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Adaptive landscape adaptive radiation the evolution of specialized species, each of which shows adaptations to a distinctive mode of life, from a generalized ancestral species. Darwin observed the adaptive radiation of finch species on the Galapagos islands. The Hawaiian archipelago shows perhaps the most spectacular examples of adaptive radiations. See Darwin's finches, Hawaiian Drosophilidae, silversword alliance.

adaptive surface, adaptive topography synonyms for adaptive landscape (q.v.).

adaptive value the property of a given genotype when compared with other genotypes that confers fitness (q.v.) to an organism in a given environment.

adaptor a short, synthetic DNA segment containing a restriction site that is coupled to both ends of a blunt-ended restriction fragment. The adaptor is used to join one molecule with blunt ends to a second molecule with cohesive ends. The restriction site of the adaptor is made identical to that of the other molecule so that when cleaved by the same restriction enzyme both DNAs will contain mutually complementary cohesive ends.

adaptor hypothesis the proposal that polynucleo-tide adaptor molecules exist that can recognize specific amino acids and also the regions of the RNA templates that specify the placement of amino acids in a newly forming polypeptide. See Appendix C, 1958, Crick; transfer RNA.

ADCC antibody-dependent cellular cytotoxicity; also known as antibody-dependent cell-mediated cy-totoxicity. Cell-mediated cytotoxicity requires prior binding of antibody to target cells for killing to occur. It does not involve the complement cascade. See K cells.

additive factor one of a group of nonallelic genes affecting the same phenotypic characteristics and each enhancing the effect of the other in the pheno-type. See quantitative inheritance.

additive gene action 1. a form of allelic interaction in which dominance is absent; the heterozygote is intermediate in phenotype between homozygotes for the alternative alleles. 2. the cumulative contribution made by all loci (of the kind described above) to a polygenic trait.

additive genetic variance genetic variance attributed to the average effects of substituting one allele for another at a given locus, or at the multiple loci governing a polygenic trait. It is this component of variance that allows prediction of the rate of response for selection of quantitative traits. See quantitative inheritance.

adducin a ubiquitously expressed protein found in the membranes of animal cells. Mammalian adducin is a heterodimeric protein whose subunits share sequence similarities and contain protease-resistant N-terminal and protease-sensitive C-terminal domains. Adducin has a high affinity for Ca2+/calmodulin and is a substrate for protein kinases. In vitro it causes actin filaments to form bundles and promotes spec-trin-actin associations in regions where cells contact one another. In Drosophila, a homolog of mammalian adducin is encoded by the hts gene. See calmodulin, fusome, hu-li tai shao (hts), heterodimer, protein kinase, spectrosome.

adduct the product of a chemical reaction that results in the addition of a small chemical group to a relatively large recipient molecule. Thus the alkylat-ing agent ethyl methane sulfonate (q.v.) can add ethyl groups to the guanine molecules of DNA. These ethylated guanines would be examples of DNA adducts.

adenine See bases of nucleic acids.

adenine deoxyriboside See nucleoside.

adenohypophysi s the anterior, intermediate, and tuberal portions of the hypophysis, which originate from the buccal lining in the embryo.

adenohypophysis hormone See growth hormone.

adenosine See nucleoside.

adenosine deaminase deficiency a rare immune deficiency disease due to mutations in a gene located on the long arm of human chromosome 20. The normal gene encodes an enzyme that controls the metabolism of purines, and ADA deficiency impairs the functioning of white blood cells. The division of T cells is depressed, and antibody production by B cells is reduced. As a result, ADA-deficient children die from viral, bacterial, and fungal infections. ADA deficiency is the first hereditary disease to be successfully treated by gene therapy. See Appendix C, 1990, Anderson; immune response.

adenosine phosphate any of three compounds in which the nucleoside adenosine is attached through its ribose group to one, two, or three phosphoric acid molecules, as illustrated here. AMP, ADP, and ATP are interconvertible. ATP upon hydrolysis yields the energy used to drive a multitude of biological processes (muscle contraction, photosynthesis, bioluminescence, and the biosynthesis of proteins, nucleic acids, polysaccharides and lipids). The most important process in human nutrition is the synthesis of ATP. Every day human beings synthesize, breakdown, and resynthesize an amount of ATP equaling their body weight. See Appendix C, 1929;

Lohmann; ATPase, ATP synthase, cellular respiration, citric acid cycle, cytochromes, electron transport chain, glycolysis, mitochondria, oxidative phosphorylation, mitochondrial proton transport.

adenovirus any of a group of spherical DNA viruses characterized by a shell containing 252 capso-meres. Adenoviruses infect a number of mammalian species including humans. See human adenovirus 2 (HAdV-2), virus.

adenylcyclase the enzyme that catalyzes the conversion of ATP into cyclic AMP (q.v.). Also called adenylate cyclase. See adenosine phosphate.

adenylic acid See nucleotide.

