from one chromosome and then insert into another. The excision of Ac may cause a break in the chro- mosome, and this is what generated the breakage- fusion-bridge cycles that McClintock observed. Ds is a defective transpon that contains a deletion in its transposase locus. Therefore the Ds transposon can move from chromosome to chromosome only if Ac is also in the nucleus to supply its transposase. Ac and Ds were originally classified as mutator genes, since they would sometimes insert into structural genes and modify their functioning. See Appendix C, 1950, McClintock; 1984, Pohlman et al.; Dotted, ge- nomic instability, mutator gene, terminal inverted re- peats (TIRs), transposon tagging.
active center in the case of enzymes, a flexible portion of the protein that binds to the substrate and converts it into the reaction product. In the case of carrier and receptor proteins, the active center is the portion of the molecule that interacts with the spe- cific target compounds. active immunity immunity conferred on an organ- ism by its own exposure and response to antigen. In the case of immunity to disease-causing agents, the antigenic pathogens may be administered in a dead or attenuated form. See also passive immunity.
active site that portion(s) of a protein that must be maintained in a specific shape and amino acid content to be functional. Examples: 1. in an enzyme, the substrate-binding region; 2. in histones or repres- sors, the parts that bind to DNA; 3. in an antibody, the part that binds antigen; 4. in a hormone, the por- tion that recognizes the cell receptor.
active transport the movement of an ion or mole- cule across a cell membrane against a concentration or electrochemical gradient. The process requires specific enzymes and energy supplied by ATP.
activin a protein first isolated from the culture fluid of Xenopus cell lines. Activin is a member of the transforming growth factor-β (q.v.) family of in- tercellular signaling molecules. It acts as a diffusible morphogen for mesodermal structures, and the type of differentiation is determined by the concentration of actin (i.e., high concentrations produce head structures, low concentrations tail structures).
actomyosin See myosin.
acute myeloid leukemia 1 gene (AML1) a gene that maps to 21q22.3 and is one of the most fre- quent targets of chromosome translocations associ- ated with leukemia. The involvement of AML1 with the oncogenic transformation of blood cells is worth noting, since acute myeloid leukemia is hundreds of
times more common in children with trisomy 21 than in other children. See Down syndrome, lozenge, myeloproliferative disease.
acute transfection infection of cells with DNA for a short period of time.
acylated tRNA a transfer RNA molecule to which an amino acid is covalently attached. Also referred to as an activated tRNA, a charged tRNA, or a loaded tRNA.
adaptation 1. the process by which organisms un- dergo modification so as to function more perfectly in a given environment. 2. any developmental, be- havioral, anatomical, or physiological characteristic of an organism that, in its environment, improves its chances for survival and of leaving descendants.
adaptive enzyme an enzyme that is formed by an organism in response to an outside stimulus. The term has been replaced by the term inducible en- zyme. The discovery of adaptive enzymes led even- tually to the elucidation of the mechanisms that switch gene transcription on and off. See Appendix C, 1937, Karstrom; regulator gene.
adaptive immunity the immunity that develops in response to an antigens (q.v.), as opposed to innate or natural immunity. Contrast with innate immunity.
adaptive landscape a three-dimensional graph that shows the frequencies of two genes, each present in two allelic forms (aA and bB in the illustration) plot- ted against average fitness for a given set of environ- mental conditions, or a comparable conceptual plot in multidimensional space to accommodate more than two loci.
adaptive melanism hereditary changes in melanin production that cause the darkening in color of pop- ulations of animals in darkened surroundings. By im- proving their camouflage, this makes them less con- spicuous to predators. For example, desert mice are preyed upon by owls, hawks, and foxes.
The mice that live among sand and light-colored rocks are tan and blend in well with their surroundings. However, the fur from populations of the same species that live among outcrops of dark, ancient lava flows is much darker. See Chaetodipus intermedius.
adaptive norm the array of genotypes (compatible with the demands of the environment) possessed by a given population of a species.
adaptive peak a high point (perhaps one of sev- eral) on an adaptive landscape (q.v.), from which movement in any planar direction (changed gene frequencies) results in lower average fitness.