A 1. mass number of an atom; 2. haploid set of autosomes; 3. ampere; 4. adenine or adenosine.

A2 See hemoglobin.

A 23187 See ionophore.

AA-AMP amino acid adenylate.

A, B antigens mucopolysaccharides responsible for the ABO blood group system. The A and B antigens reside on the surface of erythrocytes, and differ only in the sugar attached to the penultimate monosaccharide unit of the carbohydrate chain. This minor chemical difference makes the macromole-cule differentially active antigenically. The Ia, Ib, and i are alleles of a gene residing on the long arm of chromosome 9 between bands 34.1 and 34.2. The Ia and IB alleles encode A and B glycotransferases, and the difference in their specificities is due to differences in their amino acid sequences at only four positions. These in turn result from different missense mutations in the two alleles. The A and B transferases add N-acetyl galactosamine or galactose, respectively, to the oligosaccharide terminus. The i allele encodes a defective enzyme, so no additional monosaccharide is added to the chain. Glycoproteins with properties antigenically identical to the A, B antigens are ubiquitous, having been isolated from bacteria and plants. Every human being more than 6 months old possesses those antibodies of the A, B system that are not directed against its own blood-group antigens. These "preexisting natural" antibodies probably result from immunization by the ubiquitous antigens mentioned above. The A and B antigens also occur on the surfaces of epithelial cells, and here they may mask receptors that serve as binding sites for certain pathogenic bacteria. See Appendix C, 1901, Landsteiner; 1925, Bernstein; 1990, Yamomoto et al.; blood group, Helicobacter pylori, H substance, Lewis blood group, MN blood group, null allele, oligosaccharide, P blood group, Secretor gene.

ABC model See floral identity mutations.

ABC transporters a family of proteins that span the plasma membranes of cells and function to transport specific molecules into or out of the cell. The name is an abbreviation of ^TP-Binding Cassette. ABC transporters all contain an ATP binding domain, and they utilize the energy of ATP to pump substrates across the membrane against a concentration gradient. The substrates may be amino acids, sugars, polypeptides, or inorganic ions. The product of the cystic fibrosis gene is an ABC transporter. See Bacillus, cystic fibrosis (CF), Escherichia coli.

Abelson murine leukemia virus an oncogenic virus identified in 1969 by Dr. H. T. Abelson. The transforming gene v-abl has a cellular homolog c-abl. This is actively transcribed in embryos at all stages and during postnatal development. A homolog of c-abl occurs in the human genome at 9q34, and it encodes a protein kinase (q.v.). It is this gene which is damaged during the reciprocal interchange that occurs between chromosome 9 at q34 and chromosome 22 at q11, resulting in myeloid leukemia. See Philadelphia (Ph1) chromosome, myeloproliferative disease.

aberrations See chromosomal aberration, radiation-induced chromosomal aberration.

ABM paper aminobenzyloxy methyl cellulose paper, which when chemically activated, reacts cova-lently with single-stranded nucleic acids.

ABO blood group system system of alleles residing on human chromosome 9 that specifies certain red cell antigens. See AB antigens, blood groups, Bombay blood group.

abortion 1. The expulsion of a human fetus from the womb by natural causes, before it is able to survive independently; this is sometimes called a miscarriage (q.v.). 2. The deliberate termination of a human pregnancy, most often performed during the first 28 weeks of pregnancy. 3. The termination of development of an organ, such as a seed or fruit.

abortive transduction failure of a transducing ex-ogenote to become integrated into the host chromosome, but rather existing as a nonreplicating particle in only one cell of a clone. See transduction.

abortus a dead fetus born prematurely, whether the abortion was artificially induced or spontaneous. Over 20% of human spontaneous abortions show chromosomal abnormalities. See Appendix C, 1965, Carr.

abscisic acid a plant hormone synthesized by chloroplasts. High levels of abscisic acid result in the abscission of leaves, flowers, and fruits. The hormone also causes the closing of stomata in response to dehydration.

abscission the process whereby a plant sheds one of its parts, such as leaves, flowers, seeds, or fruits.

absolute plating efficiency the percentage of individual cells that give rise to colonies when inoculated into culture vessels. See relative plating efficiency.

absorbance (also absorbancy) a measure of the loss of intensity of radiation passing through an absorbing medium. It is defined in spectrophotometry by the relation log (Io/I), where Io = the intensity of the radiation entering the medium and I = the intensity after traversing the medium. See Beer-Lambert law, OD260 unit.

abundance in molecular biology, the average number of molecules of a specific mRNA in a given cell, also termed representation. The abundance, A = NRf/M, where N = Avogadro's number, R = the RNA content of the cell in grams, f= the fraction the specific RNA represents of the total RNA, and M = the molecular weight of the specific RNA in daltons.

abzymes catalytic antibodies. A class of monoclonal antibodies that bind to and stabilize molecules in the transition state through which they must pass to form products. See enzyme.

acatalasemia the hereditary absence of catalase (q.v.) in humans. Mutations in the structural gene on chromosome 11 at p13 result in the production of an unstable form of the enzyme. The gene is 34 kb in length and contains 13 exons.

acatalasia synonym for acatalasemia (q.v.).

acceleration See heterochrony.

accelerator an apparatus that imparts kinetic energy to charged subatomic particles to produce a high-energy particle stream for analyzing the atomic nucleus.

acceptor stem the double-stranded branch of a tRNA molecule to which an amino acid is attached (at the 3', CCA terminus) by a specific aminoacyl-tRNA synthetase. See transfer RNA.

accessory chromosomes See B chromosomes.

accessory nuclei bodies resembling small nuclei that occur in the oocytes of most Hymenoptera and those of some Hemiptera, Coleoptera, Lepidoptera, and Diptera. Accessory nuclei are covered by a double membrane possessing annulate pores. They are originally derived from the oocyte nucleus, but they subsequently form by the amitotic division of other accessory nuclei.

Ac, Ds system Activator-Dissociation system (q.v.).

ace See symbols used in human cytogenetics.

acentric designating a chromatid or a chromosome that lacks a centromere. See chromosome bridge.

Acer the genus of maple trees. A. rubrum, the red maple, and A. saccharum, the sugar maple, are studied genetically because of their commercial importance.

Acetabularia a genus of large, unicellular green algae. Each organism consists of a base, a stalk, and a cap. The base, which contains the nucleus, anchors the alga to the supporting rocks. The stalk, which may be 5 cm long, joins the base and the cap. The cap carries out photosynthesis and has a species-specific shape. For example, the disc-shaped cap of A. mediterranea is smooth, whereas the cap of A. crenulata is indented. Hammerling cut the base and cap off a crenulata alga and then grafted the stalk on a mediterranea base. The cap that regenerated was smooth, characteristic of the species that provided the nucleus. Heterografts like these provided some of the earliest evidence that the nucleus could send messages that directed developmental programs at distant regions of the cell. See Appendix A, Protoc-tista, Chlorophyta; Appendix C, 1943, Hammerling; graft.

Acetobacter a genus of aerobic bacilli which secure energy by oxidizing alcohol to acetic acid.

aceto-orcein a fluid consisting of 1% orcein (q.v.) dissolved in 45% acetic acid, used in making squash preparations of chromosomes. See salivary gland squash preparation.

acetylcholine a biogenic amine that plays an important role in the transmission of nerve impulses across synapses and from nerve endings to the muscles innervated. Here it changes the permeability of

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