are Dictyostelium discoideum and Polysphondylium pallidum.
acridine dyes heterocyclic compounds that in- clude 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).
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 anten- nae. See maternal polarity mutants. acrosome an apical organelle in the sperm head that is secreted by the Golgi material and that di-gests 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 dur- ing meiosis.
acrotrophic See meroistic.
acrylamide See polyacrylamide gel.
ACTH adrenocorticotropic hormone (q.v.).
actin a protein that is the major constituent of the 7-nanometer-wide microfilaments of cells. Actin mi- crofilaments (F actin) are polymers of a globular sub- unit (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 ad- dition 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. α1 is unique to skeletal muscle; α2, to cardiac muscle; α3, to smooth vascular muscle; and α4, to smooth enteric muscle. Two other actins (β and γ) 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 in- clude 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 specify- ing the trailers (q.v.) differ considerably in nucleo- tide sequences.