i G e n e t i c s R u s s e l l T h i r d iGenetics E d A Molecular Approach i t i o Peter J. Russell n Third Edition ISBN 978-1-29202-633-6 9 781292 026336 iGenetics A Molecular Approach Peter J. Russell Third Edition ISBN 10: 1-292-02633-2 ISBN 13: 978-1-292-02633-6 Pearson Education Limited Edinburgh Gate Harlow Essex CM20 2JE England and Associated Companies throughout the world Visit us on the World Wide Web at: www.pearsoned.co.uk © Pearson Education Limited 2014 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without either the prior written permission of the publisher or a licence permitting restricted copying in the United Kingdom issued by the Copyright Licensing Agency Ltd, Saffron House, 6–10 Kirby Street, London EC1N 8TS. All trademarks used herein are the property of their respective owners. The use of any trademark in this text does not vest in the author or publisher any trademark ownership rights in such trademarks, nor does the use of such trademarks imply any affi liation with or endorsement of this book by such owners. ISBN 10: 1-292-02633-2 ISBN 13: 978-1-292-02633-6 British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library Printed in the United States of America 111222332369137169591334058023601 P E A R S O N C U S T O M L I B R AR Y Table of Contents Glossary Peter J. Russell 1 1. Genetics: An Introduction Peter J. Russell 23 2. DNA: The Genetic Material Peter J. Russell 33 3. DNA Replication Peter J. Russell 64 4. Gene Function Peter J. Russell 90 5. Gene Expression: Transcription Peter J. Russell 115 6. Gene Expression: Translation Peter J. Russell 138 7. DNA Mutation, DNA Repair, and Transposable Elements Peter J. Russell 170 8. Genomics: The Mapping and Sequencing of Genomes Peter J. Russell 212 9. Functional and Comparative Genomics Peter J. Russell 263 10. Recombinant DNA Technology Peter J. Russell 296 11. Chromosomal Basis of Inheritance Peter J. Russell 350 12. Mendelian Genetics Peter J. Russell 391 I 444556666778826937036948123597123878505 13. Extensions of and Deviations from Mendelian Genetic Principles Peter J. Russell 423 14. Genetic Mapping in Eukaryotes Peter J. Russell 465 15. Genetics of Bacteria and Bacteriophages Peter J. Russell 499 16. Variations in Chromosome Structure and Number Peter J. Russell 537 17. Regulation of Gene Expression in Bacteria and Bacteriophages Peter J. Russell 571 18. Regulation of Gene Expression in Eukaryotes Peter J. Russell 602 19. Genetic Analysis of Development Peter J. Russell 633 20. Genetics of Cancer Peter J. Russell 668 21. Population Genetics Peter J. Russell 697 22. Quantitative Genetics Peter J. Russell 748 23. Molecular Evolution Peter J. Russell 785 Suggested Readings Peter J. Russell 810 Index 825 II Glossary 10-nm chromatin fiber The least compact form of chromatin. alternative polyadenylation Process for generating different It is approximately 10 nm in diameter and has a “beads-on- functional mRNAs from a single gene by cleavage and a-string” morphology. It consists of nucleosomes which polyadenylation of the primary transcript at different consist of a core of eight histone proteins around which is poly(A) sites. wrapped the DNA. Linker DNA bridges each nucleosome. alternative splicing In eukaryotes, a process for generating See also30-nm chromatin fiber. different functional mRNAs from a single precursor mRNA 30-nm chromatin fiber The next level of chromatinconden- (pre-mRNA) by incorporating different exons in the sation beyond the 10-nm chromatin fiber brought about mature mRNA. by H1 histone binding to the linker DNA and to DNA Ames test An assay that measures the ability of chemicals to bound to the histones of the nucleosome. It is about 30 nm cause mutations in certain bacteria. It can identify poten- in diameter. See also10-nm chromatin fiber. tial carcinogens. amino acid Any of the small molecules, containing a carboxyl acrocentric chromosome A chromosome with the centromere group and amino group, that are joined together to form near one end such that it has one long arm plus a stalk and polypeptides and proteins. a satellite. aminoacyl–tRNA A tRNA molecule covalently bound to an activators The major class of transcription regulatory proteins amino acid; also called charged tRNA. This complex brings in eukaryotes. Binding of these proteins to regulatory DNA the amino acid to the ribosome so that it can be used in sequences associated with specific genes determines the polypeptide synthesis. efficiency of transcription initiation. Some bacterial genes aminoacyl–tRNA synthetase An