Duffy blood group gene
Drosophila pseudoobscura the second Drosophila species to have its genome sequenced. When the se- quences of D. pseudoobscura and D. melanogaster were compared, it was observed that the vast major- ity of genes remained on the same chromosome arms. However, within each arm the gene order was extensively shuffled.
See Appendix C, 1944, Dobz- hansky; 2005, Richards et al.; inversion, syntenic genes. Drosophila targeted gene expression technique an experimental system that allows the selective ac- tivation of a chosen regulatory gene in a variety of tissues and organs during Drosophila development. For example, the technique allowed the activation of eyeless (q.v.), a gene that specifies the differentia- tion of eye tissues in ectopic imaginal discs (q.v.).
The result was extra eyes developing on antennae, legs, and wings. See Frontispiece illustration. Drosophila virilis a species with polytene chro- mosomes that are larger than those of Drosophila melanogaster and exceptionally favorable for cytolog- ical analysis. The virilis genome is one of the largest in the genus (313 mb). D. virilis and D. melanogaster are very distantly related, having diverged about 60 million years ago. The virilis karyotype is considered to be ancestral for the genus.
Its chromosomes X, 2, 3,4,5,6 correspond to the chromosome arms X, 3R, 3L, 2L, 2R, and 4 of Drosophila melanogaster. See Drosophila, Drosophila salivary gland chromo- somes. drosopterins See Drosophilia eye pigments. drug resistance genes genes which confer upon certain organisms or cell types resistance to the toxic effects of specific chemicals.
Examples of such genes are those conferring antibiotic resistance upon bacte- ria, insecticide resistance upon insects, or resistance of cancer cells to anti-cancer drugs. See ampR, chloro- quine, dichlorodiphenyltrichloroethane (DDT), meth- othrexate, penicillin, pfcrt gene. drug-resistant plasmid See R plasmid. drumstick a small protrusion from the nucleus of the human polymorphonuclear leukocyte, found in 3 to 5% of these cells in females, but not in males.
See Barr body. drupe a simple, fleshy fruit, such as an olive, de- rived from a single carpel and usually single-seeded.
Dryopithecus a genus of fossil primates from which the great apes and humans are thought to have diverged about 25 million years ago. Dscam the gene in Drosophila that encodes a ho- molog of the Down syndrome cell adhesion mole- cule (DSCAM) (q.v.). This protein plays a role in directing axons to the embryonic cells that will form Bolwig organs (q.v.). See innate immunity. dsDNA double-standed DNA. dsRNA double-stranded RNA (q.v.).
D1 trisomy syndrome See Patau syndrome. dual recognition an immunological model pro- posing that a T cell has two receptors, both of which must simultaneously bind specific molecules in order to activate the cell; one receptor binds to the anti- gen, the other binds to a self-molecule of the major histocompatibility system (q.v.); a form of associa- tive recognition (q.v.). Duchenne muscular dystrophy (DMD) a disease named after G. A. B.
Duchenne, who in 1868 pub- lished the first histological account of the pathologi- cal changes occurring in the muscles of patients. DMD, the most common type in humans, is X- linked, and it affects about 1 in 3,500 boys. The nor- mal gene, DMD, is composed of about 2,300 kilo- bases. It is the largest known gene and contains 79 exons. Over 99% of the gene is made up of introns. It takes RNA polymerase II 16 hours to traverse this giant gene. The processed mRNA is about 14 kilo- bases, and it specifies a protein named dystrophin (q.v.). Victims of Duchenne muscular dystrophy have null mutations in their dystrophin genes.
Less severe mutations allow reduced amounts of dys- trophin to be produced, resulting in a milder disease called Becker muscular dystrophy (q.v.). There is a mutational hot spot between exons 44 and 45. Or- thologs of DMD occur in both dogs and cats. See Appendix C, Hoffman, Brown, and Kunkel; Canis fa- miliaris, calveolins, continuous gene syndrome, hot spot, null allele, RNA polymerase. http://www. mdausa.org.80.
Duffy blood group gene the first human genetic locus to be localized on a specific autosome. The gene (symbolized FY) is at 1q21-22, and its nucleo- tide sequence has been determined. The gene en- codes a protein that contains 338 amino acids orga- nized into several transmembrane domains.
The protein functions as a receptor for various cytokines (q.v.) and for Plasmodium vivax merozoites. Individ- uals homozygous for mutations that repress the tran- scription of Duffy gene in erythrocytes resist the in- vasion of these malaria parasites. Practically all West Africans are Duffy negative.
