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- V -

v/v  On a volume per volume basis; the percent of the volume of a constituent in 100 units of volume, e.g., (ml/100 ml) × 100.

vaccination  See preventive immunization.

vaccine A preparation of dead or weakened pathogens, or of derived antigenic determinants, that is used to induce formation of antibodies or immunity against the pathogen.

vaccinia  The cowpox virus used to vaccinate against smallpox and, experimentally, as a carrier of genes for antigenic determinants cloned from other disease organisms.

vacuole (L. diminutive of vacuus, empty)  A cavity in a plant cell, bounded by a membrane; in which various plant products and by-products are stored.

vacuum  Vacuum is created through use of a vacuum pump or aspirator pump, to facilitate specific biological preparations, such as inclusions or disinfection of material for in vitro culture, etc.

variable domains  Regions of antibody chains that have different amino acid sequences in different antibody molecules. These regions are responsible for the antigen-binding specificity of the antibody molecule.

variable expressivity  Variation in the phenotype caused by different alleles of the same gene and/or by the action of other genes and/or by the action of non-genetic factors. cf expressivity.

variable number tandem repeat  See VNTR.

variable surface glycoprotein (VSG)  One of a battery of antigenic determinants expressed by a micro-organism to elude immune detection.

variance  In statistics: The sum of the squared deviations, divided by one less than the number of observations. A statistical measure of variation in a population.

variant  An organism that is genetically different from the wild type organism. a.k.a. mutant.

variation  Differences between individuals within a population or among populations.

variegated  Plants having both green and albino tissues. This difference in colour may result from viral infection, nutritional deficiency, or may be under genetic or physiological control.

variety  A naturally occurring subdivision of a species, with distinct morphological characters and given a Latin name according to the rules of the International Code of Nomenclature. A taxonomic variety is known by the first validly published name applied to it so that nomenclature tends to be stable. cf cultivar; pathovar.

vascular (L. vasculum, a small vessel)  Referring to any plant tissue or region consisting of or giving rise to conducting tissue, e.g., bundle, cambium, ray.

vascular bundle; fascicle  A strand of tissue containing primary xylem and primary phloem (and procambium if present) and frequently enclosed by a bundle sheath of parenchyma or fibres.

vascular cambium  In biennials and perennials, cambium giving rise to secondary phloem and secondary xylem.

vascular plants  Plants possessing organized vascular tissues.

vascular system  1. A specialized network of vessels for the circulation of fluids throughout the body tissue of an animal.

    2. The system of vascular tissue in plants.

vascular tissue  The tissue that conducts water and nutrients throughout the plant body in higher plants.

vector (L. vehere, to carry)  1. An organism, usually an insect, that carries and transmits disease-causing organisms.

    2. A plasmid or phage that is used to deliver selected foreign DNA for cloning and in gene transfer. See Ti plasmid.

vegetative propagation  Same as asexual or non-sexual propagation.

vehicle  The host organism used for the replication or expression of a cloned gene or other sequence The term is little used and is often confused with vector. See vector.

velocity density gradient centrifugation  A procedure used to separate macromolecules based on their rate of movement through a density gradient.

velogenetics  The combined use of marker-assisted selection (q.v.) and embryo technologies such as OPU (q.v.), IVM (q.v.) and IVF (q.v.), in order to increase the rate of genetic improvement in animal populations.

vermiculite  Coarse aggregate material made from expanded mica having a high cation exchange capacity and high water-holding capacity and used as a rooting medium and a soil additive.

vernalization  The process by which floral induction in some plants is promoted by exposing the plants to chilling for a certain duration.

vessel (L. vasculum, a small vessel)  1. A series of xylem elements whose function is to conduct water and nutrients in plants.

    2. A container, such as a Petri dish or test tube, used for tissue culture.

vessel element  A type of cell occurring within the xylem of flowering plants. Many are water-conducting vessels.

viability  The capability to live and develop normally.

viability test  Assay of the number or percent of living cells or plants in a population that have been given a specific treatment; such as ascertaining cell viability after cryopreservation treatment, or for seeds.

viable  Capable of germinating, living, growing and developing.

vibrio  Comma-shaped bacterium.

viral vaccines Vaccines consisting of live viruses rather than dead ones or separated parts of viruses. However, as the virus itself cannot be used, because that would simply give the patient the disease, the virus is genetically engineered so that it elicits the immune response to the viral pathogen without causing the disease itself.