enzyme that catalyzes the are controlled by activators. See alsorepressor(s). addition of a specific amino acid to the tRNA for that adenine (A) A purinebase found in DNA and RNA. In double- amino acid. stranded DNA, adenine pairs with thymine, a pyrimidine, amniocentesis A procedure in which a sample of amniotic sac by hydrogen bonding. In double-stranded RNA, adenine fluid is withdrawn from the amniotic sac of a developing pairs with uracil, a pyrimidine, by hydrogen bonding. fetus and cells are cultured and examined for chromosomal agarose gel electrophoresis An experimental procedure in abnormalities. which an electric field is used to move DNA or RNA mol- analysis of variance (ANOVA) A series of statistical proce- ecules, which are negatively charged, through a gel matrix dures for determining whether differences in the meansof of agarose from the negative pole to the positive pole. a variable in two samples are significant and for partition- allele One of two or more alternative forms of a single gene ing the varianceinto components. that can exist at the same locusin the genome. All the alle- anaphase The stage in mitosis when the sister chromatids les of a gene determine the same hereditary trait (e.g., seed separate and migrate toward the opposite poles of the cell. color), but each has a unique nucleotide sequence, which anaphase I The stage in meiosis I when the chromosomes in may result in different phenotypes (e.g., yellow or green each bivalentseparate and begin moving toward opposite seeds). See alsoDNA polymorphism. poles of the cell. allele frequency Proportion of a particular allele at a locus anaphase II The stage in meiosis II when the sister chro- within a gene pool. The sum of the allele frequencies at a matidsare pulled to the opposite poles of the cell. given locus is 1. aneuploid Referring to an organism or cell that has a chromo- allele-specific oligonucleotide (ASO) hybridization A proce- some number that is not an exact multiple of the haploid dure, using PCR primers, to distinguish alleles that differ set of chromosomes. by one base pair. aneuploidy Any condition in which the number of chromo- allelomorph Seeallele. somes differs from an exact multiple of the normal haploid allopolyploidy Condition in which a cell or organism has two number in a cell or organism. It commonly results from the or more genetically distinct sets of chromosomes that orig- gain or loss of individual chromosomes but also can result inate in different, though usually related, species. from the duplication or deletion of part(s) of a chromo- alternation of generations Type of life cycle characteristic of some or chromosomes. green plants in which haploid cells (gametophytes)alter- antibody A protein molecule that recognizes and binds to a nate with diploid cells (sporophytes). foreign substance introduced into the organism. From iGenetics: A Molecular Approach, Third Edition. Peter J. Russell. Copyright © 2010 by Pearson Education, Inc. Published by Pearson Benjamin Cummings. All rights reserved. 1 Glossary anticodon A group of three adjacent nucleotides in a tRNA base analog A chemical whose molecular structure is very sim- molecule that pairs with a codon in mRNA by comple- ilar to that of one of the bases normally found in DNA. mentary base pairing. Some chemical mutagens, such as 5-bromouracil (5BU), antigen Any large molecule that stimulates the production of are base analogs. specific antibodies or binds specifically to an antibody. base excision repair An enzyme-catalyzed process for repair- antiparallel In the case of double-stranded DNA, referring to ing damaged DNA by removal of the altered base, followed the opposite orientations of the strands, with the 5¿end of by excision of the baseless nucleotide. The correct one strand paired with the 3¿end of the other strand. nucleotide then is inserted in the gap. antisense mRNA An mRNA transcribed from a cloned gene base-modifying agent A chemical mutagenthat modifies the that is complementary to the mRNA produced by the nor- chemical structure of one or more bases normally found in mal gene. DNA. Nitrous oxide, hydroxylamine, and methylmethane apoptosis Controlled process leading to cell death that is trig- sulfonate are common base-modifying agents. gered by intracellular damage (e.g., DNA lesions) or by base-pair substitution mutation A change in the genetic external signals from neighboring cells. Also called material such that one base pair is replaced by another base programmed cell death. pair; for instance, an A-Tis replaced by