Vitamin D receptor (VDR)
of light. See chromatophore, colorblindness, cone pig- ment genes (CPGs), opsin, rhodopsin. visual purple See rhodopsin. vitalism a philosophy holding that the phenomena exhibited in living organisms are the result of special forces distinct from chemical and physical ones.
See mechanistic philosophy. vital stain a dye used to stain living cells (Janus green, methylene blue, trypan blue, etc.). vitamin an organic compound (often functioning as a coenzyme) that is required in relatively minute amounts in the diet for the normal growth of a given organism. vitamin A a fat-soluble vitamin functioning as a precursor to retinal (q.v.) and retinoic acid (q.v.). Its structure is shown at the top of the page.
Vitamin A is generated by the splitting in two of a molecule of beta carotene. See carotenoids. vitamin B complex a family of water-soluble vita- mins, including thiamin (B1), riboflavin (B2), nico- tinic acid, pantothenic acid, pyridoxin (B6), and co- balamin (B12). vitamin C ascorbic acid, an important regulator of the oxidation-reduction state of protoplasm. vitamin D calciferol; a fat-soluble vitamin required in man for the prevention of rickets.
Its structure is shown below. Vitamin D mediates the absorption
of calcium and phosphorus from the intestine and promotes the mineralization of bone. In humans 7- dehydrocholesterol is secreted at the surface of the skin, where it is converted by ultraviolet radiation from sunlight into vitamin D. A 10-minute exposure per week of the upper body to sunlight will provide sufficient vitamin D. Most naturally occurring foods lack vitamin D, but saltwater fish are an exception. See cholesterol. vitamin D receptor (VDR) a protein encoded by a gene on the long arm of human chromosome 12.
The gene contains 9 exons, and the binding to vita- min D is due to the protein segment encoded by exons 7, 8, and 9.
Exons 2 and 3 each encode zinc- finger domains, and mutations in these regions abol- ish the DNA-binding function of the protein. Thus, VDR belongs to the family of steroid, nuclear hor- mone receptors. See androgen receptor, zinc-finger protein.
vitamin D-resistant rickets a group of hereditary diseases in which patients show a reduction in the levels of calcium and phosphorous in their blood and skeletal changes characteristic of rickets, although they have adequate dietary vitamin D. An autoso- mal recessive form of the disease has been shown to result from mutations in a gene that encodes a vita- min D receptor (q.v.). vitamin E alpha tocopherol, a vitamin functioning as an antioxidant. vitamin H biotin (q.v.). vitellarium the portion of the insect ovariole pos- terior to the germarium.
Egg chambers complete de- velopment within the vitellarium. vitelline membrane a membrane that surrounds the ovum. In Drosophila the term is used specifically for the membrane that immediately surrounds the oolemma and is formed by the fusion of deposits in the intercellular space between the oocyte and the columnar follicle cells that invest it. vitellogenesis the formation of yolk. vitellogenic hormone See allatum hormones. vitellogenin a protein synthesized by vitellogenic females and incorporated into the yolk spheres of the developing oocyte.
In Xenopus laevis, vitellogen- ins are synthesized by the liver. In Drosophila mela- nogaster, vitellogenins are synthesized by abdominal and thoracic fat bodies and by the columnar follicle cells surrounding the oocyte. See lipovitellin, phos- vitin. viviparous 1. producing living young rather than eggs. Embryogenesis occurs within the mother’s body, as with most mammals. 2. bearing seeds that germinate within the fruit, as in the mangrove.
Viviparus malleatus a prosobranchiate snail showing a bizarre type of spermatogenesis with the production of oligopyrene sperm. See sperm poly- morphism. v-myc See myc. VNTR locus See variable number of tandem repeats locus. voltinism a polymorphism in terms of whether or not the embryos produced by an insect enter dia- pause (q.v.). For example, in Bombyx mori univoltine strains produce only diapause embryos.
Bivoltine strains produce a nonhibernating brood, then diapause em- bryos. von Gierke disease a hereditary glycogen storage disease in humans arising from a deficiency of the enzyme glucose-6-phosphatase. Inherited as an au- tosomal recessive. Prevalence 1/100,000. von Willebrand disease the most common blood clotting disorder of humans. It is due to a deficiency of the von Willebrand factor (vWF), which is syn- thesized by endothelial cells and megakaryocytes. The vWF consists of multimers made up of mono- mers, each containing 2,050 amino acids. The vWF is first synthesized as a precursor molecule contain- ing 2,813 amino acids.
The antihemophilic factor (AHF) (q.v.) is extremely susceptible to proteolytic degradation. In the plasma, vWF combines with AHF and stabilizes it. If vWF is reduced because of severe damage to the vWF gene, there is also a marked reduction in AHF.
The vonWillebrand fac- tor is encoded by the VWF gene which is located at the end of the short arm of human chromosome 12, which encodes the vWF. The gene is 178 kilobases in length and contains 52 exons. The first 17 exons encode the signal peptide and the propolypeptide.
The remaining 35 exons encode the mature subunit and the 3′ noncoding region of the pre-mRNA. Most von Willebrand patients have one normal vWF gene and one gene carrying a missense or nonsense muta- tion in the ORF. They make about half the usual amount of vWF, and their disease is mild.
Such het- erozygotes occur at a frequency of about 8 per 1,000 individuals. Homozygotes are very rare (less than one in a million), and they bleed uncontrollably when injured. The syndrome is named after Eric vonWillebrand, a Danish physician who first de- scribed it in 1931.
See blood clotting, hemophilia. v-sis gene See simian sarcoma virus. v-src gene See Rous sarcoma virus. vulgare wheats See wheat. Vulpes vulpes the red fox, a species bred on a large scale on ranches for its pelt. Numerous muta- tions influencing fur color are known.