Appendix A - Glossary abortive transduction An event in...

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Unformatted text preview: Glossary abortive transduction An event in which transducing DNA fails to be incorporated into the recipient chromosome. See transduction. acentric chromosome Chromosome or chromosome fragment “’lll'l IlD centromere. acquired immunodeficiency syndrome (AIDS) An in- fectious disease caused by a retrovirus named the human immun— odeficiency virus {HIV}. The disease is characterized by a gradual depiction of T lymphocytes. recurring fever, weight loss. multiple opportunistic infections. and rare forms of pneumonia and cancer associated with collapse of the immune system. acridine dyes A class of organic compounds that bind to DNA and intercalate into the double—stranded structure. producing local disruptions of base pairing. These disruptions result in nucleotide additions or deletions in the next round of replication. acrocentric chromosome Chromosome with the centrotrtere located very close to one end. Human chromosomes 13, 14. 15, 21, and 22 are acrocentric. active site That portion of a protein. usually an enzyme, whose structural integrity is required for function (e.g., the substrate bind— ing site of an enzyme). adaptation A heritable component of the phenotype that con~ tiers an advantage in survival and reproductive success. The process by which organisms adapt to the current environmental conditions. additive genes See pab'gcnic inheritance. additive variance Genetic variance that is attributed to the sub- stitution of one allele for another at a given iocus. This variance can be used to predict the. rate of response to phenotypic selection in quantitative traits. A-DNA An alternative forln of the right-handed double-helical structure of DNA in which the helix is more tightly coiled, with 11 base pairs per full turn ot'the helix. in the A form, the bases in the helix are displaced laterally and tilted in relation to the longitudi- nal axis. it is not yet clear whether this form has biological significance. albinism A condition caused by the lack of melanin production in the hair, and skin. It is most often inherited as an autosomai recessive trait in humans. aleurone layer in seeds. the outer layer of the endosperm. alkaptonuria An autosomnl recessive condition in humans caused by the lack of the enzyme honiogentisic acid oxidase. Urine of homozygous individuals turns dark upon standing because of ox— idation of excreted homogentisic acid. The cartilage of homozy- gous adults blackens from deposition of a pigment derived from homogentisic acid. Affected individuals often develop arthritic conditions. allele One of the possible mutational states ofa gene, distinguished from other alleles by phenotypic effects. A-1 allele frequency Measurement of the proportion of individuals in a population carrying a particular allele. allele-specific oligonucleotide (ASO) Synthetic nucleotides, usually 15—20 bp in length, that under carefully controlled condir tions will hybridize oniy to a perfectly matching complementary se- quence. Under these conditions, ASOs with a one-nucleotide mismatch will not hybridize. allelic exclusion in a plasma cell heterozygous for an imr munoglobulin gene. the selective action of only one allele. allelism test See complemetnation rest. allolactose A lactose derivative that acts as the inducer for the lac operon. allopatric speciation Process of speciation associated with ge- ographic isolation. allopolyploid Polyploid condition formed by the union of two or more distinct chromosome sets with a subsequent doubling of chroa mosome number. allosteric effect Contbmmiional change in the active site of a protein brought about by interaction with an effector molecule. allotctraploid Diploid for two genomes derived from different species. allozyme An allelic form of a protein that can be distinguished from other forms by electrophoresis. alpha fetoprotein (AFP) A 70—kDa glycoprotein synthesized during embryonic development by the yolk sac. High levels of this protein in the amniotic fluid are associated with neural tube defects such as spina biiida: loweHbanmormal levels may be associated with Down syndrome. alternative splicing Generation of different protein molecules from the same prermRNA by changing the number and order of exons in the mRNA product. Ala sequence An interspersed DNA sequence of approxi- mately 300 bp found in the genome 01" primates that is cleaved by the restriction enzyme Aha]. These sequences are composed of a head-to-tail dimer. The first monomer is approximately 140 hp and the second, approximately 170 bp. In humans, they are dis- persed throughout the genome and are present in 306,000—600,000 copies. constituting sortie 376 percent of the genome. See short in— te rape med 6 t' emen rs. amber codon The cotton UAG, which does not code for an amino acid but for chain termination. Ames test An assay developed by Bruce Antes to detect muta— genic and carcinogenic compounds, using reversion to histidine in- dependence in the bacterium Salmonella ryphimun'um. amino acid Any of the subunit building blocks that are cova— lently linked to form proteins. aminoacyl tRNA Covalently linked combination of an amino acid and a tRNA molecule. anmiocentcsis A procedure used to test for fetal defects in which fluid and fetal cells are withdrawn from the amniotic layer surrounding the fetus. amphidiploid See allotetrttpioirl. anabolism Tire metabolic synthesis ol'complex molecules from less complex precursors. analog A chemical compound structurally similar to another. but dif— fering by a single functional group (e.g., 5—bromodeoxyuridine is an analog of thymidine). anaphase Stage of cell division in which chromosomes begin mov- ing to opposite poles of the cell. anaphase l The stage in the first meiotic division during which mem— bers of homologous pairs of chromosomes separate from one another. aneuploidy A condition in which the chromosome number is not an exact multiple of the haploid set. angstrom (A) Unit of length equal to lt)‘ meters. annotation Analysis of genomic nucleotide seqtrence data to iden- tity the protein—coding genes. the nonprotein-coding genes. their regu— latory sequences. and their functionts). antibody Protein (immunoglobulin) produced itt response to an anti— genic stimulus with the capacit}r to bind specifically to an antigen. anticipation A phenomenon first observed in myotonic dystrophy. where the severity of the symptoms increases from generation to gen- eration and the age of onset decreases from generation to generation. This phenomenon is caused by the expansion of trinucleotide repeats within or near a gene. anticodon The nucleotide triplet in a tRNA molecule that is corn- plementary to and binds with the codon triplet in an mRNA molecule. antigen A molecule, often a cell-surface protein, that is capable of eliciting the formation ol' antibodies. antiparallel Describing molecules in parallel alignment, but run— ning in opposite directions. Most commonly used to desu‘ibe the oppo- site orientations ofthe two strands of a DNA molecule. apoptosis A genetically controlled program of cell death. activated as part of normal development or as a result of cell damage. ascospore A meiotic spore produced in certain fungi. ascus ln fungi. the sac enclosing the four or eight ascospores. asexual reproduction Production ot‘ offspring in the absence of any sexual process. assortative mating Nonrandorn mating between males and females of a species. Selection of males with the same genotype is positive: se— lection of mates with opposite genotypes is negative. ATP Adenosine triphosphate. A nucleotide that is the main energy source in cells. attached-X chromosome Two conjoined X chromosomes that share a single centromere. attenuator A nucleotith sequence between the promoter and the structural gene ol' some operons that regulates the transit of RNA poly- mcrase. reducing transcription ol'the related structural gene. autogamy A process of self—fertilization resulting in homozygosis. autoimmune disease The production of antibodies that results from an immune response to one’s own molecules, cells, or tissues. Such a response results front the inability of the immune system to distin- guish self from nonself. Diseases such as arthritis, scleroderma. sys- Icmic lupus erythematosus, and juvenile-onset diabetes are examples of autoimmune diseases. autonomously replicating sequences (ARS) Origins of repli- cation. about 100 nucleotides in length. found in yeast chromosomes. ARS elements are also present in organelle DNA. aulopolyploidy Polyploid condition resulting from the duplication of one diploid set of chromosomes. autoradiography Production of a photographic image by radioac- tive decay. Used to localize radioactively labeled compounds within cells and tissues. I0 Glossary A—2 autosomcs Chromosomes other than the sex chromosomes. In htt- mans, there are 22 pairs of autosomes. autotetraploitl An autopolyploid condition composed ol‘ tour sim- ilar genomes. In this situation. genes with two alleles (A and u) can have five genotypic classes: AAAA (quadraplex). AAAa (triplex). AAua (du- plex). Anna (simplex). and mum (nulliplex). auxotroph A mutant microorganism or cell line that requires a sub- stance for growth that can be synthesized by wild—type strains. backcross A cross involving an Fl heterozygote and one ot‘ the P, parents (or an organism with a genotype identical to one of the parents). bacteriophage A virus that infects bacteria (also. phage). bacteriophage A A member of the lambdoid family of viruses that attach to, infect. and replicate within bacterial cells. destroying the host cell in the process. Genetically modified lambda phages are used as vec- tors in recombinant DNA research. bacteriophage p. A group of phages whose genetic material be- haves as an insertion sequence that can inactivate host genes and re- arrange host chromosomes. balanced lethals Recessivc. uonallelic lethal genes. each carried on different homologous chromosomes. When organisms carrying bal— anced lethal genes are interbred, only organisms with genotypes identi- cal lo the parents (heterozygotes) survive. balanced polymorphism Genetic polymorphism maintained in a population by natural selection. Barr body Densely staining nuclear mass seen in the somatic nuclei of mammalian females. Discovered by Murray Barr. this body repre— sents an inactivated X chrtmtosome. base analog See analog. base substitution A single base change in a DNA molecule that produces a mutation. There are two types of substitutions: Irmm’limrs. in which a purine is substituted [or a purine or a pyrimidine fora pyrimi— dine: and rl‘rmsvetzw‘onx, in which a purine is substituted for a pyrimidine or vice versa. [3 -galactosidase A bacterial enzyme encoded by the lacZ gene that converts lactose into galactose and glucose. bidirectional replication A mechanism of DNA replication in which two replication forks move in opposite directions from a corn- mon origin of replication. biodiversity The genetic diversity present in populations and species of plants and animals. biometry The application of statistics and statistical methods to bi- ological problems. biotechnology Commercial and/or industrial processes that utilize biological organisms or products. bivalents Synapsed homologous chromosomes in the first prophase of meiosis. Bombay phenotype A rare variant ol'theABO system in which ali- fecled individuals do not have A or B antigens and thus appear as blood type 0. even though their genotype may carry unexpressed alleles for the A and/or B antigens. bottleneck Fluctuation in allele frequency that occurs when a pop- ulation undergoes a temporary reduction in size. BSE (bovine spongiform encephalopathy) A fatal, degener— ative brain disease ol’cattle caused by prion infection. This infection is transmissible to humans and other animals. Also know as rrrad cow disease. Bl‘dU (75- bromodeoxyuridine) A mutagcnically active analog of thymidine in which the methyl group at the 5' position in thymine is re— placed by bromine; also abbreviated BUdR. buoyant density A property of particles (and molecules) that de— pends upon their actual density, as determined by partial specific volume and degree of hydration. Provides the basis for density gradient separa- tion of molecules or particles. A-3 Glossary CAAT box A highly conserved DNA sequence found in the un- translated promoter region ol'eukaryotic genes. This sequence is rec— ognized by transcription factors. CAP Catabolite activator protein: a protein that binds CAMP and reg- tilates the activation of inducible operons. carcinogen A physical or chemical agent that causes cancer. carrier An individual heterozygous for a recessive trait. cambolism A metabolic reaction in which complex molecules are broken down into simpler forms. often accompanied by the release of energy. catabolite activator protein See CAP. catabolitc repression The selective inactivation of an operon by a metabolic product of the enzymes encoded by the operon. cdc mutation A class of cell division cycle mutations in yeasts that affect the tinting and progression through the cell cycle. cDNA DNA synthesized from an RNA template by the enzyme re- verse transcriptase. cDNA library A collection of cloned cDNA sequences. cell cycle Sum of the phases of growth of an individual cell type; divided into GI (gap 1). S (DNA synthesis). G2 (gap 2). and M (mitosis). cell-free extract A preparation of the soluble fraction of cells, tirade by lysing cells and removing the particulate matter, such as nuclei. mem- branes, and organelles. Often used to carry otit the synthesis of proteins by the addition of specific, exogenous mRNA molecules. CEN In yeasts, fragments of chromosomal DNA. about 120 bp in length. that when inserted into plasmids confer the ability to segregate during mitosis. These segments contain at least three types of sequence elements associated with centroinei'e function. centimeter (cm) A unitoflength equal to 10—2 meter. centimorgan (CM) A tinit of distance between genes on chromo- somes. One centimorgan represents a value of 1 percent crossing over between two genes. central dogma The concept that inlbrmalion flow progresses from DNA to RNA to proteins. Although exceptions are known. this idea is central to an understanding of gene function. centric fusion See Robé’I’LYUItfrlH [I‘ll/t.