a G-Cpair. aporepressor protein An inactive repressor that is activated basic research Research done to further knowledge for knowl- when bound to an effector molecule. edge’s sake. applied research Research done with the objective of develop- bidirectional replication Synthesis of DNA in both directions ing products or processes that can be commercialized or at away from an origin of replication. least made available to humankind for practical benefit. bioinformatics Application of mathematics and computer sci- Archaea Prokaryotes that constitute one of the three main ence to store, retrieve, and analyze biological data, particu- evolutionary domains of organisms. Members of this larly nucleic acid and protein sequence data. domain are called archaeans. bivalent A pair of homologous, synapsed chromosomes, con- artificial selection Process for deliberating changing the phe- sisting of four chromatids, during the first meiotic divi- notypic traits of a population by determining which indi- sion. See alsosynapsis. viduals will survive and reproduce. bootstrap procedure A method for determining confidence attenuation A regulatory mechanism in certain bacterial levels attached to the branching patterns of a phylogenetic biosynthetic operons that controls gene expression by treechosen by the parsimony approach. causing RNA polymerase to terminate transcription. bottleneck effect A form of genetic drift that occurs when a autonomously replicating sequence (ARS) A specific sequence population is drastically reduced in size and some genes in yeast chromosomes that, when included as part of an are lost from the gene pool as a result of chance. extrachromosomal, circular DNA molecule, confers on that branch-point sequence Specific sequence within introns of molecule the ability to replicate autonomously; one type of precursor mRNAs (pre-mRNAs) of eukaryotes containing eukaryotic replicator. an adenylate (A) nucleotide to which the free 5¿end of an autopolyploidy Condition in which a cell or organism has two intron binds during mRNA splicing. or more genetically distinct sets of chromosomes of the broad-sense heritability The proportion of the phenotypic same species. variancewithin a population that results from genetic dif- autosome A chromosome other than a sex chromosome. ferences among individuals. auxotroph A mutant strain of an organism that cannot synthe- size a molecule required for growth and therefore must cAMP (cyclic AMP) Adenosine 3¿,5¿monophosphate; an intra- have the molecule supplied in the growth medium for it to cellular regulatory molecule involved in controlling gene grow. Also called auxotrophic mutantor nutritional mutant. expression and some other processes in both prokaryotes auxotrophic mutant Seeauxotroph. and eukaryotes. auxotrophic mutation A mutation that affects an organism’s cancer Disease characterized by the uncontrolled and abnor- ability to make a particular molecule essential for growth. mal division of cells and by the spread of malignant tumor Also called nutritional mutation. cells (metastasis) to disparate sites in the organism. 5œ capping The addition of a methylated guanine nucleotide back mutation Seereverse mutation. (a “cap”) to the 5¿end of a precursor mRNA (pre-mRNA) Bacteria Prokaryotes that constitute one of the three main molecule in eukaryotes; the cap is retained on the mature evolutionary domains of organisms. Members of this mRNA molecule. domain are called bacteria. carcinogen Any physical or chemical agent that increases the bacterial artificial chromosome (BAC) A vector for cloning frequency with which cells become cancerous. DNA fragments up to about 200 kb long in E. coli. A BAC carrier An individual who is heterozygous for a recessive muta- contains the origin of replication of the Ffactor, a multiple tion. A carrier usually does not exhibit the mutant phenotype. cloning site, and a selectable marker. catabolite activator protein (CAP) A regulatory protein that bacteriophages Viruses that attack bacteria. Also called phages. binds with cyclic AMP (cAMP)at low glucose concentra- Barr body A highly condensed and transcriptionally inactive X tions, forming a complex that stimulates transcription of chromosome found in the nuclei of somatic cells of female the lacoperon and some other bacterial operons. mammals. See alsolyonization. catabolite repression The inactivation of some inducible base Also called nitrogenous base. Purine or pyrimidine com- operons in the presence of glucose even though the ponent of a nucleotide. operon’s inducer is present. Also called glucose effect. 