head to the anterior portion of the second thoracic segment will undergo. Mutations in the Antp gene cause the transformation of the segment that nor- mally produces the antenna into one that produces a middle leg. The gene encodes a protein character- ized by a homeobox (q.v.).
This is a segment of 60 amino acids that lies close to the C terminus of the Antp protein. The amino acid sequence of this seg- ment is shown above. It binds to target DNA se- quences by its helix-turn-helix motif (q.v.).
The complete three-dimensional structures of the Antp homeodomain as well as of the homeodomain-target DNA complex have been determined using NMR spectroscopy (q.v.) and x-ray crystallography (q.v.). See Appendix C, 1983, Scott et al.; 1989, Qian et al.; 1990, Maliki, Schughart, and McGinnis; bithorax, homeotic mutants, Hox genes, Polycomb, probos- cipedia, segment identity genes. anther the terminal portion of a stamen bearing pollen sacs.
anther culture a technique that utilizes anthers or pollen cells to generate haploid tissue cultures or even plants. See Appendix C, 1973, Debergh and Nitsch, haploid sporophytes. antheridium the male gametangium of algae, fungi, bryophytes, and pteridophytes. Contrast with oogonium. anthesis the time of flowering. anthocyanins the red, violet, or blue glycosidic pigments that give color to flowers, fruits, seeds, stems, and leaves of plants.
The common structural unit is a 15-carbon flavone skeleton to which sugars are attached. An example is pelargonidin, a scarlet pigment produced by geraniums. Unlike the carot- enoids and chlorophylls, which are lipid-soluble pig- ments of plastids, anthocyanins are water-soluble and are found dissolved in the vacuoles (q.v.) of plant cells. A primary function of anthocyanins is to attract insect pollinators to plants. See kernel, pelar- gonidin monoglucoside, R genes of maize.
anthrax a disease caused by Bacillus anthracis, a spore-forming bacterium. The condition is seen in cows, pigs, goats, horses, and sheep. In humans the condition was first called wool-sorter’s disease, and it resulted from inhalation of dust that contained spores.
The first successful artificially produced vac- cine (q.v.) was against anthrax. See Appendix A, Bac- teria, Endospora; Appendix C, 1881, Pasteur. anthropocentrism also called anthropomorphism.
1. explanation of natural phenomena or processes in terms of human values.
2. assuming humans to be of central importance in the universe or ultimate end of creation.
3. ascribing human characteristics to a non-human organism. anthropoid designating the great apes of the fam- ily Pongidae, including the gibbons, orangutans, go- rillas, and chimpanzees. anthropometry the science that deals with the measurements of the human body and its parts. antiauxin a molecule that competes with an auxin (q.v.) for auxin receptor sites. A well-known anti- auxin is 2,6-dichlorophenoxyacetic acid. antibiotic a bacteriocidal or bacteriostatic sub- stance produced by certain microorganisms, espe- cially species of the genera Penicillium, Cephalospor- ium, and Streptomyces. See actinomycin D, ampicillin, chloramphenicol, cyclohexamide, erythromycin, ka-namycin, neomycin, novobiocin, penicillin, puromy-cin, semisynthetic antibiotic, streptomycin, tetracy- cline.
antibiotic resistance the acquisition of unrespon- siveness to a specific antibiotic by a microorganism that was previously adversely affected by the drug. Such resistance generally results from a mutation or the acquisition of R plasmids (q.v.) by the micro- organism. Compare with antibiotic tolerance.