\'/()('£I!f()l!. centriole A cytoplasmic organelle composed of nine groups of mi- crotubules. generally arranged in triplets. Centrioles function in the gen~ eration of cilia and flagella and serve as foci for the spindles in cell division. centromere Specialized region of a chromosome to which sister chromatids remain attached after replication and the site to which spin- dle fibers attach during cell division. Location of the centromerc deter- mines the shape of the chromosome during the anaphase portion of cell division. Also known as the primary constriction. centrosome Region ofthe cytoplasm containing a pair ol'centrioles. chaperone A protein that icgulatcs the folding ol'a polypeptide into a functional three-dimensional shape. character An observable phenolypic attribtitc of an organism. Charon phages A group of genetically modified lambda pliang de— signed to be used as vectors (carriers) for cloning foreign DNA. Named after the feriyman in Greek mythology who carried the souls of the dead across the River Styx. chemotaxis Negative or positive response to a chemical gradient. chiasma (p1,, chiasmata) The crossed strands of nonsister chro- matids seen in diplotenc of the first meiotic division. Regarded as the cy- tological evidence for exchange of chromosomal material. or crossing over. chi-square (X2) analysis Statistical test to determine if an ob- served set of data fits a theoretical expectation. chloroplast A cytoplasmic self-replicating organelle containing chlorophyll. The site ofphotosynthesis. chorionic villus sampling (CVS) A technique of prenatal diag- nosis that intravaginally retrieves chorionic fetal cells and uses them to detect cytogenetic and biochemical defects in the embryo. chromatid One of the longitudinal subunits of a replicath chr0— mosome; it isjoincd to its sister chromaiid at the centromere. chromatin The complex of DNA. RNA. histories, turd nonhistone proteins that make tip uncoilcd chromosomes characteristic of the eu- kaiyotic interphase nucleus. chromatography ’l‘echnique for the separation of a mixture of sol- Ltbilized molecules by their differential migration over a substrate. chromocenter An aggregation ol'ccntromeres and heterochromatic elements of polytenc chromosomes. chromomere A coiled, beadlike region of a chromosome most eas- ily visualized during cell division. The aligned chromomeres of polyteue chromosomes are responsible for their distinctive bruiding pattern. chromosomal aberration Any change resulting in the duplica- tion, deletion. or rearrangement of chromosomal material. chromosomal mutation See chmnmmnmtaberration. chromosomal polymorphism Alternative structures or arrange- ments of a chromosome that are carried by members of a population. chromosome ln prokaryotes, one or more DNA molecules contain- ing the genome: in eukaryotes. a DNA molecule complexed with RNA and proteins to form a tlireadlike structure containing genetic inl’onnalion arranged in a linear sequence and visible tltiring mitosis and meiosis. chromosome banding Technique for the differential staining of mi- totic or meiotic chromosomes to produce a characteristic banding pattem. or selective staining of certain chromosomal regions such as centromeres, the nucleolus organizer regions, and GO or AT-rich regions—nor to be confused with the banding pattern present in polytene chromosomes. which is produced by the alignment ofcliromoineres. chromosome map A diagram showing the location of genes on chromosomes. chromosome puff A localized uncoiling and swelling iii a poly tene elu‘omosomc, usually regarded as a sign of active transcription. chromosome theory of inheritance The idea put forward inde- pendently by Walter Stilton and Theodore Boveri that chromosomes are the carriers of genes and the basis for the Mendelian mechanisms of segregation and independent assortment. chromosome ‘ 'alking A method for analyzing long stretches of DNA. in which the end of a cloned segment of DNA is subcloned and used as a probe to identify other clones that overlap the first clone. Cis configuration The arrangement of two genes or two mutant sites within a gene on the same homolog, such as l 2 (l (1 + + contrasts with a trans arrangement, where the mutant alleles are located on opposite ltomologs. ciS—trans test A genetic test to determine whether two mutations are located within the same cisti'on. cistron That portion ot‘a DNA molecule coding fora single polypep- tide chain; delined by a genetic test as a region within which two muta- tions cannot complement each other. Cline A gradient of genotype or phenotype distributed over a geo— graphic range. Clonal selection Theory of the immune system that proposes that an- tibody diversity precedes exposure to the antigen and that the antigen functions to select the cells containing its specific antibody to undergo prolit'enttion. clone Identical molecules, cells, or organisms derived from a single ancestor by asexual or parasexual methods, such as a DNA segment that has been cnzymalically inserted into a plasmid or Chromosome of a phage or a bacterium and replicated to form many copies. cloned DNA library A collection of cloned DNA molecules rep‘ resenting all or pan of an individual’s genome. codc See genetic code. codominancc Condition in which the phenotypic effects of a gene’s alleles we fully and simultaneously expressed in the heterozygote. codon A triplet of nucleotides that specities or encodes the informa- tion fora single amino acid. Sixty—one codons specify the amino acids used in proteins. and three codons called stop codons signal termina- tion of growth of the polypeptide chain. coefficient of coincidence A ratio ot'ttte observed number of dou— ble crossovers divided by the expected number of strch crossovers. coefficient of inbreeding The probability that two alleles present in a zygote are descended from a common ancestor. coefficient of selection (.9) A measurement of the reproductive disadvantage of a given genotype in a population. For eittunple. if for genotype an only 99 of 100 individuals reproduce, then the selection coellicient is (l. l. colchicine Art alkaloid compound that inhibits spindle formation during cell division. Used in the preparation of karyotypes to collect a large population of cells inltibited at the metaphase stage of mitosis. colinearity The linear relationship between the nucleotide sequence in a gene (or the RNA transcribed from it] and the order of amino acids in the polypeptide chain specified by the gene. competence In bacteria, the transient state or condition during which the cell can bind and internalize exogenous DNA molecules, making transformation possible. complementarity Chemical affinity between nitrogenous bases as a result of hydrogen bonding. Responsible for the base-pairing between the strands of the DNA double helix. complementation test A genetic test to determine whether two mutations occur within the same gene. If two mutations are introduced into a cell simultaneously and produce a wild-type phenotype t i.e.. they complement each other). they are often non-allelic. ll’a mutant phenotype is produced. the mutations are ttoncomplernenting and are often allelic. complete linkage A condition in which two genes are located so close to each other that no recombination occurs between them. complexity The total number of nucleotides or nucleotide pairs in a population of nucleic acid molecules as determined by reassocitttion kinetics. complex locus A gene within which a set of functionally related pscudoalleles can be identified by rccombinational analysis (e.g.. the birliurax locus in Dio.r0/;lzila). complex trait A trait whose phenotype is determined by the inter— action of multiple genes attd environmental factors. corrcatemer A chain or linear series of subunits linked together. The process of forming a concatemer is called concatenation (cg, multiple units ofa phage genome produced during replication). concordance Pairs or groups of individuals identical in their phe- notype. ln twin studies. a condition in which both twins exhibit or fail to exhibit a trait under investigation. conditional mutation A mutation that expresses a wild~type phe- notype under cettain (permissive) conditions and a mutant phenotype under other (restrictive) conditions. con ugation Temporary fusion of two sin gle—cellcd organisms for the sexual transfer of genetic material. consanguineous Related by a comtnon ancestor within the previ~ otrs few generations. consensus sequence A basically common. although not neces— sarily identical. sequence of nucleotides in DNA or amirto acids in proteins. conservation genetics The branch of genetics concerned with the preservation and maintenance of wild species of plants and animals itt their natural environments. Glossary A-4 continuous variation Phenotype variation exhibited by quantita- tive traits distributed from one phenotypic extreme to another in an over— lapping or continuous fashion. cosmid A vector designed to allow cloning of large segments of for- eign DNA. Cosmids are composed of the (or sites of phage A inserted into a plasmid. in cloning, the recombinant DNA molecules are pack— aged into phage protein coats. and after infection of bacterial cells. the recombinant molecule replicates and can be maintained as a plasmid. coupling conformation See ctr configuration. covalent bond A nonionic chemical bond formed by the sharing of electrons. Creutzfeldt—Jakob disease (CJD) A progressive degenerative and fatal disease of the brain and nervous system caused by tntrtations in the prion protein gene on chromosome 20 that produce aberrant forms of the encoded protein. CD] is inherited as art autosomal dominant trait. cri-du-cl‘lal syndrome A clinical syndrome in humans produced by a deletion of a portion of the short arm of chromosome 5. Alllicted infants have a distinctive cry that sounds like a cat. crossing over The exchange of chromosomal material (parts ofchro- mosomal arms) between homologous chromosomes by breakage and reunion. The exchange of material between nonsister chromatids during meiosis is the basis ofgenetic recombination. cross-reacting material (CRM) Nonfunctional form of an en- zyme. produced by a tnutant gene, that is recognized by antibodies tirade against the normal enzyme. C-terminal amino acid The terminal amino acid in a peptide chain that carries a free carboxyl group. C terminus The end of a polypeptide that carries a free carboxyl group of the last amino acid. By convention, the structural formula of polypeptides is written with the C terminus at the right. C value The haploid amount of DNA present in a genome. C value paradox The apparent paradox that there is no relation- ship between the size ofthc genome and the evolutionary complexity of species. For example. the C value (haploid genome size) of amphibians 'aries by a [actor of l00. cyclic adenosine monophosphnte (CAMP) An important reg- ulatory molecule in both prokaryotic and eukaryotic organisms. cyclins A class of proteins found in eukaryotic cells that are synthe— sized and degraded in synchrony with the cell cycle and regulate passage through stages of the cycle. cytogenetics A branch of biology in which the techniques of both cytology and genetics are used to study heredity. cytokinesis The division or separation of the cytoplasm during mi- tosis or meiosis. cytological map A diagram showing the location ofgcncs at par— ticular chromosomal sites. cytoplasmic inheritance Non—Mendelian form of inheritance inv volving genetic information trtmsmitte'd by self—replicating cytoplasmic organelles such as mitochondria, chloroplasts, etc. cytoskeleton An internal array ofmicrotubules, microlilantents, and intermediate filaments that con fch shape and the ability to move on a eukruyotic cell. dalton (Day) A trnit of mass equal to thal of the hydrogen atom, which is 1.67 X HT” gram. A unit used in designating molecular weights. Darwinian fitness Seefintess. deficiency A chromosomal initiation involving the loss or deletion of chromosomal material. degenerate code The genetic code. where a given amino acid may be represented by more than one codon. For example. sortie amino acids tleucine] have six codons, while others (isoleucine') have three. deletion See deficient-y. deme A local interbreeding population]. A-S Glossary denatured DNA DNA molecules that have been separated into sin— gle strands. de novo Newly arising; synthesized from less complex precursors rather than having been produced by modification of an existing molecule. density gradient ccntrifugation A method of separating macro— molecular mixtures by the use of centrifugal force and solutions of vary- ing density. in buoyant density gradient centrifugation using cesium chloride, the cesium solution establishes a gradient under the influence of the centrifugal field and a mixture of macromolecules such as DNA sediment in the gradient until the density of the cesium chloride solution equals their own. separating them by diti'erences in density. deoxyribonuclease A class of enzymes that breaks down DNA into oligonucleotidc fragments by introducing single—stranded breaks into the double helix. deoxyribonucleic