2 Glossary cDNA DNA copies made from RNA templates in a reaction cat- chromosome In eukaryotic cells, a linear structure composed alyzed by the enzyme reverse transcriptase. of a single DNA molecule complexed with protein. Each cDNA library Collection of cloned complementary DNAs eukaryotic species has a characteristic number of chromo- (cDNAs)produced from the entire mRNA population of a somes in the nucleus of its cells. Most prokaryotic cells cell. contain a single, usually circular chromosome. cell cycle The cyclical process of growth and cellular repro- chromosome library Collection of cloned DNA fragments pro- duction in unicellular and multicellular eukaryotes. The duced from a particular chromosome (e.g., the human X cycle includes nuclear division, or mitosis,and cell (cyto- chromosome). plasmic) division, or cytokinesis. chromosome theory of inheritance The theory that genes are cell division A process whereby one cell divides to produce located on chromosomes and that the transmission of two cells. See alsocytokinesis. chromosomes from one generation to the next accounts for CENsequence Nucleotide sequence of DNA in the centromere the inheritance of hereditary traits. region of yeast chromosomes. Centromeresequences differ cis-dominant Referring to a gene or DNA sequence that can among species and between chromosomes in the same control genes on the same DNA molecule but not on other species. DNA molecules. centimorgan (cM) The unit of distance on a genetic map. cis-transtest Seecomplementation test. Equivalent to map unit. classical model An early model for genetic variation that centromere The region of a chromosome containing DNA was based on the assumption that most natural popula- sequences to which mitotic and meiotic spindle fibers tions had a wild-type allele with very few mutant alleles attach. Under the microscope a centromere is seen as a present. constriction in the chromosome. The centromere region of cline A systematic change in allele frequencieswithin a con- each chromosome is responsible for the accurate segrega- tinuous population distributed over a geographic region. tion of replicated chromosomes to the daughter cells dur- clonal selection A process whereby cells that express cell- ing mitosis and meiosis. See alsokinetochore. surface antibodies specific for a particular antigen are stim- chain-terminating codon Seestop codon. ulated to proliferate and secrete that antibody by exposure character Seehereditary trait. to that antigen. charged tRNA Seeaminoacyl–tRNA. cloning (a) The production of many identical copies of a DNA charging Addition of an amino acid to a tRNA that contains an molecule by replication in a suitable host; also called DNA anticodonfor that animo acid. Also called aminoacylation. cloning, gene cloning, and molecular cloning. (b) The gener- checkpoints, cell-cycle Stages in the cell cycle at which pro- ation of cells (or individuals) genetically identical to them- gression of a cell through the cycle is blocked if there is selves and to their parent. damage to the genome or the mitotic machinery. cloning vector A double-stranded DNA molecule that is able to chiasma (plural,chiasmata) A cross-shaped structure formed replicate autonomously in a host cell and into which a during crossing-over and visible during the diplonema DNA fragment (or fragments) can be inserted to form a stage of meiosis. recombinant DNA molecule for cloning. chiasma interference Seeinterference. coactivator In eukaryotes, a large multiprotein complex that chi-square (c2) test A statistical procedure that determines interacts with activators bound at enhancers, general tran- what constitutes a significant difference between observed scription factors bound near the promoter, and RNA poly- results and results expected on the basis of a particular merase II. These interactions help stimulate transcription hypothesis; a goodness-of-fit test. of regulated genes. chloroplasts Triple-membraned, chlorophyll-containing organe- coding sequence The part of an mRNA molecule that specifies lles found in green plants in which photosynthesis occurs. the amino acid sequence of a polypeptide during translation. chorionic villus sampling A procedure in which a sample of codominance The condition in which an individual heterozy- chorionic villus tissue of a developing fetus is examined for gous for a gene exhibits the phenotypes of both homozy- chromosomal abnormalities. gotes. chromatid One of the two visibly distinct replicated copies of codon A group