the sarcolemma and causes contraction. Acetylcholine is evidently a very ancient hormone, since it is present even in protists.

acetylcholi nesterase the enzyme that catalyses the hydrolysis of acetylcholine (q.v.) into choline and acetate. Also called cholinesterase.

acetyl-coenzyme A See coenzyme A.

acetyl serine See N-acetyl serine.

achaete-scute complex a complex locus in Dro-sophila first identified by mutations that affected the development of adult bristles. Lack of the entire complex results in the failure of neurogenesis during the embryo stage. The complex contains four ORFs that encode DNA-binding proteins that contain helix-turn-helix motifs (q.v.).

achiasmate referring to meiosis without chias-mata. In those species in which crossing over is limited to one sex, the achiasmate meiosis generally occurs in the heterogametic sex.

Achilles' heel cleavage (AHC) a technique that allows a DNA molecule to be cut at a specified site. The name comes from the legend in Greek mythology where Achilles' mother dipped him in the river Styx. The waters made him invulnerable, except for the heel by which she held him. In the AHC procedure a sequence-specific DNA-binding molecule is complexed with the DNA under study. A methyl-transferase is then added to methylate all CpG sequences except those hidden under the sequence-specific DNA-binding molecule. Next, this molecule and the methyltransferases are removed, and a restriction endonuclease is added. This will cut the DNA only in the region where methylation was blocked, i.e., the "Achilles' heel."

achondroplasia a form of hereditary dwarfism due to retarded growth of the long bones. It is the most common form of dwarfism in humans (1 in 15,000 live births) and is inherited as an autosomal dominant trait. Homozygotes die at an early age. The gene responsible has been mapped to chromosome 4p16.3. The ACH gene has been renamed FGFR3, since it encodes the Fibroblast Growth Factor Receptor 3, a protein containing 806 amino acids. The gene contains 14,975 bp of DNA and produces two alternative transcripts. Homologous genes have been identified in rat, mouse, Xenopus, and zebrafish. The genes are expressed in the chondrocytes of developing bones. See bovine achondroplasia, de novo mutation, fowl achondroplasia, positional candidate approach.

achromatic figure the mitotic apparatus (q.v.). A chromosomes See B chromosomes.

acid fuchsin an acidic dye used in cytochemistry.

acidic amino acid an amino acid (q.v.) having a net negative charge at neutral pH. Those universally found in proteins are aspartic acid and glutamic acid, which bear negatively charged side chains in the pH range generally found in living systems.

acidic dye an organic anion that binds to and stains positively charged macromolecules.

Acinonyx jubatus the cheetah, a carnivore that has the distinction of being the world's fastest land animal. Cheetahs are of genetic interest because, while most other species of cats show heterozygosity levels of 10-20%, cheetahs have levels close to zero. This high degree of homozygosity is correlated with low fecundity, high mortality of cubs, and low disease resistance.

Acoelomata a subdivision of the Protostomia-con-taining species in which the space between the epidermis and the digestive tube is occupied by a cellular parenchyma. See classification.

acquired characteristics, inheritance of inheritance by offspring of characteristics that arose in their parents as responses to environmental influences and are not the result of gene action. See La-marckism.

acquired immunodeficiency syndrome See AIDS, HIV.

Acraniata a subphylum of Chordata containing animals without a true skull. See Appendix A.

acrasin a chemotactic agent produced by Dictyo-stelium discoideum that is responsible for the aggregation of the cells. Acrasin has been shown to be cyclic AMP (q.v.).

Acrasiomycota the phylum containing the cellular slime molds. These are protoctists that pass through a unicellular stage of amoebas that feed on bacteria. Subsequently, these amoebas aggregate to form a fruiting structure that produces spores. The two most extensively studied species from this phylum are Dictyostelium discoideum and Polysphondylium pallidum.

acridine dyes heterocyclic compounds that include acridine (shown below) and its derivatives. These molecules bind to double-stranded DNAs as intercalating agents. Examples of acridine dyes are acridine organe, acriflavin, proflavin, and quinicrine (all of which see).

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