acid (DNA) A macromolecule usually con- sisting of antiparallel polynucleotide chains held together by hydrogen bonds. in which the sugar residues are deoxyrihose. The primary carrier of genetic information. dcoxyribose The five—carbon sugar associated with the deoxyri- bonucleotides found in DNA. dermatoglyphics The study of the surface ridges of the skin, es- pecially of the hands and feet. determination A regulatory event that establishes a specific pat- tern of future gene activity and developmental fate fora given cell. diakinesis The final stage of meiotic prophase l in which the chro- tnosomes become tightly coiled and compacted and move toward the pe— riphery of the nucleus. dicentric chromosome A chromosome having two centromercs. dideoxynucleotide A nucleotide containing a dexoyribosc sugar lacking 3’a hydroxyl group. Stops further chain elongation when in— corporated into a growing polynucleotide; used in the Sanger method of DNA sequencing. differentiation The process of complex changes by which cells and tissues attain their adult structure and functional capacity. dihyl)rid cross A genetic cross involving two characters in which the parents possess different forms of each character (cg. yellow. round X green, wrinkled peas). diploid A condition in which each chromosome exists in pairs; hav— ing two of each chromosome. diplotene A stage of meiotic prophase inunediately after pachytene. In diplotene, one pair of sister chrornatids begins separating frorn the other, and chiasrnata become visible. These overlaps move laterally to ward the ends of the chromatids (terrninalixation). directional selection A selective force that changes the frequency of an allele in a given direction, either toward fixation or toward elimination. discontinuous replication of DNA The synthesis of DNA in discontinuous fragments on the lagging strand of the replication fork. The fragments. known as Okazaki fragments. are joined by DNA ligase to form a continuous strand. discontinuous variation Phenotypic data that fall into two or more distinct. nonoverlapping classes. discordance in twin studies, a situation where one twin expresses a trait but the other does not. disjunction The separation of chromosomes at the anaphase stage of cell division. disruptive selection Simultaneous selection for phenotypic ex- tremes in a population, usually resulting in the production of two phe- notypically discontinuous strains. dizygotic twins Twins produced from separate fertilization events; two ova fertilized independently. Also known as fi'atemal twins. DNA See (160.13ribonucleic acid. DNA fingerprinting A molecular method for identifying an indi- vidual member of a population or species. The pattern of DNA frag- ments is obtained by restriction enzyme digestion, followed by Southern blot hybn'dization using minisatellite probes. See also STR sequences. DNA footprinting Secfootprt‘nfing. DNA gyrasc One of the DNA topoisomerases that acts during DNA replication to reduce molecular tension caused by supercoiling. DNA gyrase produces, then seals double-stranded breaks. DNA ligase An enzyme that forms a covalent bond between the 5’ end ofone polynucleotide chain and the 3’ end ol'another polyntlcleotide chain. it is also called polynucleotide-joining enzyme. DNA polymerase An enzyme that catalyzes the synthesis of DNA from dcoxyribonucleotides and a template DNA molecule. DNase Deoxyribonucleosidase; an enzyme that degrades or breaks down DNA into fragments or constitutive nucleotides. dominance The expression ol'a trait in the heterozygous condition. dominant suppression A form of epistusis in which a dominant: at- lele at one locus suppresses the effect of a dominant allele at another locus, resulting in a |323 phenotypic ratio. dosage compensation A genetic mechanism that regulates the lev- els of gene products at certain loci on the X chromosome in mammals such that males and females have equal amounts of a gene product. In mammals. this is accomplished by random inactivation of one X ehrmnosome. double crossover Two separate events of chromosome breakage and exchange occurring within the same letrad. double helix The model for DNA structure proposed by James Wat— son and Francis Crick, involving two antiparallel hydrogen—bonded polynocleotide chains wound into a right-handed helical configuration. with 10 base pairs perfull turn of the double helix. Often called B—DNA. Duclrenne muscular dystrophy An X—linked recessive genetic disorder caused by a mutation in the gene for dystrophin. a protein found in muscle cells. Affected males show a progressive weakness and wast- ing of muscle tissue. Death ensues by about age '20 caused by respira— tory infection or cardiac failure. duplication A chromosomal aberration in which a segment of the chromosome is repeated. dyad The products of tetrad separation or disjunction at the first meiotic prophasc. Consists of two sister chromatids joined at the centrornere. dystrophin A protein that attaches to the inside of the muscle cell plasma membrane and stabilizes the membrane during muscle contrac- tion. Mutations in the gene encoding dystrophin cause Duchenne and Becker muscular dystrophy. See Duclrenne muscular dystrophy. effective population size The number of individuals in a popula- tion with an equal probability of contributing gametes to the next generation. effector molecule Small, biologically active molecule that regu- lates the activity of a protein by binding to a specific receptor site on the protein. electrophoresis A technique that separates a mixture of molecules by their differential migration through a stationary medium (such as a gel) in an electrical field. endocytosis The uptake by a cell of fluids, macromolecules. or particles by pi noeytosis, phagocytosis, or receptor-mediated endocytosis. endomitosis Chromosomal replication that is not accompanied by either nuclear or cytoplasmic division. cndonuclease An enzyme that hydrolyzes internal phosphodiestcr bonds in a polynucleotide chain or nucleic acid molecule. endoplasmic reticulum A membranous organelle system in the cy- toplasm of eukaryotic cells. The outer surface of the membranes may be ribosome-studded ( rough ER) or smooth ER. endopolyploidy The increase in chromosome sets that results from endornitotic replication within somatic nuclei. endosymhiont theory The proposal that self -replicating cellular or- ganelles such as mitochondria and chloroplasts were originally free living organisms that entered into a symbiotic relationship with nucleated cells. enhancer Originally identified as a 72-bp sequence in the genome of a virus. SV4U. that increases the transcriptional activity of nearby structural genes. Similar sequences that enhance transcription have been identified in the genomes of eukaryotic cells. Enhancers can act over a distance of thousands of base pairs and can be located upstream, down- stream. 5'. 3‘. or intemal to the gene they affect. and thus are different from promoters. environment The complex of geographic, climatic. and biotic fac tors within which an organism lives. enzyme A protein or complex of proteins that catalyzes a specific biochemical reaction. cpigenesis The idea that an organism develops by the appearance turd growth of new structures. Opposed to preforrmttionism, which holds that development is the growth of structures already present in the egg. episome A circular genetic element in bacterial cells that can repli- cate independently of the bacterial chromosome or integrate and replicate as part of the chromosome. epistasis Nonrecipr‘ocal interaction between genes such that orre gene interferes with or prevents the expression of another gene. In Dr'osophila for example. the recessive gene eye/ears. when homozygous. prevents the expression of eye color genes present in the genome. epitope That portion of a macromoleculc or cell that acts to elicit an antibody response; an antigenic determinant. A complex molecule or cell can contain several such sites. equational division A division of each chromosome into longitu- dinal halves that are distributed into two daughter nuclei. Chromosome division in mitosis is an example ol‘equational division. equatorial plate See meltrphase plate. euchromatin Chromatin or chromosomal regions that are lightly staining and are relatively uncoiled dtrring the interphase portion of the cell cycle. Euchromatic regions contain most of the structural genes. eugenics The improvement of the human species by selective breed- ing. Positive eugenics refers to the promotion of breeding ofpeople with favorable genes. and negative eugenics refers to the discouragement of breeding among those with undesirable traits. eukaryotes Organisms having true nuclei and membranous or- ganelles and whose cells demonstrate mitosis and meiosis. euphenics Medical or genetic intervention to reduce the impact of de— tective genotypes. ellploid Polyploid with a chromosome number that is an exact mul— tiple of a basic chromosome set. evolution The origin of plants and animals from preexisting types. Descent with modifications. excision repair Removal of damaged DNA segments followed by repair. Excision can include the removal of individual bases (base re- pair) or a stretch of damaged nucleotides (nucleotide repair). The gap cre- ated by excision is filled by DNA polymerase, and the ends are ligated to form an intact molecule. cxon (extron) The DNA segments of a gene that are transcribed and translated into proteins. exonucleasc An enzyme that breaks down nucleic acid molecules by breaking the phosphodiestcr bonds at the 3’- or 5’-terminal nucleotides. expressed sequence tags (ESTs) All or part of the nucleotide se~ quence of CDNA clones. Used as markers in construction of genetic maps. expression vector Plasmids or phages carrying promoter regions designed to cause expression of inserted DNA sequences. Glossary A-6 expressivity The degree or range in which a phenotype for a given trait is expressed. extranuclear inheritance Transmission of traits by genetic iti- fomration contained in cytoplasmic organelles such as mitochondria and chloroplasts. F_ cell A bacterial cell that does not contain a fertility factor, and that acts as a recipient in bacterial conjugation. F+ cell A bacterial cell containing a fertility factor, and that acts as a donor in bacterial conjugation. F factor An episome in bacterial cells that confers the ability to act as a donor in conjugation (also, fertilityfactor). F’ factor A fertility factor that contains a portion of the bacterial chromosome. Fl generation First filial generation: the progeny resulting from the first cross in a series. F2 generation Second lilial generation: the progeny resulting from a cross of the F1 generation. F pilus See piltrr. familial trait A trait transmitted through and expressed by inerti— bers of a family. fate map A diagram of an embryo showing the location of cells whose development fate is known. fertility factor See F flrc'lol‘. filial generations See Fl. F3 generrrlimis. fingerprint The unique pattern ol'ridges and whorls on the tip ofa htrman linger. Also. the pattem obtained by enzymatically cleaving a protein or nucleic acid and subjecting the digest to two-dimensional chromatography or electrophoresis. See also DNAfingerprin/r’ng. FISH See fluorescence in .u‘tu hybridization. fitness A measure of the relative survival and reproductive success of a given individual or genotype. fixation la population genetics, a condition in which all members of a population are homozygous for a given allele. fluctuation test A statistical test developed by Salvadore Luria and Max Delbriick to determine whether bacterial mutations arise sponta- neously or are produced in response to selective agents. fluorescence in situ hybridization (FISH) A method of in rim hybridization that utilizes probes labeled with a fluorescent tag, causing the site of hybridization to lluoresce when viewed in ultraviolet light under a microscope. flush—crash cycle A period of rapid population growth followed by a drastic reduction in population size. f-met Seefimnylmelhirmine. folded-fiber model A model ol‘eukaryotic chromosome organiza- tion in which each sister chrornalid consists of a single fiber. composed of double—stranded DNA and protein, which is wound like a tightly coiled skein of yarn. footprintng A technique for identifying a DNA sequence that binds to a particular protein, based on the idea that the phosphodiester bonds in the region covered by the proteitr are protected from digestion by deoxyribontlcleases. See also DNA firolprintt'ng. formylmethionine (f—met) A molecule derived from the amino acid methionine by attachment of a formyl group to its terminal atnino group. This is the first amino acid inserted in all bacterial polypeptides, Also known as N~formyl methionine. founder effect A form of genetic drift. The establishment ol‘a pop— ulation by a small number of individuals whose genotypes carry only a fraction of the diflerent kinds of alleles in the parental population. fragile site A heritable gap or nonstaining region ofa chromosome that can be induced to generate chromosome breaks. fragile X syndrome A genetic disorder caused by the expansion of :1 COO trimrcleotide repeat and a fragile site at Xq27.3 within Ihe FMR- I gene. Fragile X syndrome is the most common form of mental A-7 Glossary retardation. Affected males have distinctive facial features and are mett- tally retarded. Carrier females lack physical symptoms but as a group have higher rates of mental retardation than normal individuals. frameshift mutation A mutational event leading to the insertion of one or more base pairs in a gene. shifting the codon reading frame in all codons that follow the mutational site. fraternal twins See dizygmic twins. G1 checkpoint A point in