The blood group was named in 1950 after the patient whose blood con- tained antibodies against the FY gene product. See Appendix C, 1968, Donahue et al.; 1976, Miller etal.; Chaudhuri et al.; G proteins, malaria, Mendelian Inheritance in Man (MIM), Plasmodium life cycle. Duffy blood group receptor a soluble protein syn- thesized by Plasmodium vivax that binds to the Duffy receptor on human erythrocytes. The recep- tor protein contains 1,115 amino acids, and it is en- coded by a single-copy gene that has been cloned. See Appendix C, 1990, Fang et al. duplex See autotetraploid.
duplex DNA DNA molecules as described in the Watson-Crick model; that is, with the two poly- nucleotide chains of opposite 3′-5′ polarity inter- twined and annealed. duplication See chromosome aberration, gene du- plication, repeat-induced point mutation (RIP). durum wheat Triticum durum (N = 14), an ancient wheat grown since Egyptian times. Used today in the manufacture of macaroni. See wheat. dwarf a short person with abnormal proportions, usually suffering from achondroplasia (q.v.).
Com- pare with ateliosis, Laron dwarfism, midget, pituitary dwarfism. dyad 1. a pair of sister chromatids (q.v.). 2. the products resulting from disjunction of tetrads at the first meiotic division; dyads are contained in the nu- clei of secondary gametocytes. See meiosis. dynamins a family of proteins that function in the pinching off of vesicles. Specific dynamins are also recruited for the division rings of mitochondria and chroroplasts. dynein a complex protein, generally composed of two subunits, found in the armlike extensions of the A microtubules of the nine peripheral doublets of axonemes (q.v.).
Dynein functions to couple a cycle of ATP binding and hydrolysis with a mechanical binding and release cycle to generate a sliding in re- lation to one another of the microtubule doublets in axonemes. Cytoplasmic dyneins also exist and are thought to function in the translocation of organ- elles. See Appendix C, 1963, Gibbons.
dysgenesis any morphological or biochemical ab- normality that is evident at birth or during neonatal development. dysplasia the abnormal growth or development of a tissue or organ. Subsequent microscopic study of- ten shows that the component cells have abnormali- ties, but are not cancerous.
dysploidy the situation where the species in a genus have different diploid chromosome num- bers, but the numbers do not represent a polyploid series. dystrophin a protein encoded by the Duchenne muscular dystrophy (DMD) gene. This molecule contains 3,685 amino acids, and it only represents 0.002% by weight of muscle. Dystrophin is located at the cytoplasmic faces of the plasma membranes of striated, smooth, and cardiac muscle cells.
It binds cytoskeletal actin at its N-terminal end and a trans- membrane glycoprotein at its C-terminal end. A number of shorter isoforms of dystrophin are local- ized in brain liver and other tissues. These proteins are translated from mRNAs transcribed from pro- moters that reside in the DMD gene at positions to the right of the one specific for muscle transcripts. Other isoforms are the result of alternative splicing (q.v.). See gene, isoforms, muscular dystrophy, utro- phin.
E1, E2, E3 the first, second, and third generation of organisms following some experimental manipula- tion, such as irradiation with x-rays. early genes genes expressed during early develop- ment. In T4 bacteriophage, those genetic elements that function in the period of phage infection before the start of DNA replication.
Compare with late genes. ecdysis 1. molting, the periodic shedding of the cuticle of arthropods. Ecdysis is of great antiquity, since fossil trilobites (q.v.) in exuvial condition have been collected from Cambrian strata. 2. in dinoflag- ellates, the shedding of the thecal wall layer. ecdysones steriod hormones produced by the pro- thoracic gland (q.v.) of insects, required for molting and puparium formation; also called molting hor-
mones. The structural formulas for alpha and beta ecdysone are illustrated. Beta ecdysone is sometimes called 20-hydroxy-ecdysone (20 HE). The precursor of alpha ecdysone is dietary cholesterol (q.v.).
The alpha ecdysone synthesized by the prothoracic gland is converted in peripheral tissues, such as the fat body, to beta ecdysone, which is the active molting hormone. The conversion involves the addition of a hydroxyl group at the position indicated by the arrow.
The relative concentrations of ecdysone and allatum hormone (q.v.) determine whether juvenile or adult development will follow a given molt. Tem- perature-sensitive ecdysone mutants are known for Drosophila melanogaster. See Appendix C, 1965, Karl- son et al.; 1980, Gronemeyer and Pongs; ring gland.
Ecdysozoa a subdivision of the Articulata that contains animals that grow by periodically molting. It represents the largest clade in the animal kingdom, since it contains the Annelid and Arthropod phyla. The small phyla Onycophora and Tardigrada also