A Dictionary of Genetics
1. mass number of an atom;
2. haploid set of autosomes;
4. adenine or adenosine.
A˚ Angstrom unit (q.v.).
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 an- tigens reside on the surface of erythrocytes, and dif- fer 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 dif- ferences in their amino acid sequences at only four positions. These in turn result from different mis- sense 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. Glycopro- teins 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” anti- bodies 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, Se- cretor gene. ABC model See floral identity mutations. ABC transporters a family of proteins that span the plasma membranes of cells and function to trans- port specific molecules into or out of the cell. The name is an abbreviation of ATP-Binding Cassette. ABC transporters all contain an ATP binding do- main, and they utilize the energy of ATP to pump substrates across the membrane against a concentra- tion 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 vi- rus 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 en- codes a protein kinase (q.v.). It is this gene which is damaged during the reciprocal interchange that oc- curs between chromosome 9 at q34 and chromo- some 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 pa- per, which when chemically activated, reacts cova- lently with single-stranded nucleic acids. ABO blood group system system of alleles resid- ing 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 sur- vive independently; this is sometimes called a mis- carriage (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 chromo- some, 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 hor- mone 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 in- dividual cells that give rise to colonies when inocu-lated into culture vessels. See relative plating effi-ciency.
absorbance (also absorbancy) a measure of the loss of intensity of radiation passing through an ab-sorbing 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 inten-sity 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 mono-clonal antibodies that bind to and stabilize mole-cules 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 en-ergy 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 dou-ble 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 stud- ied genetically because of their commercial impor- tance.
Acetabularia a genus of large, unicellular green al-gae. 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 se-cure 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 im-portant role in the transmission of nerve impulses across synapses and from nerve endings to the mus-cles innervated. Here it changes the permeability of the sarcolemma and causes contraction. Acetylcho-line is evidently a very ancient hormone, since it is present even in protists.
acetylcholinesterase 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 he- lix-turn-helix motifs (q.v.).
achiasmate referring to meiosis without chias-mata. In those species in which crossing over is lim- ited to one sex, the achiasmate meiosis generally oc- curs 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 mythol- ogy 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 proce- dure 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 re- striction 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 domi- nant 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 Re- ceptor 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 develop- ing bones. See bovine achondroplasia, de novo muta- tion, fowl achrondroplasia, positional candidate ap- proach.
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 dis- ease resistance.
Acoelomata a subdivision of the Protostomia-con- taining species in which the space between the epi- dermis and the digestive tube is occupied by a cellu- lar parenchyma. See classification.
acquired characteristics, inheritance of inheri-tance by offspring of characteristics that arose in their parents as responses to environmental influ- ences 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 aggre- gation 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.