAGE); pulsed-field gel electrophoresis (PFGE).

electroporation  1. An electrical treatment of cells that induces transient pores, through which DNA can enter the cell.

    2. The introduction of DNA or RNA into protoplasts or other cells by the momentary disruption of the cell membrane through exposure to an intense electric field.

    Note: Although the precise mechanism of electroporation is poorly understood, pores are thought to form by the local polarization of the cell membrane when it is exposed to a high electric potential. These openings persist for a variable amount of time, depending upon the temperature at which the cell is treated. Macro-molecules, such as DNA or RNA, enter through these openings either through diffusion or through electrophoretic movement. The membrane openings then re-seal, capturing introduced DNA and preventing escape of the cell contents.

ELISA (enzyme-linked immunosorbent assay)  A sensitive technique for accurately determining specific molecules in a mixed sample. The amount of protein or other antigen in a given sample is determined by means of an enzyme-catalysed colour change, avoiding both the hazards and expense of radioactive techniques.

    It takes various forms. In the most common form, two antibody preparations are used in ELISA. An antibody (primary) specific to the test protein is adsorbed onto a solid substrate, and a known amount of the sample is added; all molecules of the test protein in the sample are bound by the antibody. A second antibody specific for a second site on the test protein is added; this is conjugated with an enzyme, which catalyses a colour change in the fourth reagent, added finally. The colour change is measured photometrically and compared against a standard curve to give the concentration of protein in the sample. ELISA is widely used for diagnostic and other purposes.

elongation factors  Soluble proteins that are required for polypeptide chain elongation.

embryo  (Gr. en, in + bryein, to swell)  An immature organism in the early stages of development. In mammals, this occurs 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 without fertilization. In aseptic cultures, adventitious embryos show polarization, followed by the growth of a shoot from one end and a root from the other end.

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

embryo culture  The culture of embryos on nutrient media.

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; if this development in culture involves somatic cells and not the zygote, it can be indicated by using the term adventitious embryogenesis or somatic embryogenesis. 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. Other chemicals have to be at suitable levels. The procedure involves the explanting of a piece of plant tissue and putting it on a high-auxin medium, where the cells grow into a mass of callus. This is then transferred to a maturation medium, where the callus starts to initialize organs, ultimately growing a root and a shoot.

embryo multiplication and transfer (EMT)  The cloning of animal embryos and their subsequent transfer to recipients (via artificial inembryonation (q.v.)). The cloned embryos can be clones of an embryo or of an adult.

embryo sac  A large thin-walled space within the ovule of the seed plant in which the egg and, after fertilization, the embryo develop; the mature female gametophyte in angiosperms. Generally a seven-celled structure. The seven cells are 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, typically by means of PCR (q.v.) involving amplification from a small sample of embryonic tissue, using primers specific for a locus on the Y 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 identicals (identical organisms) that can be produced from a single embryo is less than 10.

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

embryo transfer (ET)  See multiple ovulation and embryo transfer.

embryoid  An embryo-like body developing in vitro. It forms a complete, self-contained platelet with no vascular connection with the callus. The term embryoid is no longer commonly used. See embryo.

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

empirical  Relating to or based upon practical experience, trial and error, direct observation or observation alone, without benefit of scientific method, knowledge or theory.

EMT  See embryo multiplication and transfer.

encapsidation  The process by which a virus' nucleic acid is enclosed in a capsid. See capsid; coat protein.

encapsulation  Any method of getting something, usually an enzyme or bacterium, into a small package or capsule while it is still working or alive. It is a method for immobilizing cells for use in a bioreactor.

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

encode  To specify, after decoding by transcription and translation, the sequence of amino acids in a protein.

