adaptive radiation the evolution of specialized species, each of which shows adaptations to a dis- tinctive mode of life, from a generalized ancestral species. Darwin observed the adaptive radiation of finch species on the Galapagos islands.
The Hawai- ian archipelago shows perhaps the most spectacular examples of adaptive radiations. See Darwin’s fin- ches, Hawaiian Drosophilidae, silversword alliance. adaptive surface, adaptive topography synonyms for adaptive landscape (q.v.).
adaptive value the property of a given genotype when compared with other genotypes that confers fitness (q.v.) to an organism in a given environment.
adaptor a short, synthetic DNA segment contain-ing a restriction site that is coupled to both ends of a blunt-ended restriction fragment. The adaptor is used to join one molecule with blunt ends to a sec-ond molecule with cohesive ends. The restriction site of the adaptor is made identical to that of the other molecule so that when cleaved by the same restriction enzyme both DNAs will contain mutu- ally complementary cohesive ends.
adaptor hypothesis the proposal that polynucleo-tide adaptor molecules exist that can recognize spe-cific amino acids and also the regions of the RNA templates that specify the placement of amino acids in a newly forming polypeptide. See Appendix C, 1958, Crick; transfer RNA.
ADCC antibody-dependent cellular cytotoxicity; also known as antibody-dependent cell-mediated cy-totoxicity. Cell-mediated cytotoxicity requires prior binding of antibody to target cells for killing to oc- cur. It does not involve the complement cascade. See K cells.
additive factor one of a group of nonallelic genes affecting the same phenotypic characteristics and each enhancing the effect of the other in the pheno- type. See quantitative inheritance.
additive gene action 1. a form of allelic interac- tion in which dominance is absent; the heterozygote is intermediate in phenotype between homozygotes for the alternative alleles. 2. the cumulative contri- bution made by all loci (of the kind described above) to a polygenic trait.
additive genetic variance genetic variance attrib-uted to the average effects of substituting one allele for another at a given locus, or at the multiple loci governing a polygenic trait. It is this component of variance that allows prediction of the rate of re- sponse for selection of quantitative traits. See quanti- tative inheritance.
adducin a ubiquitously expressed protein found in the membranes of animal cells. Mammalian adducin is a heterodimeric protein whose subunits share se-quence similarities and contain protease-resistant N-terminal and protease-sensitive C-terminal domains.
Adducin has a high affinity for Ca2+/calmodulin and is a substrate for protein kinases. In vitro it causes actin filaments to form bundles and promotes spec-trin-actin associations in regions where cells contact one another. In Drosophila, a homolog of mamma-lian adducin is encoded by the hts gene. See calmod-ulin, fusome, hu-li tai shao (hts), heterodimer, protein kinase, spectrosome.
adduct the product of a chemical reaction that re-sults in the addition of a small chemical group to a relatively large recipient molecule. Thus the alkylat-ing agent ethyl methane sulfonate (q.v.) can add ethyl groups to the guanine molecules of DNA. These ethylated guanines would be examples of DNA adducts.
adenine See bases of nucleic acids.
adenine deoxyriboside See nucleoside.
adenohypophysis the anterior, intermediate, and tuberal portions of the hypophysis, which originate from the buccal lining in the embryo.
adenohypophysis hormone See growth hormone.
adenosine See nucleoside.
adenosine deaminase deficiency a rare immune deficiency disease due to mutations in a gene located on the long arm of human chromosome 20. The nor-mal gene encodes an enzyme that controls the me-tabolism of purines, and ADA deficiency impairs the functioning of white blood cells. The division of T cells is depressed, and antibody production by B cells is reduced. As a result, ADA-deficient children die from viral, bacterial, and fungal infections.
deficiency is the first hereditary disease to be suc-cessfully treated by gene therapy. See Appendix C, 1990, Anderson; immune response.
adenosine phosphate any of three compounds in which the nucleoside adenosine is attached through its ribose group to one, two, or three phosphoric acid molecules, as illustrated here. AMP, ADP, and ATP are interconvertible. ATP upon hydrolysis yields the energy used to drive a multitude of biolog- ical processes (muscle contraction, photosynthesis, bioluminescence, and the biosynthesis of proteins, nucleic acids, polysaccharides and lipids). The most important process in human nutrition is the synthe- sis of ATP. Every day human beings synthesize, breakdown, and resynthesize an amount of ATP equaling their body weight. See Appendix C, 1929;
Lohmann; ATPase, ATP synthase, cellular respiration, citric acid cycle, cytochromes, electron transport chain, glycolysis, mitochondria, oxidative phosphorylation, mitochondrial proton transport.
adenovirus any of a group of spherical DNA vi- ruses characterized by a shell containing 252 capso- meres. Adenoviruses infect a number of mammalian species including humans. See human adenovirus 2 (HAdV-2), virus.
adenylcyclase the enzyme that catalyzes the con-version of ATP into cyclic AMP (q.v.). Also called adenylate cyclase.
See adenosine phosphate.
adenylic acid See nucleotide.
ADH the abbreviation for alcohol dehydrogenase (q.v.).
adhesion plaques See vincullin.
adhesive molecules any pair of complementary cell-surface molecules that bind specifically to one
another, thereby causing cells to adhere to one an- other, as do carbohydrates and protein lectins (q.v.).
Phenomena dependent on adhesive molecules in- clude invasion of host cells by bacteria and viruses, species-specific union of sperms and eggs, and aggre-gation of specific cell types during embryological de- velopment. See cell affinity, hemagglutinins, P blood group, selectins.
adjacent disjunction, adjacent segregation See translocation heterozygote.
adjuvant a mixture injected together with an anti-gen that serves to intensify unspecifically the im- mune response. See Freund’s adjuvant.
adoptive immunity the transfer of an immune function from one organism to another through the transfer of immunologically active or competent cells. Also called adoptive transfer.