the G] phase of the cell cycle when a cell becomes committed to initiate DNA synthesis and cotttinue the cycle or withdraw into the GO resting stage. GO A point in the G1 phase where cells withdraw from the cell cycle and enter a nondividing but metabolically active state. gamete A specialized reproductive cell with a haploid number of chromosomes. gap genes Genes expressed in contiguous domains along the anterior—posterior axis of the Drosrmlu'ht embryo that regulate the process of segmentation in each domain. gene The fundamental physical unit of heredity whose existence can be confirmed by allelic variants anti which occupies a specific chromo- somal locus. A DNA sequence coding fora single polypeptide. gene amplification The process by which gene sequences are selected and differentially replicated either extrachromosomally or intrachromosomally. gene conversion The process of nonreciprocal recombination by which one allele in a heterozygotc is converted into the corresponding allele. gene duplication An event in replication leading to the production of a tandem repeat of a gene sequence. gene flow The gradual exchange of genes between two popula— tions; brought about by the dispersal of gametes or the migration of individuals. gene frequency The percentage of alleles of a given type in a population. gene interaction Production of novel phenotypes by the interac« tion of alleles of different genes. gene mutation See point mutation. gene pool The total of all alleles possessed by reproductive members of a population. generalized transduction The transduction of any gene in the bacterial genome by a phage. genetically modified organism (GMO) A plant or animal that mimics a gene from another species transferred to its genome using re- combinant DNA technology. and where the gene is expressed to produce a gene product. genetic anticipation The phenomenon of a progressively earlier age of onset and increasing severity ofsymptoms in successive genera- tions for a genetic disorder. See nntici/mrimt. genetic background All genes carried in the genome other than the one being studied. genetic code The nucleotide triplets that code for the 20 amino acids or for chain initiation or termination. genetic counseling Analysis ofrisk for genetic defects iii a famin and the presentation ot‘options available to avoid or ameliorate possible risks. genetic drift Random variation in allele frequency from generation to generation. most often observed in small populations. genetic engineering The technique of altering the genetic consti- tution of cells or individuals by the selective removal, insertion. or mod» ification of individual genes or gene sets. genetic equilibrium Maintenance ofallele frequencies at the same value in successive generations. A condition in which allele frequencies are neither increasing ttor decreasing. genetic erosion The loss of genetic diversity from a population or a species. genetic title structure lntragcnic recombinational analysis that provides mapping information at the level of individual ttucleotides. genetic load Average number of recessive lethal genes can‘ied in the heterozygous condition by an individual in a population. genetic polymorphism The stable coexistence of two or more dis- continuous genotypcs in a population. When the frequencies of two al- leles are carried to an equilibrium. the condition is called balanced polymorphism. genetics The branch of biology that deals with heredity and the ex- pression of inherited traits. genome The set of genes carried by an individual. genomic imprinting A condition where the expression of a gene depends on whether the gene has been inherited from a male or a fe- male parent. genomics The study of genomes. including nucleotide sequence. gene content. organization. and gene number. genotype The specific allelic or genetic constitution ofzui organism: often. the allelic composition ot‘onc or a limited number of genes under ittvestigation. germ line cells (eggs and sperm). germ plasm Hereditary material transmitted from generation to generation. Goldberg—Hogness box A short nucleotide sequence 20—30 bp upstream from the initiation site of eukaryotic genes to which RNA polymerase II binds. The consensus sequence is TATAAAA. Also known as TATA box. graft-versus-host disease (GVHD) ln transplants, reaction by immunologically competent cells of the donor against the antigens pre— sent on the cells ofthc ltost. Often a fatal condition in hutnan bone mar- row transplants. green revolution A program that resulted in a two- to threefold in- crease in crop yields generated by the development of new varieties of cereal plants with shorter stems and increased disease resistance. gynandromorph An individual composed ofcells with both male and female genotypes. gyrase One ofa class ofcnzyntes known as topoisomcrases. Gyrase converts closed circular DNA to a negatively supercoilcd form prior to replication. transcription. or recombination. See DNA gymse. haploid A cell or organism having a single set of unpaired chromo- somes. The gametic chromosome number. haplotype The set of alleles from closely linked loci carried by an individual and usually inherited as a unit. H ardy—Weinberg law The principle that both gene and genotype frequencies will remain in equilibrium iii an infinitely large population in the absence of mutation. migration. selection. and nonrandom mating. heat shock A transient response following exposure of cells or or ganisms to elevated temperatures. The response involves activation of a small number of loci. inactivation of some previously active loci. and se- lective translatit'm of heat shock mR NA. Appears to be a nearly univer— sal phenomenon observed in organisms ranging ftotn bacteria to humans. helicase An enzyme that participates itt DNA replication by un- winding the double helix near the replication fork. helix—turn—hclix motif The structure of a region of DNA-binding proteins in which a tttm of four atnino acids holds two a helices at right angles to each other. hemizygous Conditions where a gene is present in a single dose in an otherwise diploid cell. Usually applied to genes on the X chromosome in heterogametic males. hemoglobin (Hb') An iron-containing. oxygen-carrying protein oc— curring chiefly in the red blood cells of venebrales. hemophilia An X—linkcd trait in humans associated with defective bloodvclotting mechanisms. An embryonic cell lineage that forms the reproductive heredity Transmission of traits from one generation to another. heritability A measure of the degree to which observed phenotypic differences for a trait are genetic. helerochromatin The heavily staining. late-replicating regions of chromosomes that are condensed in interpltase. Thought to be devoid of structural genes. hctcroduplex A double-stranded nucleic acid molecule in which each polynucleotide cltaitt has a different origin. These structures may be produced as intermediates in a recombinational event or by the in vim! rcannealing of single-stranded. complementary molecules. hetcrogametic sex The sex that produces gametes containing an- like sex chromosomes. heterogeneous nuclear RNA (hnRNA) The collection ot'RNA transcripts in the nucleus. representing precursors and processing inter- mediates to rRNA. mRNA. and IRNA. Also represents RNA transcripts tltat will not be transported to the cytoplasm. such as snRNA. lteterokaryon A somatic cell containing nuclei frotn two different sottrces. helerozygote An individual with different alleles at one or more loci. Such individuals will produce unlike gametes and therefore will not breed true. llt'r A strain of bacteria exhibiting a high frequency of recombina- tiott. These strains have a chromosomally integrated F factor that is able to mobilize and transfer part of the chromosome to a recipient F‘ cell. hislocompatibility antigens See JIM. histones Proteins complexed with DNA irt tlte nucleus. They are rich in the basic amino acids arginine tutti lysine. and function in coiling DNA to form nuclcosomcs. “LA Cell-surface proteins. prodttccd by histocompatibility loci. in- volved in the acceptance or rejection of tissue and organ grafts and transplants. 1111 RNA Sec hetc’rogwicous nuclear RNA. l—lolliday structure An intermediate in bidirectional DNA recom- bination seen in the transmission electron microscope as an X-sltaped structure showing four single—stranded DNA regions. homeobox A sequence of about 180 nucleotides that encodes a se- quence of 60 atnino acids called a Itonteodomain. which is part of a DNA-binding protein that acts as a transcription factor. homeotic mutation A mutation that causes a tissue normally de— termined to form a specific organ or body part to alter its differentiation and form another structure. homogamctic sex The sex that produces gametes that do not ditl fer with respect to sex chromosome content; in mammals. the tetttale is homogametic. homologous chromosomes Chromosomes that synapse or pair during meiosis. Chromosomes that are identical with respect to their ge- netic loci and centromer'e placement. homozygote An individual with identical alleles atone or more loci. These individuals will produce identical gametes and will therefore breed true. homunculus The miniature individual imagined by prel'ormationists to be contained within the sperm or egg. H substance The carbohydrate group present on the surface of red blood cells. When unmodified, it results in blood type 0; when modified by the addition of monosaccharidcs. it results in types A. B. and AB. human immunodeficiency virus t HIV) A ltutnart retrovirus associated with the onset and progression of AIDS. See acquired im- mrurruldiciencv .t‘)‘ll([l‘um¢’. hybrid Art individual produced by crossing two parents ofdifferent genotypes. hybrid Vigor The superiority of a heterozygotc over either homozygotc for a given trait. Glossary A-S hydrogen bond An electrostatic attraction between a hydrogen atom bonded to a strongly electronegative atom such as oxygen or ni- trogen artd another atom that is electronegativc or contains an un- shared electron pair. hypervariable regions The regions ot'antil‘tody molecules that at- tach to antigens. These regions ltave a high degree ofdiversity in amino acid content. identical twins Sec nmnnqvgorr‘c Mitts. lg Seeimmmmglobu(in. imaginal disc Discrete groups of cells set aside during embryoge- nesis in holometabolous insects, which are determined to form the ex- terttal body parts of the adult. immunoglobulin (lg) ertics of antibodies. inborn error of metabolism A genetically controlled biochem- ical disorder: usually an enzyme defect that produces a clinical syndrome. inbreeding Mating hetweert closely related organisms. inbreeding depression A decrease in viability, vigor. or growth itt progeny after several generations of inbreeding. incomplete dominance Expression of heterozygous phenotype that is distinct from and often intermediate to that of either parent. incomplete linkage Occasional separation of two genes on the same chromosome by a rccombinational cvcnt. independent assortment The independent behavior of each pair of homologous chromosomes dttring their segregation in meiosis l. The random distribution of maternal and paternal homologs into gametes. inducer An effector molecule that activates transcription. indttcible enzyme system Art enzyme system under the control of a regulatory molecule. or inducer, which acts to block a repressor and allow transcription. initiation codon The triplet of nucleotides (AUG) in an ntRNA rnolectrlc that codes for the insertion of the amino acid methionine as the first amino acid in a polypeptide chain. insertion sequence See IS element. in Si!!! hybridization A technique for the cytological localiza- tion of DNA sequences complementary to a given nucleic acid or polynuclcotide. inter "alating agent A compound that inserts between bases in a DNA molecule. disrupting the alignments anti pairing of bases in the complementary strands (cg. acridinc dyes). interference A measure of the degree to which one crossover affects the incidence of another crossover in an adjacent region of the same cltromatid. Negative interference increases the chances of another crossover; positive interference redttces the probability of a second crossover event. interphase That portion of the cell cycle between divisions. intervening sequence See inn-on. intron A portion of DNA between coding regions in a gene that is transcribed bttt does not app ‘ar in the mRNA product. inversion A chromosomal aberration in which the order of a chromosomal segment has been reversed. inversion loop The chromosomal cortliguration resulting front the synapsis of homologous chromosomes. one of which carries an inversion. in vitro Literally. in glass; outside the living organism: occurring in an artificial environment. in vivo Literally. in the living: occurring within the living body of an organism. IS eletnenl A mobile DNA segment that is transposahle to any of a number ofsites in the genome. isoagglutinogen An antigenic factor or substance present on the surface of cells that is capable of inducing the formation of art antibody. The class of serum proteins having the prop- A-9 Glossary isochromosome An aberrant chromosome with two identical arms and homologous loci. isolating mechanism Any barrier to the exchange of genes be- tween different populations of a group of organisms. In general, isola- tion can be classified as spatial. environmental. or reproductive. isotopes Forms ofchemical elements that have the same number of protons and electrons but differ in the number of neutrons contained in the atomic nucleus. isozyme Any of two or more distinct forms of an enzyme that have identical or nearly identical chemical properties bttt differ in some prop— erty such as net electrical charge, pl-l optima. number and type of sub- units. or substrate concentration. K particles DNA-containing cytoplasmic particles found in certain strains of Paramecium aura/Eu. When these self-reproducing particles are transt‘en‘ed into the growth medium. they release a toxin, paramecin. that kills other sensitive strains. A nuclear gene, K. is responsible for maintaining kappa particles in the cytoplasm. karyokinesis The process of nuclear division. karyotype The chromosome complement of a cell or an individual. Often used to refer to the arrangement of metaphase chromosomes in a sequence according to length and position of the cetttromere. kilobase (kb) A ttnit of length consisting of IOOO nucleotides. kinetochore A fibrous structure with a size of about 400 ntn. lo- cated within the centromere. It appears to be the site of microtubule at- tachment during division. Klinefelter syndrome A genetic disorder in human males caused by the presence of an extra X chromosome. Klinefelter males are XXY instead of XY. This syndrome is associated with enlarged breasts, small testes, sterility, and occasionally, mild mental retardation. knockout mice ln producing knockout mice, a cloned normal gene is inactivated by the insertion ofa marker. such as an antibiotic resistance gene. The altered gene is transferred to embryonic stem cells. where the altered gene will replace the normal gene (in some cells). These cells are injected into a blastomere embtyo, producing a mouse that is bred to yield mice homozygous for the mutated gene. lac repressor protein A protein that binds to the operator in the {ac operon and blocks transcription. lagging Strand In DNA replication, the strand synthesized in a dis continuous fashion. 