5´ end  The phosphate group that is attached to the 5´ carbon atom of a sugar (ribose or deoxyribose) of the terminal nucleotide of a nucleic acid molecule.

endangered breed  In AnGR: A breed where the total number of breeding females is between 100 and 1 000 or the total number of breeding males is less than or equal to 20 and greater than five; or the overall population size is close to, but slightly above 100 and increasing, and the percentage of pure-bred females is above 80%; or the overall population size is close to, but slightly above, 1 000 and decreasing, and the percentage of pure-bred females is below 80%. (Source: FAO, 1999)

endangered-maintained breed  See critical-maintained breed.

endangered species  A plant or animal species in immediate danger of extinction because its population numbers have reached a critical level or its habitats have been drastically reduced.

endemic  1. Describing a plant or animal species whose distribution is restricted to one or a few localities.

    2. Describing a disease or a pest that is always present in an area.

end-labelling  The introduction of a radioactive atom at the end of a DNA or RNA molecule. A commonly used method is to use T4 polynucleotide kinase to introduce a 32P atom onto the end of a DNA molecule.

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. cf exocrine gland. See gland.

endocrine interference  Interference with the normal balance 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 saclike vesicle into which the material is incorporated. 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 will develop into the alimentary canal (gut) and digestive glands of the adult.

endodermis  (Gr. endon, within + derma, skin)  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. The endodermis separates the cortical cells from cells of the pericycle.

endogamy  The fusion of reproductive cells from closely related parents, i.e., inbreeding. cf exogamy.

endogenote  The part of the bacterial chromosome that is homologous to a genome fragment (exogenote) transferred from the donor to the recipient cell in the formation of a merozygote.

endogenous   (Gr. endon, within, + genos, race, kind)  Developed or added from within the cell or organism.

endomitosis  Duplication of chromosomes without division of the nucleus, resulting in increased chromosome number within a cell. Chromosome strands separate, but the cell does not divide.

endonuclease  An enzyme that breaks strands of DNA at internal positions; these enzymes are important tools in recombinant DNA technology. See nuclease.

endophyte  An organism that lives inside a plant.

endoplasmic reticulum  (Gr. endon, within + plasma, anything formed or moulded; L. reticulum, a small net)  A cytoplasmic net of membranes, adjacent to the nucleus, made visible by the electron microscope. Any system of paired membranes within the cytoplasm. Frequently abbreviated to ER. They are sites of protein synthesis.

endopolyploidy  The result of nuclear divisions without subsequent cytoplasmic division (cytokinesis); the polyploids so obtained are called endopolyploids. See polyploidy.

endoprotease  An enzyme that cleaves the peptide bonds between amino acids within a protein. Cleavage is usually at one or more specific sites.

endoreduplication  Chromosome reproduction during interphase. 4-chromatid chromosomes (diplochromosomes) are seen during this phase.

endosperm  (Gr. endon, within + sperma, seed)  Nutritive tissue that develops in the embryo sac of most angiosperms. It usually forms after the fertilization of the two fused primary endosperm nuclei of the embryo sac with one of the two male gamete nuclei. In most diploid plants, e.g., cereals, the endosperm is triploid (3n), but in some (e.g., lily) it is often consumed as the seed matures.

endosperm mother cell  One of the seven cells of the mature embryo sac, containing the two polar nuclei and, after reception of a sperm cell, 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. The enzyme is sometimes the first enzyme in a biosynthetic pathway, and the metabolite is generally 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 DNA sequence that increases the transcription of a eukaryotic gene when they are both on the same DNA molecule. a.k.a. enhancer element; enhancer sequence.

enhancer element; enhancer sequence  1. A sequence found in eukaryotes and certain eukaryotic viruses which can increase transcription of a gene when located (in either orientation) up to several kilobases from the gene concerned. These sequences usually act as enhancers when on the 5´ side (upstream) of the gene in question. However, some enhancers are active when placed on the 3´ side (downstream) of the gene. In some cases enhancer elements can activate transcription of a gene with no (known) promoter.

    2. A substance or object that increases a chemical activity or a physiological process.

    3. A major or modifier gene that increases a physiological process.

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.

environment  The aggregate of all the external conditions and influences affecting the life and development of an organism.

enzyme  (Gr. en, in + zyme, yeast or leaven)  A protein produced in living cells, 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, according to the type of reaction they catalyse: 1. Oxidoreductase; 2. Transferases; 3. Hydrolases; 4. Lyases; 5. Isomerases; 6. Ligases. The names of most individual enzymes are usually derived from the substrate on which they act, with the suffix -ase. Thus lactase is the enzyme that acts to breakdown lactose; it is classified as a hydrolase. See Enzyme Commission Number.