See ef- flux pump. antibiotic tolerance a bacterial trait characterized by the cessation of growth when exposed to penicil- lin, vancomycin, and other antibiotics but not by cell death. Antibiotic tolerance is more widespread than complete resistance to antibiotics. See antibiotic re- sistance.
antibody a protein produced by lymphoid cells (plasma cells) in response to foreign substances (an- tigens) and capable of coupling specifically with its homologous antigen (the one that stimulated the immune response) or with substances that are chem- ically very similar to that same antigen. See Appendix C, 1890, von Behring; 1900, Ehrlich; 1939, Tiselius and Kabat; abzymes, immunoglobulin. antibody-antigen reaction See antigen-antibody reaction. anticipation See genetic anticipation. anticlinal referring to a layer of cells cutting the circumference of a cylindrical plant organ at right angles. Compare periclinal.
anticoding strand See strand terminologies. anticodon the triplet of nucleotides in a transfer RNA molecule which associates by complementary base pairing with a specific triplet (codon) in the messenger RNA molecule during its translation in the ribosome. antidiuretic hormone See vasopressin. antigen 1. a foreign substance that, upon intro- duction into a vertebrate animal, stimulates the pro- duction of homologous antibodies; a complete anti- gen or immunogen.
A complex antigenic molecule may carry several antigenically distinct sites (deter- minants). 2. a substance that is chemically similar to certain parts of an immunogen and can react spe- cifically with its homologous antibody, but is too small to stimulate antibody synthesis by itself; an in- complete antigen or hapten (q.v.).
antigen-antibody reaction the formation of an in-soluble complex between an antigen and its specific antibody. In the case of soluble antigens, the com- plex precipitates, while cells that carry surface anti- gens are agglutinated. See Appendix C, 1900, Ehrlich; 1986, Amit et al. antigenic conversion
1. appearance of a specific antigen(s) on cells as a consequence of virus infec- tion.
2. antibody-induced shift in certain protozoans or parasites to a new cell-surface antigen and cessa- tion of expression of another antigen; resulting from switches in gene activity; synonymous in this sense with serotype transformation; compare with antigenic modulation. antigenic determinant a small chemical complex (relative to the size of the macromolecule or cell of which it is a part) that determines the specificity of an antigen-antibody interaction. That portion of an antigen that actually makes contact with a particular antibody or T cell receptor. An epitope.
Contrast with paratope. antigenic mimicry acquisition or production of host antigens by a parasite, enabling it to escape de- tection by the host’s immune system (as occurs in Schistosoma).
See schistosomiasis. antigenic modulation suppression of cell-surface antigens in the presence of homologous antibodies. antigen variation the sequential expression of a series of variable surface glycoproteins (VSGs) by trypanosomes while in the bloodstream of a mam- malian host. The production of VSGs allows the parasite to evade the immune defenses of the host by keeping one step ahead of the antibodies the host raises against them.
Trypanosomes contain hundreds of different genes for the individual VSGs, but in a single trypanosome only one of these genes is ex- pressed at a given time. The switch from one VSG to another is accompanied by rearrangements of the DNA that generates additional copies of the genes being expressed. The glycoprotein being synthesized forms a macromolecular coating about 15 nanome- ters thick over the body and flagellum of the para- site.
This coating functions only during the mamma- lian stage of the life cycle, since it is shed when the trypanosome enters the tsetse fly vector. See Glos- sina, Trypanosoma. antihemophilic factor a protein (also called factor 8) that participates in the cascade of reactions that results in blood clotting (q.v.). A deficiency in AHF results in classic hemophilia (q.v.). AHF contains 2,351 amino acids. The first 19 code for a leader se- quence peptide (q.v.), and the mature clotting factor
contains 2,332 amino acids. Three domains in factor 8 are also found in ceruloplasmin (q.v.). See Appen- dix C, 1984, Gitschier et al.; crossreacting material, von Willebrand disease. antimetabolite in general, a molecule that func- tions as an antagonist or metabolic poison. antimitotic agent any compound that suppresses the mitotic activity of a population of cells.
antimongolism a syndrome of congenital defects accompanying hypoploidy for chromosome 21. Such children carry a normal chromosome 21 and one that contains a large deficiency. From the standpoint of chromosome balance, mongolism and antimon- golism represent reciprocal phenomena. antimorphic mutation a mutation in which the al- tered gene product acts antagonistically to the wild type gene product.
