V
Valine
(see amino acids).
V-DNA
An in vitro form of ds cccDNA
produced by annealing two complementary ss cccDNA molecules without strand
breakage; V-DNA is thus characterized by a linking number of zero (see DNA).
Any regions of right-handed double-helical conformation must be compensated for
by negative supercoils and/or by regions of left-handed double-helical
conformation. There is evidence that V-DNA of natural base sequence can adopt a
combination of right-handed (B-DNA) and left-handed (Z-DNA) conformations.
V factor
A growth factor required by certain Haemophilus spp; the
requirement is satisfied by NAD+or NADP+ (or, minimally, by nicotinamide
mononucleotide). ‘V factor’ occurs e.g. in yeast extract and in blood, although
it is not available in whole blood since it occurs within the RBCs (which also
contain NADase); heating of blood, as in the preparation of CHOCOLATE AGAR, releases
the NAD(P)+ and destroys the NADase.
V forms
See V–W TRANSITION.
v-onc
See ONCOGENE.
V region (immunol.)
SeeVARIABLE REGION.
v strand (V strand) (virol.)
An ssDNA viral genome, or a strand homologous
to it.
V3 loop (of HIV)
See AIDS.
VA mycorrhiza
Vesicular-arbuscular MYCORRHIZA.
VA RNAs
See SPLIT GENE (e).
Vaccination
(1) immunization (sense 1) by the
parenteral administration of a vaccine.
(2) immunization (sense 1) by the oral or
parenteral administration of a vaccine (cf. inoculation (2)).
(3) syn. immunization
vaccine
A
harmless variant or derivative of a pathogen that stimulates a host's immune
system to mount defenses against the pathogen.
vacuole
[L. vacuus, empty]
A membrane-enclosed sac taking up most of the interior of a mature plant cell and containing a variety of substances important in plant reproduction, growth, and development.
A membrane-enclosed sac taking up most of the interior of a mature plant cell and containing a variety of substances important in plant reproduction, growth, and development.
vagina
Part of
the female reproductive system between the uterus and the outside opening; the
birth canal in mammals; also accommodates the male's penis and receives sperm
during copulation.
valence
shell
The
outermost energy shell of an atom, containing the valence electrons involved in
the chemical reactions of that atom.
Van der
Waals interactions
Weak
attractions between molecules or parts of molecules that are brought about by
localized charge fluctuations.
vaporization
[L. vapor, steam]
The change from a liquid to a gas; evaporation.
The change from a liquid to a gas; evaporation.
variation
Diversity
among the members of a population. Variation among individuals can exist at many
levels, including genetic, physiologic and behavioral.
vas
deferens pl. vasa
deferentia
(vass deff-er-ens)
[L. vas, a vessel + deferre, to carry down]
The tube in the male reproductive system in which sperm travel from the epididymis to the urethra.
The tube in the male reproductive system in which sperm travel from the epididymis to the urethra.
vascular
[L. vasculum, a
small vessel]
Containing or concerning vessels that conduct fluid.
Containing or concerning vessels that conduct fluid.
vascular
bundle
In
plants, a group of longitudinal supporting and conducting tissues (xylem and phloem).
vascular
cambium
[L. vasculum, a
small vessel + cambium, exchange]
A continuous cylinder of meristematic cells surrounding the xylem and pith that produces secondary xylem and phloem.
A continuous cylinder of meristematic cells surrounding the xylem and pith that produces secondary xylem and phloem.
vascular
plants
Plants
with vascular tissue, consisting of all modern species except the mosses and
their relatives.
vascular
tissue
Plant
tissue consisting of cells joined into tubes that transport water and nutrients
throughout the plant body.
vascular
tissue system
A system
formed by xylem and phloem throughout the plant, serving
as a transport system for water and nutrients, respectively.
vector
[L.
carrier]
In recombinant DNA, a small, self-replicating DNA molecule, or a portion thereof, into which a DNA segment can be spliced and introduced into a cell; generally a plasmid or a virus.
In recombinant DNA, a small, self-replicating DNA molecule, or a portion thereof, into which a DNA segment can be spliced and introduced into a cell; generally a plasmid or a virus.
vegetative
reproduction
Cloning
of plants by asexual means.
vein
[L. vena, a
blood vessel]
A vessel that returns blood to the heart.