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

Enzyme Commission (EC) Number  Systematic name and number which identify an enzyme in technical literature. Assigned by the Enzyme Commission, the EC Number consists of four numbers separated by dots: the first classifies the enzyme into one of the six broad groups: 1. Oxidoreductase; 2. Transferases; 3. Hydrolases; 4. Lyases; 5. Isomerases; 6. Ligases. 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:

      · ampometric, where the electrode is kept as near zero voltage as possible. When the enzyme catalyses its reaction, electrons flow into the electrode, and so a current flows; and

      · potentiometric, when the electrode is held at a voltage which counteracts the voltage determined by the enzyme's tendency to push electrons into its.

      Usually enzymes transfer their electrons inefficiently to the electrode, so a mediator compound is coated onto the electrode to help the transfer.

enzyme-linked immunosorbent assay  See ELISA.

enzyme stabilization using antibodies  A method of stabilizing enzymes by binding antibodies to them. The antibodies should not block the active site of the enzyme, as otherwise the protein is stabilized but is inactive as a catalyst. Monoclonal antibodies are usually used as they bind to specific bits of the protein surface. If the enzyme tries to unfold into an inactivate structure, it must not only overcome its own binding energy but also throw off all the bound antibodies; this requires more energy, and so is a correspondingly slower process.

EPD  See expected progeny difference.

epicotyl  (Gr. epi, upon + kotyledon, a cup-shaped hollow)  The upper portion of the axis of a plant embryo or seedling, above the cotyledons.

epidermis  (Gr. epi, upon + derma, skin). 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.

epigenetic  A term referring to the non-genetic causes of a phenotype.

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 fertility factor (F) 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. Plasmids and F factors are episomes.

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

epitope  A specific chemical domain on an antigen that stimulates the production of, and is recognized by, an antibody. Each epitope on a molecule such as protein elicits the synthesis of a different antibody. a.k.a. antigenic determinant.

epizootic  A disease affecting a large number of animals simultaneously.

equational division  Mitotic-type division that is usually the second division in the meiotic sequence; somatic mitosis and the non-reductional division of meiosis. A chromosome division in which the two chromatids of each duplicated chromosome separate longitudinally, prior to being incorporated into two daughter nuclei.

equatorial plate  The figure formed by the chromosomes in the centre (equatorial plane) of the spindle in mitosis.

equilibrium  A state of dynamic systems in which there is no net change.

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

equimolar  Identical molar concentrations. See molarity; mole.

ER  See endoplasmic reticulum.

Erlenmeyer flask   A conical flat-bottomed laboratory flask with a narrow neck, designed by E. Erlenmeyer. Widely used for culturing micro-organisms.

ES cells  See embryonic stem cells.

Escherichia coli  A commensal bacterium inhabiting the colon of many species. E. coli is widely used in biology, both as a simple model of cell biochemical function and as a host for molecular cloning experiments. In environmental studies, it is a key indicator of water pollution due to human sewage effluent.

E site See exit site.

essential amino acid  An amino acid that cannot be synthesized by animals and therefore has to be ingested with feed or food.

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  A nutrient is essential when it is mandatory for growth, development and reproduction. In tissue culture, it 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 stages I & II).

    2. A suspension culture subjected to several passages with a constant cell number per unit time.

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

estrogen  See oestrogen.

