is integrated into the plant genome, and here it induces changes in metabolism and tumor formation. The genome of strain 58 of A. tumefaciens was sequenced in 2001 by a group led by E. W. Nestor. Its genome contained 5.67 mbp of DNA distributed among four replicons: a circular chromosome (CC), a linear chromosome (LC), and two plasmids (pAt and pTi). The plasmids are DNA circles, and most of the genes responsible for crown galls are on pTi. The Table lists the general features of the four repli-cons.
Replicon (kbp) ORFs tRNAs rRNAs CC 2,841 2,789 40 2
pAt 543 550 0 0
pTi 214 198 0 0
Strains of A. tumefaciens carrying the plasmid may be artificially genetically engineered to introduce foreign genes of choice into plant cells, and then by growing the cells in tissue culture, whole plants can be regenerated, every cell of which contains the foreign gene. See Appendix A, Bacteria, Proteobacteria; Appendix C, 1907, Smith and Townsend; 1981, Kemp and Hall; 2001, Wood et al.; Appendix E; Ti plasmid.
Agropyron elongatum a weed related to crabgrass noted for its resistance to stem rust. Genes conferring rust resistance have been transferred from this species to Triticum aestivum (wheat).
AHC Achilles' heel cleavage (q.v.).
AHF antihemophilic factor. See blood clotting.
AI, AID, AIH See artificial insemination.
AIA anti-immunoglobulin antibodies, produced in response to foreign antibodies introduced into an experimental animal.
AIDS the acquired immunodeficiency syndrome, a disease caused by the human immunodeficiency virus (HIV). This virus attacks lymphocytes of helper T subclass and macrophages. The depletion of these cells makes the patient susceptible to pathogens that would easily be controlled by a healthy immune system. The infection is transmitted by sexual intercourse, by direct contamination of the blood (as when virus-contaminated drug paraphernalia is shared), or by passage of the virus from an infected mother to her fetus or to a suckling baby. AIDS was first identified as a new infectious disease by the U.S. Centers for Disease Control and Prevention in 1981. See Appendix C, 1983, Montagnier and Gallo. HIV, lymphocyte, retroviruses.
akinetic acentric (q.v.). ala alanine. See amino acid.
albinism 1. deficiency of chromoplasts in plants. 2. the inability to form melanin (q.v.) in the eyes, skin, and hair, due to a tyrosinase deficiency. In humans the condition is inherited as an autosomal recessive. Tyrosinase (TYR) is an essential enzyme for melanin synthesis, and some mutations in the tyrosi-nase gene (tyr) result in oculocutaneous albinism (OCA). The TYR gene is located in 11q14-21; it contains five exons, and its mRNA is 2,384 nucleotides long. More than 90 mutations have been identified, most of the missense type. One such mutation in codon 422 results in the substitution of glutamine for arginine. The changed enzyme is heat-sensitive and so mimics the temperature-sensitive enzymes known for the Himalayan strains of mice, rabbits, and other species. See Himalayan mutant, ocular albinism, temperature-sensitive mutation, tyrosinase.
albino 1. a plant lacking chromoplasts. 2. an animal lacking pigmentation. See melanin.
albomaculatus referring to a variegation consisting of irregularly distributed white and green regions on plants resulting from the mitotic segregation of genes or plastids.
albumin a water-soluble 70-kilodalton protein that represents 40-50% of the plasma protein in adult mammals. It is important both as an osmotic and as a pH buffer and also functions in the transport of metal ions and various small organic molecules. Albumin is synthesized and secreted by the liver. In the mouse the albumin gene resides on chromosome 5, separated from the alpha fetoprotein gene by a DNA segment about 13.5 kilobases long. In humans, these two genes are in the long arm of chromosome 4. See Appendix C, 1967, Sarich and Wilson; alpha fetoprotein.
alcaptonuria alkaptonuria (q.v.).
alcohol any hydrocarbon that carries one or more hydroxyl groups. The term is often used to refer specifically to ethyl alcohol, the product of yeast-based fermentations. Hereditary differences in alcohol preference are known to exist in mice. See Appendix C, 1962, Rodgers and McClearn.
alcohol dehydrogenase (ADH) a zinc-containing enzyme found in bacteria, yeasts, plants, and animals that reversibly oxidizes primary and secondary alcohols to the corresponding aldehydes and ketones. In the case of yeast, ADH functions as the last enzyme in alcoholic fermentation. In Drosophila melanogas-
ter, ADH is a dimeric protein. By suitable crosses between null activity mutants it is possible to generate heteroallelic individuals that exhibit partial restoration of enzyme activity. This is often due to the production of a heterodimer with improved functional activity. The gene is of interest to developmental geneticist because its expression is controlled by two promotors. The proximal promotor lies adjacent to the initiation codon and switches the gene on during the larval stage. The distal promotor lies 700 base pairs upstream and controls the production of ADH in the adult. See allelic complementation, promotor.
aldehyde any of a class of organic compounds having the general formula CnH2nO and containing H
aldosterone an adrenal corticosteroid hormone that controls the sodium and potassium balance in the vertebrates.
aleurone the outer layer of the endosperm of a seed. Genes controlling the inheritance of aleurone color in maize provided early examples of epistasis (q.v.) and parental imprinting (q.v.). See kernel.
aleurone grain a granule of protein occurring in the aleurone.