ADH the abbreviation for alcohol dehydrogenase (q.v.).

adhesion plaques

See vincullin.

adhesive molecules any pair of complementary cell-surface molecules that bind specifically to one another, thereby causing cells to adhere to one another, as do carbohydrates and protein lectins (q.v.). Phenomena dependent on adhesive molecules include invasion of host cells by bacteria and viruses, species-specific union of sperms and eggs, and aggregation of specific cell types during embryological development. See cell affinity, hemagglutinins, P blood group, selectins.

adjacent disjunction, adjacent segregation See translocation heterozygote.

adjuvant a mixture injected together with an antigen that serves to intensify unspecifically the immune response. See Freund's adjuvant.

adoptive immunity the transfer of an immune function from one organism to another through the transfer of immunologically active or competent cells. Also called adoptive transfer.

Adenosine phosphate agammaglobulinemia 11

ADP adenosine diphosphate.

phate.

See adenosine phos-

adrenal corticosteroid a family of steroid hormones formed in the adrenal cortex. There are more than 30 of these hormones, and all are synthesized from cholesterol by cortical cells that have been stimulated by the adrenocorticotropic hormone (q.v.).

adrenocorticotropic hormone a single-chain peptide hormone (39 amino acids long) stimulating secretion by the adrenal cortex. It is produced by the adenohypophysis of vertebrates. Abbreviated ACTH. Also called corticotropin.

Adriamycin an antibiotic produced by Streptomy-ces peucetius that interacts with topoisomerase. DNA isolated from Adriamycin-poisoned cells contains single- and double-strand breaks. See gyrase, mitotic poison.

adult tissue stem cells See stem cells advanced in systematics, the later or derived stages or conditions within a lineage that exhibits an evolutionary advance; the opposite of primitive.

adventitious embryony the production by mitotic divisions of an embryonic sporophyte from the tissues of another sporophyte without a gametophytic generation intervening.

Aedes a genus of mosquitoes containing over 700 species, several of which transmit important human diseases. A. aegypti, the vector of yellow fever, has a diploid chromosome number 6, and about 60 mutations have been mapped among its three linkage groups. Among these are genes conferring resistance to insecticides such as DDT and pyrethrins (both of which see).

Aegilops a genus of grasses including several species of genetic interest, especially A. umbellulata, a wild Mediterranean species resistant to leaf rust. A gene for rust resistance has been transferred from A. umbellulata to Triticum vulgare (wheat).

aerobe an organism that requires molecular oxygen and lives in an environment in contact with air.

aestivate to pass through a hot, dry season in a torpid condition. See also hibernate.

afferent leading toward the organ or cell involved. In immunology, the events or stages involved in activating the immune system. Compare with efferent.

affinity in immunology, the innate binding power of an antibody combining site with a single antigen binding site. Compare with avidity.

affinity chromatography a technique for separating molecules by their affinity to bind to ligands (e.g., antibodies) attached to an insoluble matrix (e.g., Sepharose). The bound molecules can subsequently be eluted in a relatively pure state.

afibrinogenemia an inherited disorder of the human blood-clotting system characterized by the inability to synthesize fibrinogen; inherited as an autosomal recessive.

aflatoxins a family of toxic compounds synthesized by Aspergillus flavus and other fungi belonging to the same genus. Aflatoxins bind to purines, making base pairing impossible, and they inhibit both DNA replication and RNA transcription. These my-cotoxins are highly toxic and carcinogenic, and they often are contaminants of grains and oilseed products that are stored under damp conditions. The structure of aflatoxin G, is shown.

Aflatoxin B1 has a CH2 substituted for the O at the position marked by the arrow. Aflatoxin B2 and G2 are identical to B1 and Gj, except that the ring labeled with an asterisk lacks a double bond.

African bees Apis mellifera scutellata, a race of bees, originally from South Africa, that was accidentally introduced into Brazil in 1957 and has spread as far as the southern United States. African bees are poor honey producers and tend to sting much more often than European bees. Because of daily differences in flight times of African queens and European drones, hybridization is rare. See Apis mellifera.