5’ to 3' away from the replication fork. Each short piece of DNA synthesized in this fashion is called an Okazaki fragment. lampbrush chromosomes Meiotic chromosomes characterized by extended lateral loops, which reach maximum extension during diplotene. Although most intensively studied in amphibians, these structures occur in meiotic cells ol'organistns ranging from insects to humans. lariat structure A structure formed by an intron via a 5' to 3’ bond during processing and removal of that intron from an mRNA molecule. leader sequence That portion of an mRNA molecule from the 5’ end to the beginning codon; may contain regulatory or ribosome bind- ing sites. leading strand During DNA replication, the strand synthesized continuously 5' to 3’ from the origin of replication toward the repli- cation fork. leptotene The initial stage of meiotic prophase 1. during which the chromosomes become visible and are often arranged in a bouquet con- figuration, with one or both ends of the chromosomes gathered at one spot on the inner nuclear membrane. lethal gene A gene whose expression results in death. leucine zipper A structural motif in a DNA-binding protein that is characterized by a stretch of leucine residues spaced at every seventh amino acid residue, with adjacent regions of positively charged amino acids. Leucine zippers on two polypeptides may interact to fortn a dimer that binds to DNA. linkage Condition in which two or ntore nonallelie genes tend to be inherited together. Linked genes have their loci along the same chro- mosome; they do not assott independently, but can be separated by cross- in g over. linkage group A group of genes that have their loci on the same chromosome. linking number The number oftimes that two strands ofa closed. circular DNA duplex cross over each other. locus [pl., loci) The site or place on a chromosome where a pattie- ular gene is located. [or] score A statistical method used to determine whether two loci are linked or unlinked. A lod (log ol'the odds) score of 4 indicates that link- age is lt),()()() times more likely than nonlinkage. By convention. lod scores of 3—4 are signs of linkage. long interspersed elements (LINES) Repetitive sequences found in the genomes ol'higher organisms, such as the 6-H) Ll sequences found in primate genomes. long terminal repeat (LTR) Sequence of several hundred ba5c pairs found at the ends of retroviral DNAs. Lyon hypothesis The random inactivation of the matemal or pa- ternal X chromosome in somatic cells of mammalian females early in development. All daughter cells will have the same X chromosome inactivated, producing a mosaic pattern of expression of genes on the X chromosome. lysis The disintegration of a cell brought about by the rupture of its membrane. lysogenic bacterium A baclerial cell carrying the DNA ofa tem- perate bacteriophage integrated into its chromosome. lysogeny The process by which the DNA of an infecting phage be- comes repressed and integrated into the chromosome of the bacterial cell it infects. Iytic phase The condition in which a temperate bacteriophage loses its integrated status in the host chromosome (becomes induced). repli- cates. and lyses the bacterial cell. major histocompatibility (MHC) loci complex; and in mice, the [[2 complex. Inappng functions Map distance estimates from recombination when the recombination frequency in a region exceeds 15—20 percent. and double crossovers are undetectable. map unit A measure of the genetic distance between two genes. cor- responding to a recombination frequency of 1 percent. See cenrt'morgun. maternal effect Phenotypic effects on the offspring produced by the maternal genome. Factors transtnitted through the egg cytoplasm that produces a phenotypic effect in the progeny. maternal influence See maternal ejj'ect. maternal inheritance The transmission of traits via cytoplasmic genetic factors such as mitochondria or chloroplasts. mean The arithmetic average. median The value in a group of numbers below and above which there is an equal ntttnbcr of data points or measurements. meiosis The process in gametogenesis or sporogenesis during which one replication of the chromosomes is followed by two nuclear divi- sions to produce four haploid cells. melting profile (T m) The temperature at which a population of doublestrandcd nucleic acid molecules is half-dissociated into single strands. This is taken to be the melting temperature for that species of nucleic acid. merozygote A partially diploid bacterial cell containing. in addition to its own chromosome, a chromosome fragment introduced into the cell by transformation, transduction. or conjugation. messenger RNA (mRNA) An RNA molecule transcribed from DNA and translated into the amino acid sequence of a polypeptide. In humans, the HLA metabolism The sum of chemical changes in living organisms by which energy is generated and used. n‘tetacentric chromosome A chromosome with a centrally lo- calcd ccntromerc. producing chromosome arms of equal lengths. tnetafemale ln DmsrnJhIIa, a poorly developed female of low via- bility in which the ratio of X chromosomes to sets of autosomes exceeds LO. Previously called a supertemale. metamale In [)msophilu. a poorly developed male of low viability in which the ratio ofX chromosomes to sets of autosomcs is less than 0.5. Previously called a supermale. melaphase The stage of cell division in which condensed chro— mosomcs lie in a central plane between the two poles of the cell and during which the chromosomes become attached to the spindle fibers. ntctaphase plate The arrangement of mitotic or meiotic chromo~ stunes at the equator of the cell during metaphase. methylation Enzymatic transfer of methyl groups from S adcnosylmethionine to biological molecules including phospho— lipids, proteins, RNA. and DNA. Methylation of DNA is associated with reduction in gene expression and with epigenetic phenomena such as imprinting. M HC See major lti.s‘l‘()('()lttpulibt/ity loci. micrometer (pm) A ttttit of length equal to l X Ill—‘7' meter. Pre- viously called a micron. micron Seemicmmct‘er. migration coefficient An expression of the proportion of migrant genes entering the population per generation. millimeter (mm) A unit ot‘length equal to l X 10—3 meter. minimal medium A medium containing only those nutrients that will support the growth and reproduction of wild-type strains of an organism. minisatellite Short tandem repeats of 10—100 nucleotides widely dispersed in the genome of eukaryotes. The number of repeats at each locus is variable: these loci zuc known as variable number tandem repeats [VNTRs). Each variation represents a VNTR allele. and many loci have dozens of alleles. VNTRs are used in DNA fingerprinting. See also STR sequences. mismatch repair A process of excision repair, dttring which an un- paired base or bases is excised, followed by the synthesis of a new seg- ment, using the complementary strand as a template. missense mutation A mttlalion that alters a codon to that of an- other :tmino acid, causing an altered translation product to be tnade. mitochondrial DNA (mtDNA) Double—stranded. self-replicating circular DNA found in mitochondria. mtDNA encodes mitochondrial ribosomal RNAS. transfer RNAs. and proteins used in oxidative respi‘ ratory functions of the organelle. tnitochondrion Found in the cells of eukaryotes. a cytoplasmic, self—reproducing organelle that is the site ofATP synthesis. mitogen A substance that stimulates mitosis in nondividing cells ('c.g., phytohemagglutinin). mitosis A form of cell division resulting in the production of two cells. each with the same chromosome and genetic complement as the parent cell. mode In a set of data, the value occurring in the greatest frequency. tnonohybrid cross A genetic cross between two individuals in- volving only one character (e.g.. AA X on). monosomic An ancuploid condition in which one member ofu chro mosome pair is missing: having a chromosome mtmber of Zn — l. monozygotic twins Twins produced from a single fertilization event; the first division of the zygote produces two cells. each of which develops into an embryo. Also known as identical twins. In RNA See messenger RNA. mtDNA See mitochomlriu/ DNA. Glossary A-1O mulligene family A gene set descended from a common ancestor by duplication and subsequent divergence from a common ancestor. The globin genes are an example ofa mulligcne fatnily. multiple alleles Three or tnorc alleles ofthc same gene. multiple-factor inheritance Sec [Io/menic inheritance. multiple infection Simultaneous infection of a bacterial cell by more than one bacteriophage, often ofdiffcrcnt genotypes. mu ((1.) phage A phage group itt which the genetic material be- haves like an insertion sequence; capable of insertion, excision, trans- position. inactivation of host genes. and induction of chromosomal rearrangements. See bacteriophage fJ.. mutagen Any agent that causes an increase in the rate of mutation. mutant A cell or organism carrying an altered or mutant gene. mutation The process that produces art alteration in DNA or chro- mosome structure; the source of most alleles. mutation rate The frequency with which mutations take place at a given locus or in a population. muton The smallest unit of Imitation in a gene, corresponding to a sin- gle base change. nanometer (nm) A unit of length equal to | X 10—9 meter. natural selection Differential reproduction ot'some members ol‘a species resulting from variable fitness conferred by genotypic differences. neutral mutation A mutation with no immediate adaptive signif- icance or phenotypic effect. nonaulonomous transposon functional transposase gene. noncrossover gamete A gamete that contains no chromosomes that have undergone genetic recombination. nondisjnnction chromosomes (in meiosis) or the sistcrchrolnatids [in mitosis) fail to scp- aratc and migrate to opposite poles; responsible for defects such as monosomy and trisomy. nonsense codon The nucleotide triplet in an mRNA molecule that signals the termination of translation. Three such termination codons are known: UGA, UAG. and UAA. nonsense mutation A mutation that changes an atnino acid cotton into a tennination codon: UAG. UAA, or UGA. Leads to prematttrc Icr- minalion during translation of mRNA. NOR Sec uric/color organizer region. normal distribution A probability function that approximates the distribution of random variables. The normal curve. also known as a Gaussian or bell-shaped curve, is the graphic display of the “mural distribution. northern blot A technique in which RNA molecules are separated by electrophoresis and transferred by capillary action to a nylon or ni- trocellttlose membrane. Specific RNA molecules can be identified by hy— bridization to a labeled nucleic acid probe. N-terminal amino acid The terminal amino acid in a peptide chain that carries a free atnino group. N terminus The free atnino group of the first amino acid in a polypeptide. By convention, the structural formula of polypeptides is written with the N terminus at the left. nuclease An enzyme that breaks bonds in nucleic acid molecules. nucleoid The DNA-containing region within the cytoplasm in prokaryotic cells. nucleolar organizer region (NOR) A chromosomal region con— taining the genes for rRNA; most often found in physical association with the nucleolus. nucleolus A nuclear organelle. that is the site of ribosomc biosyn— thesis: usually associated with or formed in association with the NOR. nucleosidc A purine or pyrimidine base covalently linked to a ri- bose or deoxyribose sugar molecule. A transposable element that lacks a An error during cell division in which homologous A-11 Glossary nucleosome A complex of four historic molecules, each present in duplicate. wrapped by two trims of a DNA molecule. One of the basic units of eukaryotic chromosome structure. nucleotide A nucleoside covalently linked to a phosphate group. Nucleotides are the basic building blocks of nucleic acids. The nucleotides commonly found in DNA are deoxyadcnylic acid. deoxycytidylic acid, deoxyguanylic acid. and dcoxythymidyli ‘ acid. The nucleotides in RNA are adenylic acid, cytidylic acid. guanylic acid. and uridylic acid. nucleotide pair The pair ol'nuclcotidcs [A and T or G and C) in op- posite strands ofa DNA molecule that are hydrogen-bonded to eaclt other. nucleus The membrane-bound cytoplasmic organelle ol' eukaryotic cells that contains the Chromosomes and nucleolus. null allele A mutant allele that produces no functional gene product. Usually inherited as a recessive trait. null hypothesis Used in statistical tests. it states that there is no dif- ference between the observed and expected data sets. Statistical meth- ods such as chi-square analysis are used to test the probability of this hypothesis. nullisomic Describes an individual with a chromosomal aberration in which both members ot‘a chromosome pair are missing. Okazaki fragment The small. discontinuous strands of DNA pro- duced on the lagging strand during DNA synthesis. oligonucleotide A linear sequence of about 10—20 nucleotides con- nected by 5'—3' phosphodiester bonds. oncogene A gene whose activity promotes uncontrolled prolifera- tion in eukaryotic cells. open reading frame (ORF) A nucleotide sequence organized as triplets that encodes the amino acids of a protein. Located between an initiation codon and a tennination codon within a gene. operator region Ar‘egion ot‘a DNA molecule that interacts with a specific repressor protein to control the expression of an adjacent gene or gene set. operon A genetic unit consisting of one or more structural genes en< coding polypeptides. and an adjacent operator gene that controls the transcriptional activity of the structural gene or genes. origin of replication ('ori) Sites along the length of a chromo- some where DNA replication begins. outbreeding depression Reduction in fitness in the offspring pro- duced by mating genetically diverse parents. It is thought to result from a lowered adaptation to local environmental conditions. overdominance The phenomenon where Iteterozygotes have a phe- notype that is more extreme than either homozygous genotype. overlapping code A genetic code first proposed by George Gamow in which any given nucleotide is shared by three adjacent codons. pachyteuc The Stage in meiotic prophasc I when the synapsed ho- mologous chromosomes split longitudinally (except at the ccntromere). producing a group of four chromatids called a tetrad. pair-rule genes Genes expressed as stripes around the blastodcnn embryo during development of the Droraplri/a embryo. palindrome A word. number. verse. or sentence that reads the same backward or forward (cg. It's [mu (m a visit). In nucleic acids. a se- quence in which the base pairs read the same on complementary strands in the 5’ to 3’ direction. For example: 5’-GAAT'I‘C-3’ 3’-C'I‘TAAG-5’ These often occur as sites for restriction endonuclease recognition and cutting. pangenesis A discarded theory of development that postulated the existence of pangenes. small particles from all parts of the body that concentrated in the gametes. passing traits from generation to generation. blending the traits ofthe parents in the offspring. paracentric inversion A chrotnosomal inversion that does not in- clude the centromere. parental gamete See nancmssm-‘er gamete. parthenogenesis Development of an egg without fertilization. partial diploids See nwruzygale. partial dominance See incomplete dominance. patroclinous inheritance A I'onn ol'genetic transmission in which the offspring have the phenotype ol‘the lather. pedigree In human genetics, :1 diagram showing the ancestral rela- tionships and transmission of genetic traits over several generations in a lamily. P element 'l‘ransposablc DNA clcrncnt found in Drosopliiln re- sponsible for hybrid dysgcnesis. penett‘ance The frequency, expressed as a percentage. with which in- dividuals ol'a given genotype manil'est at least some degree ol'a specilic mutant phenotype associated with a trait. peptide bond Thc covalent bond between the amino group of one amino acid and the carboxyl grortp of another amino acid. pericentric inversion A chromosomal inversion that involves both arms of the chromosome and thus involves the centromere. phage See bacteriophage. phenotype The observable properties of an organism that are ge» netically controlled. phenylketonuria (PKU) A hereditary condition in humans asso- ciated with the inability to metabolize the amino acid phenylalanine. The most common form is caused by the lack of the enzyme pheny- lalanine hydroxylase. Philadelphia chromosome The product of a reciprocal tranle— cation that contains the short arm ol'chromosome 9 carrying the C-ABL oncogene and the long arm ol' chromosome 22 carrying the. BCR gene. phosphodicster bond ln nucleic acids. the covalent bond between a phosphate group and adjacent nucleotides. extending from the 5' car- bon of one pcntosc (ribose or dcoxyribosc) to the 3‘ carbon of the pen- tose in the neighboring nucleotide. l’ltospltodiester bonds form the backbone of nucleic acid molecules. photoreactivation enzyme (PRE) An exonuclease that cat- alyzes the light-activated excision ol‘ ultraviolet-induced thymine dimers l'rorn DNA. photoreactivation repair Light-induced repair ofdamage caused by exposure to ultraviolet light. Associated with an intracellular enzyme system. phyletic evolution The gradual transfomrarion of one species into another over time; vertical evolution. pilus A filamentlike projection from the surface of a bacterial cell. Often associated with cells possessing F t'actors. plaque A clear area on an otherwise opaque bacterial lawn. caused by the growth and reproduction of phages. plasmid Art cxtrachrtmrosomal. circular DNA molecule (ol'ten carrying genetic information) that replicates independently ol’ the host clu‘ornosomc. pleiotl‘opy Condition in which a single mutation simultaneously at- l’ects several character's. ploidy Term referring to the basic chromosome set or to multiples of that set. point mutation A tnutation that can be mapped to a single locus. At the molecular level. a mutation that results in the substitution of one nucleotide for another. polar body A cell produced in females at either the first or second meiotic division. which contains almost no cytoplasm as a result of an unequal cytokinesis. polycistronic mRNA A messenger RNA molecule that encodes the amino acid sequence of two or more polypeptide chains in adjacent structural genes. polygenic inheritance The transmission of a phenotypic trait whose expression depends on the additive effect of a number of genes. polylinker A segment of DNA that has been engineered to contain rntrltiple sites for restriction enzyme digestion. i’olylinkers are ustrally found in engineered vectors such as plasmids. polymerase chain reaction (PCR) A method for amplifying DNA segments tlral rrses cycles of denaturation. annealing to primers. and DNA polymerase-directed DNA synthesis. polymerases Enzymes that catalyze the formation of DNA and RNA from deoxynucleotides and ribontrcleotides. respectively. polymorphism The existence of two or more discontinuous. seg- regating phenotypes in a population. polynucleotide A linear sequence of more tltan 20 nucleotides. joined by 5 '—3" phosphodiester bonds. See also oligorzucteotidc. polypeptide A trrolecule made up ofarnirro acitlsjoincd by covalent peptide bonds. This term is used to denote the amino acid chain before it assumes its functional three-dimensional configuration. polyploid A cell or individual having more than two sets of chromosomes. polyribosome See/1(J[_t'.v()lrte. polysome A structure composed of two or more ribosomes associated with rnRNA. engaged in translation. Formerly called polyribosome. pol ytene chromosome A chromosome that has undergone several rounds of DNA replication without separation of the replicated chro- mosomes. thus formin g a giant. thick chromosome with aligned chro« rnomeres producing a characteristic banding pattern. population A local group of individuals belonging to the same species. which are actually or potentially interbreeding. position effect Change in expression of a gene associated with a change irt the gene’s location within the genome. postzygotie isolation mechanism A factor that prevents or re- dtrccs inbreeding by acting after fertilization to produce nonviable. ster- ile hybrids or hybrids of lowered fitness. preadaptive mutation A mutational event that later becomes of adaptive significance. prcformationisrn 'l‘hc discredited idea that an organism develops by growth of structures already present in the egg or sperm. prezygotic isolation mechanism A factor that reduces inbreed- ing by preventing courtship, mating. or fertilization. Pl'ih now box A 6~hp sequence upstream front the beginning of tran- scription in prokaryotic genes. to which the a subunit of RNA poly— merase binds. The consensus sequence for this box is TA'I‘AA'l‘. primary protein structure The sequence of amino acids in a polypeptide chain. primary sex ratio Ratio of males to females at fertilization. primer 1n nucleic acids. a short length of RNA or single-stranded DNA required for the functioning of polymerases. prion An infectious pathogenic agerrl devoid of nucleic acid and com- posed of a protein, PrP, with a molecular weight of 27,000—30.(l{)0 Da. Pr'ions are known to cause scrapie. a degenerative rteurological disease in sheep; bovine spongiform encephalopathy (BSE or mad cow disease) in cattle; and similar diseases in humans. including kurtr and Crcutzfeldt-Jakob disease. probability Ratio of the frequency of a given event to the frequency ol'all possible events. prohand Art individual in whom a genetically determined trait of interest is first detected. Fomierly known as a propositus. probe A macromolecule such as DNA or RNA that has been labeled and can be detected by an assay such as autoradiography or fluores- cence microscopy. Probes are used to identify target molecules. genes. or gene products. Glossary A-12 product law In statistics. the law that holds that the probability of two independent events occurring simultaneously is the prodtrct of their independent probabilities. progeny The offspring produced from a mating. prokuryotes Organisms lacking nuclear membranes. meiosis. and mitosis. Bacteria and blue—green algae are examples of prokztr'yotic organisms. promoter Region having a regulatory function and to which RNA polymerase binds prior to the initiation of transcription. proofreading A molecular mechanism for correcting errors in repli- cation. transcription. or translation. Also known as editing. prophage A phage genome integrated into a bacterial chromosome. Bacterial cells carrying prophages are said to be lysogertic. proposittrs (female, proposita) See prolnrnd. protein A molectrlc composed of one or more polypeptides. each composed of amino acids covalently linked together. protcomics The study of the expressed proteins present in a cell at a given time. protooncogene A gene that normally functions to initiate or main- tain cell division. Protoorrcogenes can be converted to oncogenes by al- terations in structure or expression. protoplast A bacterial or plant cell with the cell wall removed. Some- times called a spheroplast. protolroph A strain (usually microorganisms) that is capable of growth on a defined. minimal medium. Wild—type strains are usually re— garded as prototrophs. pseudoalleles Genes that behave as alleles to one another by corri- plementation, but cart be separated from one another by recombination. pseudoautosomal inheritance Inheritance of alleles located within the regions of the Y chromosome that are homologous to the X chromosome. Because these alleles are located on both the X and Y chromosome. their pattern of inheritance is indistinguishable from that of autosomal inheritance. pseudodominance The expression ofa recessive allele on one ho- molog caused by the deletion of the dominant allele on the other hornolog. psclldogene A nonfunctional gene with sequence homology to a known structural gene present elsewhere in the genome. They differ from their functional relatives by insertions or deletions and by the pres- ence of flanking direct repeat sequences of l0—20 nucleotides. puff See clzmlrrosomc puff. punctuated equilibrium A patient in the fossil record of long pe- riods of species stability. punctuated with brief periods of species divergence. quantitative inheritance See pofygr'lric inheritance. quantitative trait loci (QTL) Two or more genes that act on a sitt- gle polygenic trait. quantum SpECintiOl] Formation ofa new species within a single or a few generations by a combination of selection and drift. quaternary protein structure Types and modes of interaction between two or more polypeptide chains within a protein molecule. race A genotypically or geographically dislirrct subgroup within a species. rad A trnit ofabsorbed dose of radiation with an energy eqtral to l()0 ergs per gram of irradiated tissue. radioactive isotope One of the forms of an element. differing in atomic weight and possessing an unstable nucleus that emits ionizing ra- diation during decay. random amplified polymorphic DNA (RAPD) A PCR method that uses random primers about It) nucleotides in length to arri- plify unknown DNA sequences. random mating Mating between individuals without. regard to genotype. A-13 Glossary reading frame Linear sequence ofcodons in a nucleic acid. reannealing Formation ofdouble-stranded DNA molecules from dissociated single strands. recessive A term describing an allele that is not expressed in the her- erozygous condition. reciprocal cross A paired cross in which the genotype of the fe- male in the first cross is present as the genotype of the male in the sec- ond cross. and vice versa. reciprocal translocatiorr A chromosomal aberration in which trott- homologous chromosomes