    Two genetic engineering methods can be used. The first is to make the disease virus harmless, but still able to replicate in cultured animal cells. This is similar to producing an `attenuated' virus, i.e., one which has been grown in the laboratory until it loses its ability to cause disease. However, the genetic engineering route seeks to make sure that the attenuated virus has no chance of mutating back to a wild type, pathogenic virus, by deleting whole genes or replacing key regions of genes with completely different genetic material.

    The second approach is to clone the gene for a protein from the pathogenic virus into another, harmless virus, so that the result `looks' like the pathogenic virus but does not cause disease.

vir genes  A set of genes on a Ti plasmid that prepare the T-DNA segment for transfer into a plant cell.

viral coat protein  Protein present in the outer layer of a virus.

viral oncogene  A gene in the viral genome that contributes to malignancies in vertebrate hosts. See oncogene.

viral pathogen  A disease-causing virus.

virion  An infectious virus particle. A plant pathogen that consists of a naked RNA molecule of approximately 250-350 nucleotides, whose extensive base pairing results in a nearly correct double helix. See satellite RNA.

viroid  An infectious entity similar to a virus but smaller, consisting only of a strand of nucleic acid without the protein coat characteristic of a virus. See satellite RNA.

virulence (L. virulentia, a stench)  The degree of ability of an organism to cause disease. The relative infectiousness of a bacterium or virus, or its ability to overcome the resistance of the host metabolism.

virulent phage  A phage (virus) that destroys the host (bacterium).

virus (L. virus, a poisonous or slimy liquid)  An infectious particle composed of a protein capsule and a nucleic acid core (DNA or RNA), which is dependent on a host organism for replication. The DNA or a double-stranded DNA copy of an RNA virus genome is integrated into the host chromosome during lysogenic infection or replicated during the cystic cycle. See coat protein; DNA; genome; host; nucleic acid; prion; RNA; tumour virus; viroid.

virus-free  Plant, animal, cell, tissue or meristem which exhibits no viral symptoms or contains no identifiable virus-particles.

virus-tested  Description of a organism or a cell stock certified as being free of certain specified viruses following recognized procedures of virus diagnosis.

visible light  The part of the electromagnetic spectrum with wavelengths between 380 nm and 750 nm and perceived by the human eye.

vitamin B complex  A large group of water soluble vitamins that function as co-enzymes, including thiamine (B1); riboflavin or vitamin G (B2); niacin or nicotinic acid (B3); pantothenic acid (B5); pyridoxine (B6); cyanocobalamin (B12); biotin or vitamin H; folic acid or vitamin M (Bc); inositol; choline; and others.

vitamin C  See ascorbic acid.

vitamin H  See biotin; vitamin B complex.

vitamins (L. vita, life + amine)  Naturally occurring organic substances required by living organisms in relatively small amounts to maintain normal health, and which are added to tissue culture media to enhance growth, usually acting as enzyme co-factors.

vitrified; water soaked  Cultured tissue having leaves and sometimes stems with a glassy, transparent or wet and often swollen appearance. The process of vitrification is a general term for a variety of physiological disorders that lead to shoot tip and leaf necrosis.

vivipary  1. A form of reproduction in animals in which the developing embryo obtains its nourishment directly from the mother via a placenta or by other means.

    2. A form of asexual reproduction in certain plants, in which the flower develops into a budlike structure that forms a new plant when detached from the parent.

    3. The development of young plants in the inflorescence of the parent plant.

Vmax   The maximal rate of an enzyme-catalysed reaction. Vmax is the product of Eo (the total amount of enzyme) times the value of Keat (the catalytic rate constant).

VNTR  Variable number tandem repeat. A short DNA sequence that is present as tandem repeats and in highly variable copy number.

V regions  Variable regions in antibodies. See CDR.

volatilization  The conversion of a solid or liquid into a gas or vapour.

VSG  See variable surface glycoprotein.

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