ET  See multiple ovulation and embryo transfer.

ethanol (ethyl alcohol) (C2H6O; f.w. 46.07)  Commonly used to disinfest plant tissues, glassware utensils and working surfaces in tissue culture manipulations. The concentration used is 70% (v/v) for disinfecting and 95% (v/v) when flaming tools. Ethanol is also used to dissolve water-insoluble additions (addendums) to culture media.

ethephon  (2-chloroethyl) phosphonic acid (ClC2PO3H6; f.w. 144.50) Through a spontaneous degradation of ethephon, ethylene is produced. Ethephon is a synthetic compound commonly used to treat cultured cells or unripened fruit with ethylene (q.v.).

ethidium bromide  A fluorescent dye used to stain DNA and RNA. The dye fluoresces when exposed to UV light.

ethyl alcohol See ethanol.

ethylene  (C2H4)  A gaseous plant growth regulator regulating various aspects of vegetative growth, fruit ripening and abscission of plant parts.

ethylenediamine tetraacetic acid  (EDTA)  A chelating compound. In tissue culture it is used to keep nutrients, such as iron, bound in a form that leaves them still available to the plant but which prevents them from precipitating out.

etiolation  An abnormal increase in stem elongation, accompanied by poor or absent 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 caused by disease.

ETL  See economic trait locus.

eucaryote; eucaryotic  See eukaryote.

euchromatin  Genetic material that is stained less intensely by certain dyes during interphase, and that comprises many different kinds of genes. cf heterochromatin.

eugenics  The application of the principles of genetics to the "improvement" of humankind.

eukaryote  (Gr. eu, true + karyon, true nucleus)  Any organism characterized by having the nucleus enclosed by a membrane. Eukaryotic organisms include animals, plants, fungi and some algae. They also possess membrane-bound functional organelles, such as mitochondria and chloroplasts, in the cytoplasm of their cells. cf prokaryote.

euploid  An organism or cell having a chromosome number that is an exact multiple of the monoploid (n) number. Terms used to identify different levels in an euploid series are diploid, triploid, tetraploid, and so on. cf aneuploid.

evaluation   In AnGR: Measurement of the characteristics that are important for production and adaptation, either of individual animals or of populations, most commonly in the context of comparative evaluation of the traits of animals or of populations. (Source: FAO, 1999)

evapotranspiration (L. evaporare, e, out of, + vapor, vapour + F. transpirer, to perspire)  The process of water loss in vapour form from a unit surface of land both directly and through leaf surfaces during a specific period of time.

evolution  The process by which the present diversity of plant and animal life arose from the earliest organisms, a process believed to have been continuing for at least 3 000 million years.

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

excision  1. The natural or in vitro enzymatic release (removal) of a DNA segment from a chromosome or cloning vector.

    2. Cutting out and preparing a tissue, organ, etc., for culture.

    3. Removing adventitious shoots from callus tissue.

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

excrete  To transport a compound out of a cell. a.k.a. to secrete; to export.

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

ex novo  See de novo.

exo III  See ase III.

exocrine gland  In animals, a gland that secretes through a duct. cf endocrine gland. See gland.

exodeoxyribonuclease III  See exonuclease III.

exogamy  The fusion of reproductive cells from distantly related or unrelated organisms, i.e., outbreeding. cf endogamy.

exogenote  Chromosomal fragment homologous to an endogenote and donated to a merozygote.

exogenous (Gr. exe, out, beyond + genos, race, kind)  Produced outside of, originating from, or due to external causes. Opposite of endogenous.

exogenous DNA  DNA that has been derived from a source organism and has been cloned into a vector and introduced into a host cell. 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. See DNA; intron; split gene; splicing.

exon amplification  A procedure that is used to amplify exons.

exonuclease  An enzyme that digests DNA or RNA, beginning at the ends of strands. It requires a free end in order to degrade a DNA or RNA molecule. 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 the molecule in the opposite direction. cf endonuclease.

exonuclease III  An E. coli enzyme that removes nucleotides from the 3´-hydroxyl ends of double stranded DNA. a.k.a. exo III; exodeoxyribonuclease III.

exopolysaccharide  A high-molecular-weight polymer that is composed of sugar residues and 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 (EPD)  The predicted performance of the future offspring of an animal for a particular trait, calculated from measurement(s) of the animal'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 animal in question is mated to a sample of animals whose average genetic merit equals that of the base population. The predicted performance of the offspring of the mating between any two animals is the sum of their EPDs.

explant  A plant part aseptically excised and prepared for culture in a nutrient medium. cf donor plant.

