d 1. dextrorotatory. 2. the dalton unit. 2,4D 2,4 dichlorophenoxyacetic acid (q.v.). daf-2 a gene in Caenorhabditis that regulates its life span. See insulin-like growth factors 1 and 2 (IGF- 1 and IGF-2). dalton a unit equal to the mass of the hydrogen atom (1.67 × 10−24 g) and equal to 1.0000 on the atomic mass scale. The unit is named after John Dal- ton (1766-1844), who developed the atomic theory of matter. Abbreviated Da. daltonism See color blindness.
dam the female parent in animal breeding. Com- pare with sire. Danaus plexippus the Monarch butterfly. See au- tomimicry. Danio rerio the fish that has become a model or- ganism for the genetic study of vertebrate develop- ment. The fish has a 3-month life cycle and produces large, transparent embryos. Large-scale mutagenesis experiments have generated a wealth of mutations that produce a dazzling array of abnormal pheno- types.
The genome contains about 1,700 mbp of DNA distributed among 25 chromosomes. See Ap- pendix A, Chordata, Osteichthyes, Neopterygii, Cyprinidontiformes; Appendix C, 1993, Mullins and Nu¨sslein-Volhard; Appendix E. DAPI 4′,6-diamidino-2-phenylindole, a fluorescent dye that binds to DNA. DAPI-staining of chromo- somes within nuclei can be followed with the collec- tion of three dimensional data sets obtained by re- cording serial images at these, linearized maps of all the chromosomes can be constructed. The structure of the DAPI molecule is shown below.
dark-field microscope a microscope designed so that the entering center light rays are blacked out and the peripheral rays are directed against the ob- ject from the side. As a result, the object being viewed appears bright upon a dark background. dark reactivation repair of mutagen-induced ge- netic damage by enzymes that do not require light photons for their action. See photoreactivating en- zyme. Darwinian evolution See Darwinism. Darwinian fitness synonymous with adaptive value (q.v.).
Darwinian selection synonymous with natural se- lection (q.v.). Darwinism the theory that the mechanism of bio- logical evolution involves natural selection of adap- tive variations. See gradualism, Origin of Species. Darwin on the Web the most extensive collection of Darwin’s writings (http://pages.britishlibrary.net/ charles.darwin/).
Darwin’s finches a group of finches observed and collected by Charles Darwin during his visit to the Galapagos Islands in 1835. Birds of all 14 species are seed eaters, but they are subdivided into one genus of ground finches (Geospiza) and two genera of tree finches (Camarhynchus and Cactospiza). The species differ in beak morphology, coloration of plumage, size, and habitat preferences.
Darwin was the first to suggest that the modern populations of these birds are the end product of an adaptive radiation from a single ancestral species. The evolutionary divergences resulted from adaptations that allowed different pop- ulations to utilize different food sources on different islands and to avoid competition. This adaptive radia- tion occurred in less than 3 million years.
Recent DNA analyses suggest that the ancestor to Darwin’s finches was phenotypically similar to a warbler finch, Certhidea olivacea, that currently inhabits many of the islands. See Appendix C, 1835, Darwin; 1947, Lack; 1999, Petren, Grant, and Grant. Dasypus a genus of armadillos that contains six species, all of which are always polyembryonic, pro- ducing four genetically identical offspring per litter. The nine-banded armadillo, Dasypus novemcinctus, is the most studied species. See cloning.
Datura stramonium the Jimson weed, a species belonging to the nightshade family of plants. It is found all over North America as a roadside weed. The plant is dangerous to eat, since it synthesizes a variety of toxic and hallucinogenic alkaloids (q.v.). D. stramonium has 12 pairs of chromosomes. A set of trisomics was developed, each with a different chromosome in triplicate. Each primary trisomic dif- fered from normal and from each other in character- istic ways.
This suggested that each chromosome contained genes with morphogenetic effects and that the abnormal phenotype that characterized each trisomic was the result of increases in the rela- tive doses of these genes.
See Appendix A, Plantae, Angiospermae, Dicotyledonae, Solanales; Appendix C, 1920, Blakeslee, Belling, and Farnham; aneu- ploidy, haploid sporophytes, haploidy, polyploidy. dauermodification an environmentally induced phenotypic change in a cell that survives in the gen- erative or vegetative descendants of the cell in the absence of the original stimulus. However, with time the trait weakens and eventually disappears. daughter cells (nuclei) the two cells (nuclei) re- sulting from division of a single cell (nucleus).
