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acriflavin an acridine dye that produces reading frame shifts (q.v.).

acritarchs spherical bodies thought to represent the earliest eukaryotic cells, estimated to begin in the fossil record about 1.6 billion years ago. Most acritarchs were probably thick-walled, cyst-forming protists. See Proterozoic.

acrocentric designating a chromosome or chroma-tid with a nearly terminal centromere. See telocentric chromosome.

acromycin See tetracycline.

acron the anterior nonsegmented portion of the embryonic arthropod that produces eyes and antennae. See maternal polarity mutants.

acrosome an apical organelle in the sperm head that is secreted by the Golgi material and that digests the egg coatings to permit fertilization.

acrostical hairs one or more rows of small bristles along the dorsal surface of the thorax of Drosophila.

acrosyndesis telomeric pairing by homologs during meiosis.

acrotrophic See meroistic. acrylamide See polyacrylamide gel.

ACTH adrenocorticotropic hormone (q.v.). actidione cycloheximide.

actin a protein that is the major constituent of the 7-nanometer-wide microfilaments of cells. Actin microfilaments (F actin) are polymers of a globular subunit (G actin) of Mr 42,000. Each G actin molecule has a defined polarity, and during polymerization the subunits align "head to tail," so that all G actins point in the same direction. F actin grows by the addition of G actin to its ends, and cytochalasin B (q.v.) inhibits this process. All the actins that have been studied, from sources as diverse as slime molds, fruit flies, and vertebrate muscle cells, are similar in size and amino acid sequence, suggesting that they evolved from a single ancestral gene. In mammals and birds, there are four different muscle actins. a is unique to skeletal muscle; a2, to cardiac muscle; a3, to smooth vascular muscle; and a4, to smooth enteric muscle. Two other actins (P and y) are found in the cytoplasm of both muscle and nonmuscle cells. See alternative splicing, contractile ring, fibro-nectin, hu-li tai shuo (hts), isoform, kelch, myosin, ring canals, spectrin, stress fibers, tropomyosin, vin-culin.

actin-binding proteins a large family of proteins that form complexes with actin. Such proteins include certain heat-shock proteins, dystrophin, myo-sin, spectrin, and tropomyosin (all of which see).

actin genes genes encoding the various isoforms of actin. In Drosophila, for example, actin genes have been localized at six different chromosomal sites. Two genes encode cytoplasmic actins, while the other four encode muscle actins. The amino acid-encoding segments of the different actin genes have very similar compositions, but the segments specifying the trailers (q.v.) differ considerably in nucleo-tide sequences.

Activator-Dissociation system 7

actinomycete any prokaryote placed in the phylum actinobacteria (see Appendix A). Actinomycetes belonging to the genus Streptomyces produce a large number of the antibiotics, of which actinomycin D (q.v.) is an example.

actinomycin D an antibiotic produced by Streptomyces chrysomallus that prevents the transcription of messenger RNA. See RNA polymerase.

activated macrophage a macrophage that has been stimulated (usually by a lymphokine) to enlarge, to increase its enzymatic content, and to increase its nonspecific phagocytic activity.

activating enzyme an enzyme that catalyzes a reaction involving ATP and a specific amino acid. The product is an activated complex that subsequently reacts with a specific transfer RNA.

activation analysis a method of extremely sensitive analysis based on the detection of characteristic radionuclides produced by neutron activation.

activation energy the energy required for a chemical reaction to proceed. Enzymes (q.v.) combine transiently with a reactant to produce a new complex that has a lower activation energy. Under these circumstances the reaction can take place at the prevailing temperature of the biological system. Once the product is formed, the enzyme is released unchanged.

activator a molecule that converts a repressor into a stimulator of operon transcription; e.g., the repres-sor of a bacterial arabinose operon becomes an activator when combined with the substrate.

Activator-Dissociation system a pair of interacting genetic elements in maize discovered and analyzed by Barbara McClintock. Ac is an autonomous element that is inherently unstable. It has the ability to excise itself from one chromosomal site and to transpose to another. Ac is detected by its activation of Ds. Ds is nonautonomous and is not capable of excision or transposition by itself. Ac need not be adjacent to Ds or even on the same chromosome in order to activate Ds. When Ds is so activated, it can alter the level of expression of neighboring genes, the structure of the gene product, or the time of development when the gene expresses itself, as a consequence of nucleotide changes inside or outside of a given cistron. An activated Ds can also cause chromosome breakage, which may yield deletions or generate a breakage-fusion-bridge cycle (q.v.). It is now known that Ac is a 4,500 bp segment of DNA that encodes a transposable element (q.v.) which contains within it the locus of a functional transposase (q.v.). The transposase gives Ac the ability to detach

Actinomycin D

from one chromosome and then insert into another. The excision of Ac may cause a break in the chromosome, 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, genomic instability, mutator gene, terminal inverted repeats (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 specific target compounds.

active immunity immunity conferred on an organism 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 repressors, the parts that bind to DNA; 3. in an antibody, the part that binds antigen; 4. in a hormone, the portion that recognizes the cell receptor.

active transport the movement of an ion or molecule 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-P (q.v.) family of intercellular 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 frequent targets of chromosome translocations associated 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 undergo modification so as to function more perfectly in a given environment. 2. any developmental, behavioral, 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 enzyme. The discovery of adaptive enzymes led eventually 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) plotted against average fitness for a given set of environmental 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 populations of animals in darkened surroundings. By improving their camouflage, this makes them less conspicuous 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 several) on an adaptive landscape (q.v.), from which movement in any planar direction (changed gene frequencies) results in lower average fitness.

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