exchange parts. recombinant DNA A DNA molecule formed by joining two het— erologous molecules. Usually applied to DNA molecules produced by in vii/‘0 ligation of DNA from two different organisms. recombinant gamete A gamete containing a new combination of genes produced by crossing over during meiosis. recombination The process that leads to the formation ofncw gene combinations on chromosomes. recon A term coined by Seymour Benzer to denote the smallest ge— netic units between which recombination can occur. reductional division The chromosome division that halves the diploid chromosome number. The first division of meiosis is a reduc- tional division. redundant genes Gene sequences present in more than one copy per haploid genome (e.g.. r'ibosornal genes). regulatory Site A DNA sequence that is involved itt the control of expression of other genes. usually involving an interaction with another molecule. rem Radiation equivalent in man; the dosage of radiation that will cause the same biological effect as one roentgen of X—rays. renaturation The process by which a denatured protein or nucleic acid returns to its normal three-dimensiorurl structure. repetitive DNA sequences DNA sequences present in many copies in the haploid genome. replicating form (RF) Double-stranded nucleic acid molecules present as am intemrediate during the reproduction of certain viruses. replication The process of DNA synthesis. replication fork The Y-slraped region of a chromosome associated with the site of replication. rcplicon A chromosomal region or free genetic element containing the DNA sequences necessary for the initiation of DNA replication. replisome The term used to describe the complex of proteins. in- cluding DNA polymerase, that assembles at the bacterial replication fork to synthesize DNA. repressihlc enzyme system An enzyme or group of enzymes whose synthesis is regulated by the intracellulzu‘ concentration ofcertain metabolites. repressor A protein that binds to a regulatory sequence adjacent to a gene and blocks transcription ot'the gene. reproductive isolation Absence of interbreeding between popu- lations, subspecies, or species. Reproductive isolation can be brought about by extrinsic factors, such as behavior. and intrinsic barriers, such as hybrid inviability. resistance transfer factor (RTF) A component of R plasmids that confers the ability for cell—cell transfer of the R plasmid by conjugation. resolution In art optical system, the shortest distance between two points or lines at which they can be perceived to be two points or litres. restriction endonuclease Nuclease that recognizes specitic nu- cleotide sequences in a DNA molecule and cleaves or nicks the DNA at those sites. Derived from a variety of microorganisms. those enzymes that cleave both strands of the DNA are used in the construction of re— cornbinant DNA molecules. restriction fragment length polymorphism (RFLP) Variation in the length of DNA fragments generated by a restriction endonuclcase. These variations are caused by mutations that create or abolish cutting sites for restriction enzymes. RFLPs are inherited in a codotninant fashion and can be used as genetic markers. restrictive transduction See specialized transduction. retrovirus Viruses with R NA as genetic material that utilize the en- zyme reverse transcriptase during their life cycle. reverse transcriptase A polymerase that uses RNA as a template to transcribe a single-stranded DNA molecule as a product. reversion A mutation that restores the wild-type phenotype. R factor (R plasmid) Bacterial plasmids that may antibiotic re- sistzutce genes. Most R plasmids have two components: an r-determinamr. which carries the antibiotic resistance genes, and the resistance transfer factor (RTF ). RFLP See restrictionfragment length poty/nm'plrism. Rh factor An antigenic system first described in the rhesus monkey. Recessive rj'r individuals prodtlcc no Rh antigens and are Rh negative. while R/R and R/r individuals have Rh antigens on the surface of their red blood cells and are classilicd as Rh positive. ribonucleic acid (RNA) A nucleic acid characterized by the sugar ribose and the pyrimidine uracil. usually a single—stranded polynu- clcotidc. Several forms are recognized. including ribosomal RNA. mes- senger RNA. transfer RNA. and heterogeneous nuclear RNA. ribose The live-carbon sugar associated with the ribonucleotidcs found in RNA. I'ibosotnal RNA ('r'RNA) The RNA molecules that arcthc structural components of the ribosomal strbtr nits. in prokaryotes. these are the 165. 233. and SS molecules: in cukaryotes. they are the 185. 285. and SS molecules. ribosome A ribonucleoprotein organelle consisting oftwo subunits. each containing R NA and protein. Ribosomes are the site oftranslation of mRNA codons into the amino acid sequence of a polypeptide. chain. RNA See rt'bmmcleic acid. RNA editing Alteration of the nucleotide sequence of an rnRNA molecule after transcription and before translation. There are two main types of editing: substitution editing, which changes individual nu- cleotides. and insertion/deletion editing, in which individual nucleotides we added or deleted. RNA polymerase An enzyme that catalyzes the formation of an RNA polyrrrrcleotide strand using the base sequence of a DNA molecule as a template. RNase A class ofenzyrncs that hydrolyzes RNA. Robertsonian translocation A form of chromosomal aberration that involves the fusion of the long arms of acroccntric chromosomes at the centromere. roentgen (R) A unit of measure of the amount of radiation corre— sponding to the generation of 2.083 X 10” ion pairs in one cubic cen— timeter of air at ()“C at an atmospheric pressure of 760 mm of mercury. rolling circle model A model of DNA replication in which the growing point or replication fork rolls around a circular template strand: in each pass around the circle, the newly synthesized strand displaces the strand from the previous replication. producing a series of contiguous copies of the template strand. R point The point during the G1 stage of the cell cycle when either a commitment is made to DNA synthesis and another cell cycle or the cell withdraws from the cycle and becomes quiescent. Also known as the restriction point. I‘RNA See riboromctt RNA. RTF See resistance Int/rsflerfltc'tnrz $1 nuclease A deoxyribonuclease that cuts and degrades single- stranded molecules of DNA. satellite DNA DNA that forms a minor band when genomic DNA is centrifuged in a cesium salt gradient. This DNA usually consists of short sequences repeated many times in the genome. SCE See sitter ('Irmmutirl exchange. secondary protein structure The a-helical or B—pleated-sheet form of a protein molecule brought about by the formation of hydrogen bonds between amino acids. secondary sex ratio The ratio of males to females at birth. secretor An individual having soluble forms of the blood group anti- gens A and/or 8 present in saliva and other body fluids. This condition is caused by a dominant. autosontal gene unlinked to the A80 locus (I locus). sedimentation coefficient See St't’dbt’lg caefiicieur um’r. segment polarity genes Genes that regulate the spatial pattern of differentiation within each segment of the developing [)mrnp/zila embryo. segregation The separation of homologous chromosomes into dif- ferent gametes during meiosis. selection The force that brings about changes in the frequency of al- leles and genotypes in populations through differential reproduction. selection coefficient (5) A quantitative measure of the relative fitness of one genotype compared with another. See coefficient of selection. selling In plant genetics. the fertilization of ovules of a plant by pollen produced by the same plant. Reproduction by self-fertilization. semiconservative replication A model of DNA replication in which a double-stranded molecule replicates in such a way that the daughter molecules are composed of one parental (old) and one newly synthesized strand. semisterility A condition in which a percentage of all zygotes is inviable. sex chromatin body See Barr body. sex chromosome A chromosome, such as the X orY in humans. which is involved in sex dctennination. sextluction Transmission of clu‘omosomal genes from a donor bac- terium to a recipient cell by the F factor. sex-influenced inheritance Phenotypic expression that is condi- tioned by the sex of the individual. A heterozygote may express one phenotype in one sex and the alternate phenotype in the other sex. sex-limited inheritance A trait that is expressed in only one sex even though the trait may not be X—linked. sex ratio Sec primary and secondary sex ratio. sexual reproduction Reproduction through the fusion of gametes. which are the haploid products of meiosis. Shine-Dalgarno sequence The nucleotides AGGAGG present in the leader sequence of prokaryotic genes that serve as a ribosome bind- ing site. The 165 RNA of the small ribosontal subunit contains a com- plementary sequence to which the mRNA binds. short interspersed elements (SINES) Repetitive sequences found in the genomes of higher organisms, such as the 300-bp Alu sequence. Shotgun experiment The cloning of rzutdom fragments of genomic DNA into a vehicle such as a plasmid or phage. usually to produce a li— brary from which clones of specific interest CtLll be selected. sibling species Species that are morphologically almost identical, but which are reproductively isolated from one another. sickle-cell anemia A genetic disease in humans caused by an autoso— mnl recessive gene, fatal in the homozygous condition if untreated. Caused by an alteration in the amino acid sequence of the [3 chain of globin. sickle-cell trait The phenotype exhibited by individuals heterozy- gous for the sickle—cell gene. sigma (0') factor A polypeptide subunit of the RNA polymerase that recognizes the binding site for the initiation of transcription. single-stranded binding proteins (SSBs) In DNA replication. proteins that bind to and stabilize single-stranded regions of DNA that result from the action of unwinding proteins. Glossary A-14 sister chromatid exchange (SCE) A crossing over event that can occur in meiotic and mitotic cells; involves the reciprocal exchange of chromosomal material belwsen sister chromatids joined by a common ccntromcre. Such exchanges can be detected cytologically after BrdU in- corporation into the replicating chromosomes. site-directed mutagenesis A process that uses a synthetic oligonu- clcotide containing a mutant base or sequence as a primer for inducing a mutation at a Specific site in a cloned gene. small nuclear RNA (snRNA) Species of RNA molecules rang- ing in size from 90 to 400 nucleotides. The abundant snRNAs are pre- sent in l X 104 to l X if)6 copies per cell, associated with proteins. and fonn RNP particles known as snRNPs or .murps. Six uridinc-ricli snRNAs known as Ul—U6 are located in the nucleoplasrn, and their complete nucleotide sequence is known. snRNAs have been implicated in the processing of prc-mRNA and may have a range of Cleavage and ligation functions. snurps See small nnciem‘ RNA. solenoid structure A level of eukaryotic chromosome structure generated by the supercoiling of nucleosomes. somatic cell genetics The use of cultured somatic cells to investi- gate genetic phenomena by parasexual techniques involving the fusion of cells from different organisms. somatic cells All cells other than the germ cells or gametes in an organism. somatic mutation somatic cell. somatic pairing The pairing of homologous chromosomes in so- matic cells. SOS response The induction of enzymes to repair damaged DNA in Ert‘ltcrichia calf. The response involves activation of an enzyme that cleaves a repressor, activating a series of genes involved in DNA repair. Southern blot A technique developed by Edward Southern in which DNA fragments produced by restriction enzyme digestion are separated by electrophoresis and transferred by capillary action to a nylon or ni- trocellulose membrane. Specific DNA fragments can be identitied by hy— bridization to a labeled nucleic acid probe. spacer DNA DNA sequences found between genes, usually repet- itive DNA segments. specialized transduction Genetic transfer of only specific host genes by transducing phages. speciation The process by which new species of plants and ani- mals arise. species A group of actually or potentially interbreeding individuals that is reproductively isolated from other such groups. spheroplast See pmmphm. Spindle fibers Cytoplasmic fibrils formed during cell division that are involved with the separation of chrornatids at anaphase and their movement toward opposite poles in the cell. spliceosome The nuclear macromoleculc complex within which splicing reactions occur to remove introns from prc-mRNAs. spontaneous mutation A mutation that is not induced by a muta- genic agent. spore A unicellular body or cell encased in a protective coat that is produced by some bacteria. plants, and invertebrates: it is capable of survival in unfavorable environmental conditions; and it can give rise to a new individual upon germination. In plants. spores are the haploid products of meiosis. SRY The sex-determining region of the Y gene found near the pseudoautosomal boundary of the Y chromosome. Accumulated evi- dence indicates that this gene is the testis-determining factor (TDF). stabilizing selection Preferential reproduction of those individuals having genotypes close to the mean for the population. A selective elim- ination of genotypes at both extremes. A nonheritablc mutational event occurring in u A-15 Glossary standard deviation A quantitative measure ofthe arttortnt of vari- ation in a sample of measurements from a population. standard error A quantitative measure of the amount of variation in a sample of measurements