Pref- erably called sibling or offspring cells (nuclei). day-neutral referring to a plant in which flowering is not controlled by photoperiod. See phytochrome. DBM paper diazobenzyloxymethyl paper that binds all single-stranded DNA, RNA, and proteins by means of covalent linkages to the diazonium group; used in situations where nitrocellulose blot- ting is not technically feasible. See Appendix C, 1977, Alwine et al. DEAE-cellulose diethylaminoethyl-cellulose, a sub- stituted cellulose derivative used in bead form for chromatography of acidic or slightly basic proteins at pH values above their isoelectric point.
deamination the oxidative removal of NH2 groups from amino acids to form ammonia. decarboxylation the removal or loss of a carboxyl group from an organic compound and the formation of CO2. decay of variability the reduction of heterozygos- ity because of the loss and fixation of alleles at vari- ous loci accompanying genetic drift. deciduous 1. designating trees whose leaves fall off at the end of the growing season, as opposed to evergreen. 2. designating teeth that are replaced by permanent teeth. decoy protein See sporozoite.
dedifferentiation the loss of differentiation, as in the vertebrate limb stump during formation of a blastema. In the regenerating mammalian liver, cells undergo partial dedifferentiation, allowing them to reenter, the cell cycle while maintaining all critical differentiation functions. See differentiation, regen- eration. defective virus a virus that is unable to reproduce in its host without the presence of another “helper” virus (q.v.). deficiency in cytogenetics, the loss of a microscop- ically visible segment of a chromosome. In a struc- tural heterozygote (containing one normal and one deleted chromosome), the nondeleted chromosome forms an unpaired loop opposite the deleted seg- ment when the chromosomes pair during meiosis.
See Appendix C, 1917, Bridges; cat cry sydrome. deficiency loop in polytene chromosomes, defi- ciency loops allow one to determine the size of the segment missing. The illustration on page 115 shows a portion of the X chromosome from the nucleus of a salivary gland cell of a Drosophila larva structurally heterozygous for a deficiency. Note that bands C2- C11 are missing from the lower chromosome. defined medium a medium for growing cells, tis- sues, or multicellular organisms in which all the chemical components and their concentrations are known.
definitive host the host in which a parasite attains sexual maturity. deformylase an enzyme in prokaryotes that re- moves the formyl group from the N-terminal amino acid; fMet is never retained as the N-terminal amino acid in functional polypeptides. See start codon. degenerate code one in which each different word is coded by a variety of symbols or groups of letters. The genetic code is said to be degenerate be- cause more than one nucleotide triplet codes for the same amino acid. For example, the mRNA triplets GGU, GGC, GGA, and GGG all encode glycine.
When two codons share the same first two nucleo- tides they will encode the same amino acids if the third nucleotide is either U or C and often if it is A or G. See amino acids, codon bias, genetic code, wob- ble hypothesis. degrees of freedom the number of items of data that are free to vary independently. In a set of quan- titative data, for a specified value of the mean, only (n − 1) items are free to vary, since the value of the
nth item is then determined by the values assumed by the others and by the mean. In a chi-square test (q.v.) the number of degrees of freedom is one less than the number of phenotypic classes observed. dehiscent designating fruit that opens when ripe to release seeds. Deinococcus radiodurans a Gram-positive red- pigmented, nonmotile, aerobic bacterium that is ex- tremely resistant to a number of agents that damage DNA (ionizing radiation, ultraviolet radiation, and hydrogen peroxide). D. radiodurans can tolerate 3 million rads of ionizing radiation (the human lethal dose is about 500 rads). The D. radiodurans genome is composed of four circular molecules:
chromosome 1 (2,649 kb), chromosome 2 (412 kb), a megaplas- mid (177 kb), and a plasmid (46 kb). The genome contains 3,187 ORFs, with an average size of 937 kb, and these occupy 91% of the genome. The spe- cies possesses a highly efficient DNA repair system that involves about 40 genes, many of which are present in multiple copies. See Appendix A, Bacteria, Deinococci; Appendix C, 1999, White et al.; Appen- dix E; haploidy. delayed dominance See dominance.
delayed hypersensitivity a cell-mediated immune response manifested by an inflammatory skin re- sponse 24-48 hours after exposure to antigen. Com- pare with immediate hypersensitivity. delayed Mendelian segregation See Lymnaea peregra. deletion the loss of a segment of the genetic mate- rial from a chromosome. The size of the deletion can vary from a single nucleotide to sections containing a number of genes. If the lost part is at the end of a chromosome, it is called a terminal deletion. Other- wise, it is called an intercalary deletion. See indels.
deletion mapping tions to localize the position of an unknown gene on a chromosome or linkage map. 2. the establishment of gene order among several phage loci by a series of matings between point mutation and deletion mu- tants whose overlapping pattern is known. Recombi- nants cannot be produced by crossing a strain bear- ing a point mutant with another strain carrying a deletion in the region where the point mutant re- sides. See Appendix C, 1938, Slizynska; 1968, Davis and Davidson. deletion method a method of isolating specific messenger RNA molecules by hybridization with DNA molecules containing genetic deletions. deletion-substitution particles a specialized trans- ducing phage in which deleted phage genes are sub- stituted by bacterial genes. Delta the capital Greek letter (∆) used in molecu- lar biology to indicate a deletion of one or more amino acids in a polypeptide chain. See cystic fibrosis (CF).