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

explant donor  The source plant or mother plant from which is taken the explant used to initiate a culture. cf donor plant.

exponential phase  The growth stage where cells undergo their maximum rate of cell division. The exponential phase follows the lag phase, and precedes the linear growth phase. See growth phases. a.k.a logarithmic phase.

export  To transport a compound out of a cell. a.k.a. to secrete; to excrete.

express  To transcribe and translate a gene's message into a peptide product.

expressed sequence tag (EST)  Short cDNA sequence. So-called because it represents part of the sequence (i.e., the "tag" of a sequence) of an expressed gene.

expression library  A population of different DNA molecules encoding peptides, that has been cloned into one kind of expression vector. See library.

expression system  Combination of host and vector which provides a genetic context for making a cloned gene function, 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 RNA 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. Such promoters are termed high level; examples include P1 (the leftward promoter of phage l) and the promoter of the yeast PGK (phosphoglycerate kinase) gene.

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

ex situ conservation  A conservation method which entails the actual removal of germplasm resources (seeds, pollen, sperm, individual organisms) from the original habitat or natural environment. cf gene bank; cryobiological preservation; in situ conservation.

ex situ conservation of farm animal genetic diversity  In AnGR: All conservation of genetic material in vivo, but out of the environment in which it developed, and in vitro including, inter alia, the cryoconservation of semen, oocytes, embryos, cells or tissues. Note that ex situ conservation and ex situ preservation are considered here to be synonymous. (Source: FAO, 1999)

ex situ preservation See ex situ conservation.

extension  Single-stranded DNA region consisting of one or more nucleotides at the end of a strand of duplex DNA. a.k.a. protruding end; sticky end; overhang; cohesive end. See extension.

3´-extension  A short single-stranded nucleotide sequence on the 3´-hydroxyl end of a double-stranded DNA molecule. a.k.a. 3´ protruding end; 3´ sticky end; 3´ overhang.

5´-extension  A short single-stranded nucleotide sequence on the 5´-hydroxyl end of a double-stranded DNA molecule. a.k.a. 5´ protruding end; 5´ sticky end; 5´ overhang.

extinct breed  In AnGR: A breed where it is no longer possible to recreate the breed population. Extinction is absolute when there are no breeding males (semen), breeding females (oocytes), nor embryos remaining. (Source: FAO, 1999)

external guide sequence (EGS)  See guide sequence.

extinction  The irreversible condition of a species or other group of organisms of having no living representatives in the wild, which follows the death of the last surviving individual of that species or group. Extinction may occur on a local or global level; it can result from various human activities, including the destruction of habitats or the overexploitation of species that are hunted or harvested as a resource.

extrachromosomal  Things that are not part of the chromosomes; DNA units in the cytoplasm that control cytoplasmic inheritance.

extrachromosomal inheritance  See cytoplasmic inheritance.

extrachromosomes  Self-replicative genetic elements separate from main chromosome(s) of a cell. This definition usually excludes viruses, but the division is somewhat arbitrary. In bacteria, plasmids are the principal extrachromosomes; they encode functions which are not essential to the growth and division of the host cell. In eukaryotes, extrachromosomes may be either essential or dispensable. They may inhabit (i) the nucleus, e.g., extrachromosomal rDNA molecules, yeast 2mm plasmid; (ii) the cytosol, e.g., dsRNA molecules in fungi; or (iii) the cytoplasmic organelles, e.g., mitochondrial DNA, chloroplast DNA. Eukaryotic extrachromosomal elements may be recognized genetically by their failure to show segregation at meiosis.

exude  Slowly discharge; leak liquid material (exudate such as tannins or oxidized polyphenols) through pores or cuts, or by diffusion into the medium. In some woody plant species, exudation is associated with a lethal browning of explants.

ex vitro (L. "from glass")  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. After culturing, the transformed cells are introduced back into the individual by transfusion, infusion or injection, to alleviate a genetic disorder.

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