Table Of Contenti
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