Antimorphic mutations behave as dominant or semidominant alleles. See Hunting- ton disease (HD). antimutagen a compound (generally a purine nu- cleoside) that antagonizes the action of mutagenic agents (generally purines or purine derivatives) on bacteria. Some antimutagens reduce the spontane- ous mutation rate. anti-oncogenes a class of genes (also called tumor suppressor genes) that are involved in the negative regulation of normal growth.
The loss of these genes or their products leads to malignant growth. The Rb gene of humans is an example of an anti-oncogene. See Appendix C, 1969, Harris et al.; 1971, Knudson; 1990, Baker et al.; breast cancer susceptibility genes, retinoblastoma, TP53, Wilms tumor.
antioxidant enzymes enzymes that prevent the buildup of reactive oxygen molecules in cells. Super- oxide dismutase (q.v.) and catalase (q.v.) are the pri- mary enzymes involved in this process. Superoxide dismutase converts the superoxide radical (O−) to H2O2 and catalase breaks down H2O2 into water and O2. Transgenic Drosophila containing additional cop- ies of the genes encoding superoxide dismutase and catalase have increased longevity. See Appendix C, 1994, Orr and Sohol; free radical theory of aging. antiparallel
1. the opposite strand orientations with which all nucleic acid duplexes (DNA-DNA, DNA-RNA, or RNA-RNA) associate; if one strand is oriented left to right 5′ to 3′, the complementary strand is oriented left to right 3′ to 5′ antiparallel to it.
2. two segments of a polypeptide chain may lie
antiparallel with one end N-terminal to C-terminal and the other end C-terminal to N-terminal, respec- tively.
See Appendix C, 1952, Crick; 1961, Josse, Kai- ser, and Kornberg; deoxyribonucleic acid, strand termi- nologies. antipodal one of a group of three haploid nuclei formed during megasporogenesis. In corn, these di- vide mitotically and eventually form a group of 20 to 40 antipodal cells, which aid in nourishing the young embryo. See double fertilization. antirepressor See cro repressor.
Antirrhinum a genus of decorative flowers com- monly called snapdragons. The scientific name was given by Linnaeus in the first (1753) edition of the Species Plantarum. A. majus has 8 chromosomes, and it was a classical species for the study of multiple alleles. Antirrhinum research has made important contributions to the understanding of the biosynthe- sis of floral pigments and of photosynthetic path- ways and their regulation. Studies of alleles of pig- ment genes that produced variegated color patterns led to the discovery of transposable elements (q.v.).
One of these, Tam1, was one of the first plant trans- posons to have its nucleotide sequence determined. Transposon tagging (q.v.) has been used to identify genes that control floral organ identity.
The base se- quences are known for Antirrhinum genes that en- code RNases, phytochromes, rRNAs, the large rubi- sco subunit, chloroplast proteins, and ubiquitin. See Appendix A, Plantae, Angiospermae, Dicotyledonae, Scrophulariales; anthocyanins, floral organ identity mutations. antisense RNA (asRNA) a single-stranded RNA molecule with a nucleotide sequence complemen- tary to a sense strand RNA, i.e., messenger RNA. The combining of sense and antisense RNA in a cell is a powerful silencer of gene expression. asRNAs occur naturally in many organisms (e.g., as small in- terfering RNAs (q.v.) or small temporal RNAs (q.v.)).
These natural asRNAs silence genes at the transcriptional, post-transcriptional, or translational level. asRNAs can also be experimentally introduced into a cell or organism by such means as transforma- tion (q.v.) with an artificial gene that transcribes as- RNA or by injecting double-stranded RNA (q.v.) into cells or embryos. This is a powerful means of creating loss of function mutations in distinct genes. See Morpholinos, RNA interference (RNAi), strand ter- minologies. antisense strand See strand terminologies. antiserum a serum containing antibodies.