of three adjacent nucleotides in an mRNA mol- each chromosome that becomes visible between early ecule that specifies either one amino acid in a polypeptide prophase and metaphase of mitosis and is joined to its sis- chain or the termination of polypeptide synthesis. ter chromatid at their centromeres. codon usage bias A disproportionate use of one or a few syn- chromatin The DNA–protein complex that constitutes eukary- onymous codons within a codon family for a particular otic chromosomes and can exist in various degrees of fold- gene or across a genome. ing or compaction. coefficient of coincidence A measure of the extent of chiasma chromatin remodeling Alteration of the structure of chro- interference throughout a genetic map; ratio of the matin in the vicinity of a core promoter in a way that observed to the expected frequency of double crossovers. stimulates or represses transcription initiation. Remodel- See alsointerference. ing is carried out by enzymes catalyzing histone acetyla- combinatorial gene regulation In eukaryotes, control of tran- tion or deacetylation and by nucleosome remodeling scription by the combined action of several activators and complexes. repressors, which bind to particular gene regulatory chromosomal aberration Seechromosomal mutation. sequences. chromosomal mutation The variation from the wild-type con- comparative genomics Comparison of the nucleotide dition in chromosome number or structure. sequences of entire genomes of different species, with the 3 Glossary goal of understanding the functions and evolution of correlation coefficient A statistical measure of the strength of genes. Such comparisons can identify which genome the association between two variables. See alsoregression. regions are evolutionarily conserved and likely to represent cotransduction The simultaneous transduction of two or more functional genes. bacterial genes, a good indication that the bacterial genes complementary base pairs The specific A-T and G-C base are closely linked. pairs in double-stranded DNA. The bases are held together coupling In individuals heterozygous at two genetic loci, the by hydrogen bonds between the purine and pyrimidine arrangement in which the wild-type alleles of both genes base in each pair. are on one homologous chromosome and the recessive complementary DNA See cDNA. mutant alleles are on the other; also called cis configuration. complementation test A test used to determine whether two See alsorepulsion. independently isolated mutations that confer the same covariance A statistical measure of the tendency for two vari- phenotype are located within the same gene or in two dif- ables to vary together; used to calculate the correlation ferent genes. Also called cis-trans test. coefficientbetween the two variables. complete dominance The condition in which an allele is phe- CpG island DNA region containing many copies of the dinu- notypically expressed when one or both copies are present, cleotide CpG. Many genes in eukaryotic DNA have CpG so that the phenotype of the heterozygote is essentially islands in or near the promoter. Methylation of the indistinguishable from that of the homozygote. cytosines (C) in these islands represses transcription. complete medium For a microorganism, a medium that sup- crisscross inheritance Transmission of a gene from a male par- plies all the ingredients required for growth and reproduc- ent to a female child to a male grandchild. tion, including those normally produced by the wild-type cross The fusion of male gametes from one individual and organism. female gametes from another. complete recessiveness The condition in which an allele is cross-fertilization Seecross. phenotypically expressed only when two copies are present. crossing-over The process of reciprocal chromosomal interchange conditional mutation A mutation that results in a wild-type that occurs frequently during meiosis and gives rise to phenotype under one set of conditions but a mutant phe- recombinant chromosomes. notype under other conditions. Temperature-sensitive C-value The amount of DNA found in the haploid set of chro- mutations are a common type of conditional mutation. mosomes. conjugation In bacteria, process of unidirectional transfer of cyclin Any of a group of proteins whose concentrations genetic material through direct cellular contact between a increase and decrease in a regular pattern through the cell donor (“male”) cell and a recipient (“female”) cell. cycle. The cyclins act in conjunction with cyclin- consensus sequence The series of