developmental homeostasis canalization (q.v.). developmental homology anatomical similarity due to derivation from a common embryological source; e.g., the halteres of flies are developmentally homologous to the hind wings of moths. deviation the departure of a quantity (derived from one or more observations) from its expected value (usually the mean of a series of quantities). Devonian the Paleozoic period during which car- tilagenous and bony fishes evolved.
On land, lyco- pods, sphenophytes, and ferns were the abundant plants and amphibians and wingless insects the most common animals. A mass extinction occurred late in the period. See geologic time divisions. dex dextrorotatory. See optical isomers. dextran a polysaccharide (composed of repeating D-glucose subunits) synthesized by certain lactic acid bacteria. dextrose glucose (q.v.). df, d.f., D/F degrees of freedom (q.v.). DHFR dihydrofolate reductase (q.v.).
See amplicon. diabetes insipidus (DI) excessive excretion of normal urine; brought about because of inadequate output of vasopressin (q.v.) or its receptor. In hu- mans, autosomal dominant DI is caused by muta- tions in a gene that encodes the vasopressin precur- sor protein. DI inherited as an X-linked recessive is due to mutations in a gene that encodes a vasopres- sin receptor. This belongs to the family of G pro- tein-coupled receptors. See aquaporins, G proteins. diabetes mellitus a disease in humans marked by glucose intolerance.
It exists in two forms: type 1, insulin-dependent diabetes mellitus (IDDM), and type 2, non-insulin-dependent diabetes mellitus (NIDDM). Since type 1 diabetes usually occurs be- fore age 20, it is often called “juvenile-onset diabe- tes.” It is usually caused by the autoimmune destruc- tion of the beta cells of the pancreas, which secrete insulin (q.v.). Type 1 diabetes can also result from mutations in the coding region of the insulin gene and from variations in the number of tandem repeats of a segment containing 14 to 15 nucleotides that resides upstream of the coding region. This region may regulate the rate of transcription of insulin mRNA.
Since type 2 diabetes usually begins be- tween the ages of 40 and 60, it is often called “matu- rity-onset diabetes.” Genes on at least 10 different chromosomes have been identified that increase sus- ceptibility to NIDDM. This disease is far more com- mon than IDDM, and its prevalence is rising in af- fluent societies throughout the world where people get little exercise, overeat, and tend to become obese. J. V. Neel’s “thrifty gene hypothesis” (q.v.) provides an evolutionary explanation for the pres- ence in human populations of genes that predispose their bearers to type 2 diabetes. See Appendix C, 1962, Neel; obese. diakinesis See meiosis.
diallelic referring to a polyploid in which two dif- ferent alleles exist at a given locus. In a tetraploid, A1A1A2A2 and A1A2A2A2 would be examples. dialysis the separation of molecules of differing size from a mixture by their differential diffusibility through a porous membrane. In the procedure knowns as equilibrium dialysis, soluble molecules of the same size are allowed to reach equivalent con- centrations on either side of a semipermeable mem- brane.
At equilibrium, if more molecules are de- tected on one side of the membrane, it indicates that they have become bound to some other larger mole- cules (e.g., repressor proteins, transport proteins, an- tibodies, etc.) present only on that side of the mem- brane, and thus are too large to pass through the pores of the membrane. This procedure is also used in immunology as a method of determining associa- tion constants for hapten-antibody reactions. 2,6-diaminopurine a mutagenically active purine analog. See bases of nucleic acids.
diapause a period of inactivity and suspension of growth in insects accompanied by a greatly decreased metabolism. In a given species, diapause usually takes place in a specific stage in the life cycle, and it often provides a means of surviving the winter.
diaspora the dispersion of an originally homoge- neous group of people from their homeland into for- eign territories. Also, the people who have dispersed from their homelands (e.g., those Jews who live in communities outside the biblical land of Israel). diasteromer epimer (q.v.).
diauxy the adaptation of a microorganism to cul- ture media containing two different sugars. The or- ganism possesses constitutive enzymes for one of the
sugars, which it utilizes immediately. Induced en- zyme synthesis is required before the second sugar can be metabolized. dicentric designating a chromosome or chromatid having two centromeres. Dicer a nuclease (q.v.) that processes endogenous or exogenous double-stranded RNA (dsRNA) (q.v.) precursors to small interfering RNAs RNAs (q.v.).
The Dicer protein is evolutionarily conserved and is found in fungi, plants, worms, flies, and humans. The enzyme structure includes a heli- case domain, domains related to the bacterial dsRNA-specific endonuclease, RNase III, and RNA- binding domains. Inactivation of Dicer in vertebrates results in the cessation of microRNA (q.v.) produc- tion, leading to early developmental arrest or lethal- ity. In Caenorhabditis elegans inactivation of the Dicer gene causes developmental timing defects. See RNA interference (RNAi). dichlorodiphenyltrichloroethane (DDT) an insec- ticide to which many insect species have developed resistant races.
2,6-dichlorophenoxyacetic acid an antiauxin (q.v.). dichogamous referring to flowers or hermaphro- ditic animals characterized by male and female sex organs that become mature at different times. dichroism See circular dichroism. Dicotyledoneae one of the two classes of flower- ing plants (see Appendix A, Kingdom 5, Plantae). The seeds of all dicots produce two primary leaves. See cotyledon, Monocotyledoneae.
dictyosome 1. a synonym for Golgi apparatus (q.v.). 2. one of the flattened vesicles that make up the Golgi apparatus. Most eukaryotes contain a Golgi of stacked dictyosomes, but fungal cells gener- ally contain dispersed dictyosomes.
