double cross the technique used for producing hybrid seed for field corn. Four different inbred lines (A, B, C, and D) are used. A × B → AB hybrid and C×D→CD hybrid. The single-cross hybrids (AB and CD) are then crossed and the ABCD seed is used for the commercial crop.
double crossover See double exchange. double diffusion technique synonymous with Ouchterlony technique (q.v.). double exchange breakage and interchange occur- ring twice within a tetrad involving two, three, or four of the chromatids. double fertilization a type of fertilization that dis- tinguishes flowering plants from other seed plants. As shown in the illustration, the pollen grain upon
germination forms a male gametophyte that injects two sickle-shaped haploid sperm nuclei into the fe- male gametophyte. The egg nucleus and the polar nuclei are haploid and genetically identical. The union of one sperm nucleus with the egg nucleus produces a diploid nucleus from which the embryo develops. The two polar nuclei fuse with the other sperm nucleus to form a triploid nucleus. The endo- sperm (q.v.) develops by the mitotic activity of this 3N nucleus. See Appendix A, Plantae, Pteropsida, Angiospermae; Appendix C, 1898, Navashin; kernel, pollen grain, synergid.
double haploids plants that are completely homo- zygous at all gene loci, generated when haploid germ cells, grown in tissue culture, double their chromo- some sets. See anther culture. double helix the Watson-Crick model of DNA structure, involving plectonemic coiling (q.v.) of two hydrogen-bonded polynucleotide, antiparallel (q.v.) strands wound into a right-handed spiral configura- tion.
See deoxyribonucleic acid. double infection infection of a bacterium with two genetically different phages. double-sieve mechanism a model that explains the rarity of misacylation of amino acids by propos- ing that an amino acid larger than the correct one is rarely activated because (1) it is too large to fit into the active site of the tRNA synthetase (first sieving), and (2) the hydrolytic site of the same synthetase is too small for the correct amino acid (second siev- ing). Thus, an amino acid smaller than the correct one can be removed by hydrolysis. double-stranded RNA (dsRNA) an RNA duplex in which a messenger RNA (q.v.) is bound to an anti- sense RNA (q.v.) containing a complementary se- quence of bases. Endogenous or exogenous dsRNAs provide a powerful means of silencing gene expres- sion. See Morpholinos, RNA interference (RNAi). double transformation See cotransformation.
double X in Drosophila melanogaster, an acrocen- tric, double-length X chromosome arising as a radia- tion-induced aberration. Such double X chromo- somes are superior to the ordinary metacentric, attached X chromosomes (q.v.) for most stockkeep- ing operations, since they do not break up by cross- ing over with the Y. See also detached X. doubling dose that dose of ionizing radiation that doubles the spontaneous mutation rate of the spe- cies under study.
doubling time the average time taken for the cell number in a population to double. The doubling time will equal the generation time (q.v.) only if (1) every cell in the population is capable of forming two daughter cells, (2) every cell has the same aver- age generation time, and (3) there is no lysis of cells. The doubling time is generally longer than the gen- eration time. Dowex trademark of a family of ion-exchange resins.
down promoter mutations promoter alterations that decrease the frequency with which transcrip- tion is initiated relative to wild type; promoters with this property are called low-level or weak pro- moters. downstream See strand terminologies, transcription unit. Compare with upstream. downstream genes genes whose expression is subordinate to developmental control genes (q.v.).
Downstream genes are switched on or off at various times and in different tissues by transcription factors (q.v.) encoded by upstream regulatory genes. A mas- ter controlling gene like eyeless (q.v.) will activate some downstream genes that encode their own tran- scription factors. The result will be a cascade of pro- teins that each regulate their own subsets of down- stream genes. See gene networking, Hox genes, selector genes.
Down syndrome a type of mental retardation due to trisomy of autosome 21. The syndrome is named after the British physician, John Langdon Down, who identified it in 1866. The underlying trisomy was discovered 107 years later. Since the eyelid openings of the patient are oblique and the inner corner of the eyelid may be covered by an epicanthic fold, the condition is sometimes called mongolism. The frequency of such trisomic births increases with advancing maternal age as shown in the table.
Although most patients with Down syndrome have a complete third copy of chromosome 21, the phe- notype is due primarily to a segment, the Down syn-drome chromosome region (DCR), between bands 21q 22.2 and 22.3. Within it lie five genes that seem critical to the syndrome. Chromosome 21 and chro- mosome 22 are similar in size, and both are acrocen- trics. Chromosome 21 contains 225 genes, aligned along a 33.8 Mb DNA molecule, whereas chromo- some 22 contains 545 genes on a 33.4 Mb DNA molecule. The relatively low density of genes on chromosome 21 is consistent with the observation that trisomy 21 is viable, while trisomy 22 is not.
See Appendix C, 1959, Lejeune, Gautier, and Turpin; 1968, Henderson and Edwards; 1999, Dunham et al.; 2000, Hattori et al.; Alzheimer disease; AML1 gene; translocation Down syndrome. http://www. nads.org. Down syndrome cell adhesion molecule (DSCAM) a protein encoded by a gene in band 22 of the long arm of human chromosome 21. The gene contains multiple exons which allow multiple mRNAs to be transcribed by alternative splicing (q.v.).
The tran- scripts are differentially expressed in different sub- structures of the adult brain. The DSCAM is a member of the immunoglobulin domain superfamily (q.v.). These isoforms may be involved in the pat- terning of neural networks by selective adhesions be- tween axons. See innate immunity.
DPN diphosphopyridine nucleotide. NAD is the preferred nomenclature. draft sequence in genome sequencing, a prelimi- nary DNA sequence that has enough accuracy and continuity to allow an initial genomic analysis and annotation, but that is incomplete. It is separated by small gaps of unknown sequence, and the order and orientation of all the sequenced fragments are not always fully determined. A draft sequence of the hu- man genome was published in 2001.
Compare with finished sequence. See Appendix C, 2001, The Inter- national Human Genome Sequencing Consortium, Venter et al.; genomic annotation. drift genetic drift (q.v.). dRNA DNA-like RNA. RNA molecules that are not included in the rRNA and tRNA classes. Much of the dRNA is of high molecular weight, short half- life, and never leaves the nucleus. See hnRNA. Drosophila a genus of flies containing about 900 described species.
The most extensively studied of all genera from the standpoint of genetics and cytol- ogy. The genus is subdivided into eight subgenera: (1) Hirtodrosophila, (2) Pholadoris, (3) Dorsilopha, (4) Phloridosa, (5) Siphlodora, (6) Sordophila, (7) So- phophora, and (8) Drosophila. D. melanogaster, the multicellular organism for which the most genetic.