nucleotides found most fre- dependent kinasesto regulate cell-cycle progression. quently at each position in a particular DNA sequence cyclin-dependent kinase (Cdk) Any of a group of protein among different species. kinases, activated by binding of specific cyclins, that regu- conservative model A model for DNA replication in which the late cell-cycle progression. two parental strands of DNA remain together and serve as cytokinesis Division of the cytoplasm following mitosis or a template for the synthesis of a new daughter double meiosis I and II during which the two new nuclei com- helix. The results of the Meselson–Stahl experiment did partmentalize into separate daughter cells. not support this model. cytosine (C) A pyrimidinefound in RNA and DNA. In double- constitutive gene A gene whose expression is unregulated. The stranded DNA, cytosine pairs with guanine, a purine, by products of constitutive genes are essential to the normal hydrogen bonding. functioning of the cell and are always produced in growing cells regardless of the environmental conditions. dark repair Seeexcision repair. constitutive heterochromatin Condensed chromatin that is Darwinian fitness (w) The relative reproductive ability of always transcriptionally inactive and is found at homolo- individuals with a particular genotype. gous sites on chromosome pairs. daughter chromosomes Detached sister chromatids after they continuous trait Seequantitative trait. separate at the beginning of mitotic anaphase or meiotic contributing allele An allele that affects the phenotype of a anaphase II. quantitative trait. deamination Removal of an amino group from a nucleotide in coordinate induction The simultaneous transcription and DNA. translation of two or more genes brought about by the degeneracy In the genetic code, the existence of more than action of an inducer. one codon corresponding to each amino acid. core enzyme The portion of E. coliRNA polymerase that is the degradation control The regulation of the rate of breakdown active enzyme; it is bound to the sigma factor, which (turnover) of RNA molecules in the cell. directs the enzyme to the promoterregion of genes. deletion A chromosomal mutation resulting in the loss of a seg- corepressor In eukaryotes, a large multiprotein complex that ment of a chromosome and the gene sequences it contains. interacts with repressors bound at enhancers, general tran- deoxyribonuclease (DNase) An enzyme that catalyzes the scription factors bound near the promoter, and RNA poly- degradation of DNA to nucleotides. merase II. These interactions help inhibit transcription of deoxyribonucleic acid (DNA) A polymeric molecule consist- regulated genes. ing of deoxyribonucleotide building blocks that in a core promoter In eukaryotic genomes, the gene regulatory ele- double-stranded, double-helical form is the genetic mater- ments closest to the transcription start site that are required ial of all living organisms. for RNA synthesis to begin at the correct nucleotide. deoxyribonucleotide Any of the nucleotides that make up 4 Glossary DNA, consisting of a sugar (deoxyribose), a base, and a progeny DNA segments interspersed. The results of the phosphate group. Meselson–Stahl experiment did not support this model. deoxyribose The pentose (five-carbon) sugar found in DNA. DNA A polymeric molecule consisting of deoxyribonucleotide depurination Loss of a purine base (adenine or guanine) from building blocks that in a double-stranded, double-helical a nucleotide in DNA. form is the genetic material of all living organisms. determination Process early in development that establishes DNA chip SeeDNA microarray. the fate of a cell, that is, the differentiated cell type it will DNA-dependent RNA polymerase The more complete name for become. RNA polymerase,the enzyme responsible for transcription, development Overall process of growth, differentiation, and the process of RNA synthesis using a DNA template. See morphogenesis by which a zygote gives rise to an adult RNA polymerase. organism. It involves a programmed sequence of pheno- DNA fingerprinting SeeDNA typing. typic events that are typically irreversible. DNA helicase An enzyme that catalyzes unwinding of the diakinesis The final stage in prophase I of meiosis, during DNA double helix at a replication fork during DNA which the replicated chromosomes (bivalents) are most replication. condensed, the nuclear envelope breaks down, and the DNA ladder Also known as DNA size markers, a set of DNA spindle begins to form. molecules of known size used in agarose gel electrophore- dicentric bridge Seedicentric chromosome. sis experiments. dicentric chromosome A homologous chromosome pair in DNA ligase An enzyme that catalyzes the formation of a phos- meiosis I in which one chromatid has two centromeres as phodiester bond between the 5¿end of one DNA chain and the result of crossing-over within a paracentric inversion. 