Dictyostelium discoideum a protoctist that has the ability to alternate between unicellular and multicellular life-styles. Individual Dictyostelium amoebas live in forest soil and eat bacteria and yeasts. However, when challenged by adverse condi- tions, such as starvation, groups of up to 100,000 cells signal each other by secreting acrasin (q.v.). This chemical attractant causes the amoebas to ag- gregate, forming a motile slug that is surrounded by a slimy extracellular matrix.
At the apex of the mound, a fruiting body that produces spores differ- entiates. Dictyostelids are placed in the phylum Acrasiomycota (q.v.) and go by common names such as slime molds, social amoebas, or amoebozoans.
They represent one of the earliest branches from the last common ancestor of all eukaryotes. Slime molds diverged after the split between the plants and opis- thokonts (q.v.), but before the split of the fungi and animals. Therefore the slime molds, fungi, and meta- zoa are sister groups. D. discoidium has a genome size of 34 mb of DNA distributed among six chro- mosomes. The number of genes is about 12,500, and many of these have orthologs among the genes of opisthokonts. For example, there are 64 genes that are orthologs of human disease genes, such as Tay- Sachs, G6PD deficiency, and cystic fibrosis.
The Dic- tyostelium genome contains genes that encode cell adhesion and signaling molecules (normally exclu- sive to animals) and genes that encode proteins con- trolling cellulose deposition and metabolism (nor- mally exclusive to plants). See Appendix C, 2005, Eichinger et al. dictyotene stage a prolonged diplotene stage of meiosis seen in oocytes during vitellogenesis. The chromosomes that have already undergone crossing over may remain in this stage for months or even years in long-lived species. 2′,3′-dideoxynucleoside triphosphates analogs of normal 2′-deoxyribonucleoside triphosphates used in a modified “minus” technique for base sequencing of DNA molecules.
Because these analogs have no oxygen at the 3′ position in the sugar, they act as specific chain-terminators (q.v.) for primed synthesis techniques (see DNA sequencing techniques). Nucle- otides in which arabinose is substituted for deoxyri- bose also exhibit this chain-terminating effect. differential affinity the failure of two partially ho- mologous chromosomes to pair during meiosis when
Differential gene expression
a third chromosome is present that is more com- pletely homologous to one of the two. In its absence, however, pairing of the partially homologous chro- mosomes can occur. See autosyndesis, homoeolo- gous chromosomes. differential gene expression the principle that all the cells of a multicellular organism have the same genetic content, but differ from one another in the sets of genes that they express.
differential segment See pairing segment. differential splicing See alternative splicing. differentiation the complex of changes involved in the progressive diversification of the structure and functioning of the cells of an organism.
For a given line of cells, differentiation results in a continual re- striction of the types of transcription that each cell can undertake. See development, morphogenesis. Compare with dedifferentiation, determination. differentiation antigen a cell-surface antigen that is expressed only during a specific period of embryo- logical differentiation. diffuse centromere (kinetochore) See centromere. diffusion the tendency for molecules because of their random heat motion to move in the direction of a lesser concentration, and so make the concentra- tion uniform throughout the system.
digenetic descriptive of organisms of the subclass Digenea of the class Trematoda within the flatworm phylum Platyhelminthes. The term means “two be- ginnings,” referring to a life cycle with alternation of generations, one parasitic and the other free-living. Digenea is the largest group of trematodes and the most important medically and economically.
All members are endoparasitic with two or more hosts in the life cycle, the first host usually being a mol- lusc. The digenetic flukes include blood flukes and schistosomes that are generally considered to be the most serious helminthic human parasite.
See Appen- dix A; schistosomiasis. dihaploid a diploid cell, tissue, or organism having arisen from a haploid cell by chromosome doubling. dihybrid a genotype characterized by heterozygos- ity at two loci. Mendel found that crosses between pure lines of peas that differed with respect to two unrelated traits produced genetically uniform F1 di- hybrid offspring. Intercrossing F1 dihybrids produced parental and recombinant types in the F2 population. dihydrofolate reductase (DHFR) an enzyme es- sential for de novo thymidylate synthesis.
It regener- ates an intermediate (tetrahydrofolate) in thymidylate synthesis and is also essential for other biosynthetic events that depend on tetrahydrofolate, such as the synthesis of purines, histidine, and methionine. See amplicon, folic acid. dihydrouridine See rare bases. 2,5-dihydroxyphenylacetic acid homogentisic ac- id (q.v.). dimer a chemical entity consisting of an associa- tion of two monomeric subunits; e.g., the association of two polypeptide chains in a functional enzyme. If the two subunits are identical, they form a homodi- mer; if nonidentical, they form a heterodimer.
Hex- osaminidase (q.v.) is an example of a heterodimeric enzyme. dimethylguanosine See rare bases. dimethyl sulfate protection a method for identi- fying specific points of contact between a protein (such as RNA polymerase) and DNA based on the principle that, within an endonuclease-protected re- gion (see DNAase protection), the adenines and gua- nines in the site of contact are not available to be methylated by exposure to dimethyl sulfate.
dimorphism the phenomenon of morphological differences that split a species into two groups, as in the sexual dimorphic traits distinguishing males from females. dinitrophenol (DNP) a metabolic poison that pre- vents the uptake of inorganic phosphate and the production of energy-rich phosphorus compounds like ATP. DNP is a commonly used hapten in im- munological experiments.
dioecious having staminate or pistillate flowers on separate unisexual plants. Compare with monoe- cious. See flower. diphtheria toxin a protein produced by certain ly- sogenic strains of Corynebacterium diphtheriae that is responsible for the symptoms of diphtheria. The structural gene for the toxin is carried by certain bacteriophages (e.g., corynephages beta, omega, and