A vessel that returns blood to the heart.
vena cava
(vee-na cah-va)
[L. blood vessel + hollow]
A large vein that brings blood from the tissues to the right atrium of the four-chambered mammalian heart. The superior vena cava collects blood from the forelimbs, head, and anterior or upper trunk; the inferior vena cava collects blood from the posterior body region.
A large vein that brings blood from the tissues to the right atrium of the four-chambered mammalian heart. The superior vena cava collects blood from the forelimbs, head, and anterior or upper trunk; the inferior vena cava collects blood from the posterior body region.
ventilation
Any
method of increasing contact between the respiratory medium and the respiratory
surface.
ventral
[L. venter, belly]
Pertaining to the undersurface of an animal that holds its body in a horizontal position; to the front surface of an animal that holds its body erect.
Pertaining to the undersurface of an animal that holds its body in a horizontal position; to the front surface of an animal that holds its body erect.
ventricle
[L. ventriculus, the
stomach]
A muscular chamber of the heart that receives blood from an atrium and pumps blood out of the heart, either to the lungs or to the body tissues.
A muscular chamber of the heart that receives blood from an atrium and pumps blood out of the heart, either to the lungs or to the body tissues.
venule
A very
small vein. See also Vein.
vertebral
column
[L. vertebra, joint]
The backbone; in nearly all vertebrates, it forms the supporting axis of the body and protects the spinal cord.
The backbone; in nearly all vertebrates, it forms the supporting axis of the body and protects the spinal cord.
vertebrate
A
chordate animal with a backbone: the mammals, birds, reptiles, amphibians, and
various classes of fishes.
vesicle
[L. vesicula, a
little bladder]
A small, intracellular membrane-bound sac.
A small, intracellular membrane-bound sac.
vessel
element
[L. vas, a
vessel]
A specialized short, wide cell in angiosperms; arranged end to end, they form continuous tubes for water transport.
A specialized short, wide cell in angiosperms; arranged end to end, they form continuous tubes for water transport.
vestigial
organ
A type of
homologous structure that is rudimentary and of marginal or no use to the
organism.
viable
[L. vita, life]
Able to live.
Able to live.
villus pl. villi
[L. a
tuft of hair]
In vertebrates, one of the minute, fingerlike projections lining the small intestine that serve to increase the absorptive surface area of the intestine.
In vertebrates, one of the minute, fingerlike projections lining the small intestine that serve to increase the absorptive surface area of the intestine.
viroid
(vy-roid)
A plant pathogen composed of molecules of naked RNA only several hundred nucleotides long.
A plant pathogen composed of molecules of naked RNA only several hundred nucleotides long.
virulent
Capable
of overcoming a host's defense mechanisms and causing a disease sometimes of
rapid onset and severe symptoms.
virus
[L.
slimy, liquid, poison]
A submicroscopic, noncellular particle composed of a nucleic acid core and a protein coat (capsid); parasitic; reproduces only within a host cell.
A submicroscopic, noncellular particle composed of a nucleic acid core and a protein coat (capsid); parasitic; reproduces only within a host cell.
viscera
[L.
internal organs]
The collective term for the internal organs of an animal.
The collective term for the internal organs of an animal.
visceral
muscle
Smooth
muscle found in the walls of the digestive tract, bladder, arteries, and other
internal organs.
visible
light
That
portion of the electromagnetic spectrum detected as various colors by the human
eye, ranging in wavelength from about 400 nm to about 700 nm.
vitalism
The
belief that natural phenomena are governed by a life force outside the realm of
physical and chemical laws.
vitamin
[L. vita, life]
An organic molecule required in the diet in very small amounts; vitamins serve primarily as coenzymes or parts of coenzymes.
An organic molecule required in the diet in very small amounts; vitamins serve primarily as coenzymes or parts of coenzymes.
viviparous
(vy-vip-er-us)
Referring to a type of development in which the young are born alive after having been nourished in the uterus by blood from the placenta.
Referring to a type of development in which the young are born alive after having been nourished in the uterus by blood from the placenta.
voltage-gated
channel
Ion
channel in a membrane that opens and closes in response to changes in membrane
potential (voltage); the sodium and potassium channels of neurons are examples.