3¿end of another DNA chain during DNA replication and As the two centromeres begin to migrate to opposite poles, DNA repair. a dicentric bridge stretching across the cell forms and even- DNA markers Sequence variations among individuals in a spe- tually breaks. cific region of DNA that are detected by molecular analysis dideoxynucleotide (ddNTP) A modified nucleotide that has a of the DNA and can be used in genetic analysis. See also 3¿-H on the deoxyribose sugar rather than a 3¿-OH. A gene markers. ddNTP can be incorporated into a growing DNA chain, but DNA microarray An ordered grid of DNA molecules of known no further DNA synthesis can occur because no phospho- sequence—probes—fixed at known positions on a solid diester bond can be formed with an incoming nucleotide. substrate, either a silicon chip, glass, or less commonly, a See also dideoxy sequencing. nylon membrane. Labeled free DNA molecules—targets— dideoxy sequencing A method for rapidly sequencing DNA are added to the unlabeled fixed probes to analyze identi- molecules in which the DNA to be sequenced is used as the ties or quantities of target molecules. DNA microarrays template for in vitro DNA synthesis in the presence of allow for the simultaneous analysis of thousands of DNA dideoxynucleotides (ddNTPs).When a dideoxynucleotide target molecules. is incorporated into a growing DNA chain, no further DNA DNA molecular testing A type of genetic testing that focuses synthesis occurs, generating a truncated chain in which the on the molecular nature of mutations associated with a terminal dideoxynucleotide corresponds to the normal particular disease. nucleotide that occurs at that point in the sequence. DNA polymerase Any enzyme that catalyzes the polymeriza- differentiation Series of cell-specific changes in gene expres- tion of deoxyribonucleotides into a DNA chain. All DNA sion by which determined cells give rise to cell types with polymerases synthesize DNA in the 5¿to 3¿direction. characteristic structures and functions. DNA polymerase I (DNA Pol I) One of several E. colienzymes dihybrid cross A cross between two individuals of the same that catalyze DNA synthesis; originally called the Kornberg genotype that are heterozygous for two pairs of alleles at enzyme. two different loci (e.g., Ss Yy!Ss Yy). DNA polymorphism Variation in the nucleotide sequence or dioecious Referring to plant species in which individual plants number of tandem repeat units at a particular locus in the possess either male or female sex organs. See also genome. Most commonly, this term is used for DNA mark- monoecious. ers, variations that are located outside of genes and that are diploid (2N) A cell or an individual with two copies of each detected by molecular analysis. chromosome. DNA primase An enzyme that catalyzes formation of a short diplonema The stage in prophase I of meiosis during which RNA primer in DNA replication. the synaptonemal complexdisassembles and homologous DNA profiling SeeDNA typing. chromosomes begin to move apart. DNA typing Molecular analysis of DNA polymorphisms to discontinuous trait A heritable characteristic that exhibits a identify individuals based on the unique characteristics of small number of distinct phenotypes, which commonly are their DNA. Also called DNA fingerprinting. determined by variant alleles at a single locus. See also domain shuffling Proposed mechanism for evolutionof genes quantitative trait. with new functions by the duplication and rearrangement dispersed repeated DNA Repetitive DNA sequences that are of exons encoding protein domains in different combina- distributed at irregular intervals in the genome. tions. Also called exon shuffling. dispersive model A model for DNA replication in which the dominant Describing an allele or phenotype that is expressed parental double helix is cleaved into double-stranded DNA in either the homozygous or the heterozygous state. segments that act as templates for the synthesis of new dominant lethal allele An allele that results in the death of an double-stranded DNA segments, which are reassembled organism that is homozygous or heterozygous for the into complete DNA double helices, with parental and allele. 5