from a population. sterile The condition of being rrnable to reproduce. Also. free from contaminating microorganisms. strain A group with common ancestry that has physiological or morphological characteristics of interest for genetic study or domestication. STR sequences Short tandem repeats of 2—9 base pairs found within minisatellite sequences. These sequences are trsed to prepare DNA profiles in forensics, patemity identification, and other applications. structural gene A gene that encodes the amino acid sequence of a polypeptide chain. subletltal gene A mutation causing lowered viability, with death be— fore maturity in less than 50 percent ofthe individuals carrying the gene. submetacerrtric chromosome A chromosome with the cen- trornere placed so that one arm of the chromosome is slightly longer than the other. subspecies A morphologically or geographically distinct inter- breeding population of a species. sum law The law that holds that the probability of one or the other of two mutually exclusive events occurring is the sum of their individ— ual probabilities. supercoiled DNA A form of DNA structure in which the helix is coiled upon itself. Such structures can exist in stable forms only when the ends of the DNA are not free. as in a covalently closed circular DNA molecule. superi‘emale See metafemulc. supermale Seemetumale. suppressor mutation A mutation that acts to completely or partially restore the function lost by a previous mutation at an- other site. Svedberg coefficient unit (5') A unit of measure for the rate at which particles (molecules) sediment in a centrifugal field. This unit is a function of several physicochemical properties. including size and shape. A sedimentation value of l X 10"3 sec is defined as one Sved- : berg coefficient unit. symbiont An organism coexisting irt a mutually beneficial relation— ship with another organism. sympatric speciation Process of speciation involving populations that inhabit, at least in part. the same geographic range. synapsis The pairing of homologous chromosomes at meiosis. synaptonemal complex (SC) An organelle consisting of a tri- panite nucleoprotein ribbon that forms between the paired homologous chromosomes in the pachyterre stage of the first meiotic division. syndrome A group of signs or symptoms tltat occur together and characterize a disease or abnormality. synkaryon The nucleus of a xygole that results from the frrsion of two gametic nuclei. Also used in somatic cell genetics to describe the prod- uct of nuclear fusion. syntenic test In somatic cell genetics. a method for determining whether two genes are on the same chromosome. TATA box See Goldberg—Hague” bar. tautomeric shift A reversible isorrterization in a molecule. brought about by a shift in the localization of a hydrogen atom. in nucleic acids, tautotneric shifts in the bases ofnucleotides cart cause changes in other bases at replication and are a source of mutations. TDF (testis-determining factor) The product ofthc SR l" gene on the Y chromosome that controls the developmental switch point for the development of the indifferent gonad into a testis. telocentric chromosome A chromosome irt which the cerrtromere is located at the end of the chromosome. tel()merase The enzyrtte that adds short. tandemly repeated DNA sequences to the ends ofeukaryotic chromosomes. telomere The terminal chromomere ofa chromosome. telophase The stage ol'cell division in which the daughter chromo— somes reach the opposite poles of the cell and re-form nuclei. Telophase ends with the completion of cytokinesis. telnphase l in the first meiotic division. when duplicated chromo— somes reach the poles of the dividing cell. temperate phage A bacteriophage that can become a prophage and confer lysogeny upon tltc host bacterial cell. temperature-sensitive mutation A conditional mutation that produces a mutant phenotype at one temperature range and a wild-type phenotype at another temperature range. template The sin gle-stranded DNA or RNA molecule that specifies the nucleotide sequence of a strand synthesized by a polymerase molecule. terminalization The movement of chiasrnata toward the ends of chromosomes during the diplotene stage of the first meiotic division. tertiary protein structure The three-dimensional structure of a polypeptide chain brought about by folding upon itself. lest cross A cross between an individual whose genotype at one or more loci may be unknown artd art individual who is homozygous re- cessive for the genes in question. tetrad The four chrornatids that make up paired homologs in the prophase of the lirst meiotic division. The four haploid cells produced by a single meiotic division. tetrad analysis Method for the analysis of gene linkage and re- combination, using the four haploid cells produced in a single meiotic division. tetranucleotide hypothesis An early theory of DNA structure proposing that the molecule was composed of repeating units. each cott- sisting of the four nucleotides containing adenine, thymine, cytosine, and guanine. theta (0) Structure Art intermediate in the bidirectional replica- tion of circular DNA molecules. At about midway through the cycle of replication. the intermediate resembles the Greek letter theta. thymine dimer A pair of adjacent thymine bases in a single polynu- cleotide strand that have chemical bonds formed between carbon atoms 5 and 6. This lesion. usually caused by exposure to ultraviolet light. in hibits DNA replication unless repaired by the appropriate enzymes. T," See melting profile. topoisomerase A class of enzymes that convert DNA front one topological form to another. During DNA replication. these enzymes facilitate the unwinding of thc double—helical structure of DNA. totipotent The ability ofa cell or embryo part to give rise to all adult structures. This capacity is usually progressively restricted during development. trait Arty detectable phenotypic variation of a particular inherited character. trans configuration posite homologs, such as The arrangement of two mutant sites on op- (r' + + (12 contrasts with a cis ru'rartgcment, where the sites are located on the same homolog. transcription Transfer of genetic information front DNA by the synthesis of art RNA molecule copied front a DNA template. transcriptome The set omeNA molecules present in a cell at any given time. transdetermination group of cells. transduction Virally mediated genes transfer from one bacterium to another or tlte transfer of eukaryotic genes mediated by retrovirus. Change in developmental fate of a cell or transfer RNA (tRNA) A small ribonucleic acid molecule tltat con- tains a three—base segment (anticodon) that recogniyies a codon in mRNA. a binding site fora specific amino acid. and recognition sites l‘or inter- action with the ribosomes and the enzyme that links it to its specific amino acid. transformation Heritable change in a cell or an organism brought about by exogenous DNA. transgenic organism An organism whose genome has been mod- ified by the introduction of external DNA sequences into the germline. transition A mutational event in which one purine is replaced by another or one pyrimidine is replaced by another. translation The derivation of the amino acid sequence of a polypep- tide fi'om the base sequence of an mRNA molecule in association with a ribosome. translocation A chromosomal mutation associated with the trans- fer ol‘ a chromosomal segment from one chromosome to another. Also used to denote the movement of mRNA through the ribosome during translation. transmission genetics The lield of genetics concerned with the mechanisms by which genes are transferred from parent to offspring. transposable element A DNA segment that trtmslocates to other sites in the genome, essentially independent of sequence homology. Usually such elements are flanked by short inverted repeats of 20—40 base pairs at each end. Insertion into a structural gene can produce a mutant phenotype. Insertion and excision of transposable elements de- pends on two enzymes. transposase and resolvase. Such elements have been identified in both prokaryotes and eukaryotes. transversion A mutational event in which a purine is replaced by a pyrimidine or a pyrimidine is replaced by a purine. trinncleotide repeat A tandemly repeated cluster of three nu- cleotides (such as CTG‘) in or near a gene. which undergoes an expan- sion in copy number. resulting in a disease phenotype. tt'iploidy The condition in which a cell or organism possesses three haploid sets of chromosomes. trisonly The condition in which a cell or organism possesses two copies of each chromosome, except for one, which is present in three copies. The general form for trisomy is therefore 2): + l. tRNA See transfer RNA. tumor-suppressor gene A gene that encodes a gene product that normally functions to suppress cell division. Mutations in tumor-suppressor genes result in the activation of cell division and tttmor formation. Turner syndrome A genetic condition in human females caused by a 45, X genotype. Such individuals are phenotypically female but are sterile because of undeveloped ovaries. unequal crossing over A crossover between two improperly aligned homologs. producing one homolog with three copies of a re- gion and the other with one copy of that region. unique DNA DNA sequences that are present only once per genome. universal code The assumption that the genetic code is used by all life forms. In general. this is true; some exceptions are found in mito- chondria, ciliates. zutd mycoplasmas. unwinding proteins Nuclear proteins that not during DNA repli- cation to destabilize and unwind the DNA helix ahead of the repli- cating fork. variable number tandem repeats (VNTRS) Short, repeated DNA sequences (2720 nucleotides) present as tandem repeats between two restriction enzyme sites. Variations in the number ol‘ repeats cre- ates DNA fragments of differing lengths following restriction enzyme digestion. variable region Portion of an immunoglobulin molecule that ex- hibits many amino acid sequence diflerences between antibodies of dif- I‘ering specil'icities. Glossary A-16 variance A statistical measure of the variation of values from a cert- tral value, calculated as the square of the standard deviation. variegation Patches of differing phenotypes. such as color. in a tissue. vector In recombinant DNA, an agent such as a phage or plasmid into which a foreign DNA segment will be inserted. Viability The measure of the number of individuals in a given phe- notypic class that survive. relative to another class [usually wild type). virulent phage A bacteriophage that inl‘ects and lyscs the host bacterial cell. VNTRS See variable nmnber tandem repeats. western blot A technique in which proteins are separated by gel electrophoresis and trrulsferred by capillzuy action to a nylon membrane or nitrocellulose sheet. A specific protein can be identified through hy- bridization to a labeled antibody. wild type The most commonly observed phenotype or genotype. designated as the norm or standard. wobble hypothesis An idea proposed by Francis Crick. stating that the third base in an anticodon can align in several ways to allow it to rec- ognize more than one base in the codons of mRNA. writhing number The number ol'times that the axis ofa DNA du- plex crosses itself by supercoiling. W, Z chromosomes Sex chromosomes in species where the fe- male is the heterogarnetic sex (WZ). X chromosome Sex chromosome present in species where females are the homogametic sex (XX). X inactivation in mammalian females, the random cessation ol' transcriptional activity of one X chromosome. This event. which occurs early in development. is a mechanism ol‘dosagc compensation. Molec- ular basis ol' inactivation is unknown, but involves a region called the X- inactivation center (XIC) on the proximal end of the p arm. Sonic loci on the tip of the short arm 01‘ the X can escape inactivation. See also Barr/70(1):. Lynn /i_,t-pntliesis. XIST A locus in the X-chromosomc inactivation center that may con- trol inactivation of the X chromosome in mammalian females. X—l'tnkage The pattern of inheritance resulting from genes located on the X chromosome. X-ray crystallography A technique to determine the three- dimensional structure 01‘ molecules through dil'l'raction patterns pro- dtrced by X—ray scattering by crystals of the molecule under study. YAC A cloning vector in the form of a yeast artiticial chromosome. constructed using chromosomal elements including telomeres (l'rom a ciliale). centromeres. origin ol'replication. and marker genes from yeast. YACs are used to clone long stretches ot‘cukaryotic DNA. Y chromosome Sex chromosome in species where the male is het- erogametic (XY). Y-linkage Mode of inheritance shown by genes located on the Y chromosome. Z-DNA An alternative structure of DNA in which the two antiparal- lel polynucleotide chains form a left-handed double helix. Z-DNA has been shown to be present in chromosomes and may have a role in reg- ulation of gene expression. zein Principal storage protein of maize endosperm. consisting ot‘two major proteins. with molecular weights of 19.000 and 21,000 Da. zinc finger A DNA»binding domain of a protein that has a character- istic pattern ot‘cysteine and histidine residues that complex with line ions. throwing intermediate amino acid residues into a series of loops or fingers. zygote The diploid cell produced by the fusion of haploid gametic nuclei. zygotene A stage of meiotic prophase I in which the homologous chromosomes synapse and pair along their entire length. forming biva- lents. The synaptonemal complex terms at this stage. ...
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Appendix A - Glossary abortive transduction An event in...

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