Aleutian mink an autosomal recessive mutation in Mustela vison producing diluted pigmentation of the fur and eyes. The homozygotes show a lysosomal defect similar in humans to the Chediak-Higashi syndrome (q.v.).
alga (plural algae) any of a large group of aquatic, chlorophyll-bearing organisms ranging from single cells to giant seaweeds. See Appendix A: Cyano-bacteria, Dinoflagellata, Euglenophyta, Xanthophyta, Chrysophyta, Bacillariophyta, Phaecophyta, Rhodo-phyta, Gamophyta, Chlorophyta.
algorithm a set of simple mathematical procedures that are followed in a specified order to solve a problem in a finite period of time. Computers are instructed to perform tasks with programs containing one or more algorithms.
alien addition monosomic a genome that contains a single chromosome from another species in addition to the normal complement of chromosomes.
alien substitution replacement of one or more chromosomes of a species by those from a different species.
aliphatic designating molecules made up of linear chains of carbon atoms.
aliquot a part, such as a representative sample, that divides the whole without a remainder. Two is an aliquot of six because it is contained exactly three times. Loosely used for any fraction or portion.
alkali metal any of five elements in Group IA of the periodic table: lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and cesium (Cs).
alkaline earth any element of Group IIA of the periodic table: beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium
alkaline phosphatase an enzyme that removes 5'-P termini of DNA and leaves 5'-OH groups. The alkaline phosphatase of E. coli is a dimer made up of identical protein subunits encoded by a single structural gene. In vitro complementation was demonstrated using this enzyme. See allelic complementation.
alkaloid any member of a group of over 3,000 cyclic, nitrogenous, organic compounds, many of which have pharmacological properties. They occur mainly in plants, but are also synthesized by some fungi, amphibians, and arthropods. They include caffeine, cocaine, quinine, morphine, nicotine, reser-pine, strychnine, and theobromine.
alkapton 2,5-dihydroxyphenylacetic acid. See ho-mogentisic acid.
alkaptonuria (also alcaptonuria) a relatively benign hereditary disease in humans due to a recessive gene located on the long arm of chromosome 3. Al-kaptonurics cannot make the liver enzyme homo-gentisic acid oxidase. Therefore, homogentisic acid (q.v.) is not broken down to simpler compounds but is excreted in the urine. Since the colorless homo-gentisic acid is readily oxidized to a black pigment, the urine of alkaptonurics darkens when exposed to air. This disease enjoys the historic distinction of being the first metabolic disease studied. See Appendix C, 1909, Garrod.
alkylating agent a compound causing the substitution of an alkyl group (usually methyl or ethyl) for an active hydrogen atom in an organic compound. According to the number of reactive groups they contain, alkylating agents are classified as mono-, bi-, or polyfunctional. Many chemical mutagens are alkylating agents. See busulfan, chlorambucil, cyclo-phosphamide, epoxide, ethylmethane sulfonate, mel-phalan, Myleran, nitrogen mustard, sulfur mustard, TEM, Thio-tepa, triethylenethiophosphoramide.
allelic exclusion 15
alkyl group a univalent radical having the general formula CnH2n+1 derived from a saturated aliphatic hydrocarbon by removal of one atom of hydrogen. Named by replacing the ending -ane of the hydrocarbon with -yl (e.g., methane becomes methyl).
allantois a saclike outgrowth of the ventral side of the hindgut present in the embryos of reptiles, birds, and mammals. The allantois represents a large and precocious development of the urinary bladder.
allatum hormones hormones synthesized by the insect corpus allatum. The titer of allatum hormones influences the qualitative properties of each molt in holometabolous insects. At high concentrations, larval development ensues; at lower levels, the insect undergoes pupal metamorphosis, and in the absence of the allatum hormones adult differentiation takes place. The allatum hormones thus have a juveniliz-ing action and for this reason have also been called juvenile hormones (JHs). The structural formulas for three of the juvenile hormones are illustrated on page 14. In adult females, the allatum hormone is required for vitellogenesis. The JH analog, ZR515 (q.v.), is often used as a substitute for natural JHs in Drosophila experiments. See Appendix C, 1966, Roller et al.; ring gland, status quo hormones.
allele a shorthand form of allelomorph, one of a series of possible alternative forms of a given gene (cistron, q.v.), differing in DNA sequence, and affecting the functioning of a single product (RNA
and/or protein). If more than two alleles have been identified in a population, the locus is said to show multiple allelism. See heteroallele, homoallele, isoal-lele, null allele, silent allele.
allele-specific oligonucleotide testing a technique used to identify a specific mutation in a collection of DNA fragments isolated from a mutant organism. An oligonucleotide is synthesized that has a base sequence complementary to the segment under study, and it is used as a probe. All segments binding to the probe are then collected and analyzed.
allelic complementation the production of nearly normal phenotype in an organism carrying two different mutant alleles in trans configuration. Such complementation is sometimes caused by the reconstruction in the cytoplasm of a functional protein from the inactive products of the two alleles. When such a phenomenon can be demonstrated by mixing extracts from individuals homozygous for each allele, the term in vitro complementation is used. Synonymous with intra-allelic complementation. See Appendix C, 1963, Schlesinger and Levinthal; alcohol dehydrogenase, alkaline phosphatase, transvec-tion.
allelic exclusion the situation in a diploid nucleus where either the parental or the maternal allele, but not both, is expressed, even though both parental alleles are capable of being transcribed and may even be identical. This situation is seen during recombina-
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