African Eve See mitochondrial DNA lineages.

African green monkey See Cercopithecus aethiops.

agamete a haploid, asexual reproductive cell resulting from meiosis in an agamont. Agametes disperse and grow into gamonts (q.v.).

agammaglobulinemia the inability in humans to synthesize certain immunoglobulins. The most common form is inherited as an X-linked recessive trait, which is symbolized XLA (X-linked agammaglobulinemia) in the early literature. When O. C. Bruton described the condition in 1952, it was the first hereditary immune disease to be reported. The disease is now known to be caused by mutations in a gene at Xq21.3-q22. The gene is 36,740 bp long, and it encodes a protein containing 659 amino acids. The protein is a tyrosine kinase that has been named in Bruton's honor, and the gene is now symbolized BTK, for its product, the Bruton tyrosine kinase. The enzyme is a key regulator in the development of B lymphocytes. Boys with XLA lack circulating B cells. The bone marrow contains pre-B cells, but they are unable to mature. See antibody.

agamogony the series of cellular or nuclear divisions that generates agamonts.

agamont the diploid adult form of a protoctist that also has a haploid adult phase in its life cycle. An agamont undergoes meiosis and produces agametes. See gamont.

agamospermy the formation of seeds without fertilization. The male gametes, if present, serve only to stimulate division of the zygote. See apomixis.

agamous See floral identity mutations.

Agapornis a genus of small parrots. The nest building of various species and their hybrids has provided information on the genetic control of behavior patterns.

agar a polysaccharide extract of certain seaweeds used as a solidifying agent in culture media.

agarose a linear polymer of alternating D-galactose and 3,6-anhydrogalactose molecules. The polymer, fractionated from agar, is often used in gel elec-trophoresis because few molecules bind to it, and therefore it does not interfere with electrophoretic movement of molecules through it.

agar plate count the number of bacterial colonies that develop on an agar-containing medium in a petri dish seeded with a known amount of inoculum. From the count, the concentration of bacteria per unit volume of inoculum can be determined.

age-dependent selection selection in which the values for relative fitness of different genotypes vary with the age of the individual.

agglutination the clumping of viruses or cellular components in the presence of a specific immune serum.

agglutinin any antibody capable of causing clumping of erythrocytes, or more rarely other types of cells.

agglutinogen an antigen that stimulates the production of agglutinins.

aggregation chimera a mammalian chimera made through the mingling of cells of two embryos. The resulting composite embryo is then transferred into the uterus of a surrogate mother where it comes to term. See allophenic mice.

aging growing old, a process that has a genetic component. Hereditary diseases are known in humans that cause premature aging, and mutations that speed up or delay aging have been isolated in Saccharomyces, Caenorhabditis, and Drosophila. See Appendix C, 1994, Orr and Sohal; 1995, Feng et al.; antioxidantenzymes, apoptosis, daf-2, free radical hypothesis of aging, Indy, methuselah, Podospora anse-rina, progeria, SGSI, telomerase, senescence, Werner syndrome.

agonistic behavior any social interaction between members of the same species that involves aggression or threat and conciliation or retreat.

agouti the grizzled color of the fur of mammals resulting from alternating bands of yellow (phaeo-melanin) and black (eumelanin) pigments in individual hairs. The name is also given to the genes that control the hair color patterns. In the mouse more than 20 alleles have been described at the agouti locus on chromosome 2. The gene encodes a cysteine-rich, 131 amino acid protein that instructs the me-lanocytes in the hair follicle when to switch from making black to yellow pigment. The protein is translated by nearby follicle cells rather than in the melanocytes themselves. Therefore, the agouti protein acts as a paracrine-signaling molecule. See Appendix C, 1905, Cuenot; autocrine, MC1R gene, melanin.

agranular reticulum See smooth endoplasmic reticulum (SER).

agranulocytes white blood cells whose cytoplasm contains few or no granules and that possess an un-lobed nucleus; mononuclear leucocytes including lymphocytes and monocytes.

agriculturally important species See Appendix B.

Agrobacterium tumefaciens the bacterium responsible for crown gall disease (q.v.) in a wide range of dicotyledonous plants. The bacterium enters only dead, broken plant cells and then may transmit a tumor-inducing plasmid into adjacent living plant cells. This infective process is a natural form of genetic engineering, since the bacterium transfers part of its DNA to the infected plant. This

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