14
Mechanisms of Genetic Variation
CHAPTER OVERVIEW
This chapter begins with a discussion of mutation and genetic variation and includes molecular mechanisms of
mutation and repair. A general discussion of bacterial recombination, plasmids, and transposable elements
follows, with examination of the acquisition of genetic information by conjugation, transformation, and
transduction.
CHAPTER OBJECTIVES
After reading this chapter you should be able to:
•
discuss the nature and causes of mutations
•
discuss the various genetic repair mechanisms and their limitations
•
discuss the nature of prokaryotic recombination
•
distinguish horizontal gene transfer from vertical gene transfer
•
compare and contrast conjugation, transformation, and transduction
•
discuss how transposable elements can move genetic material between bacterial chromosomes and within
a chromosome to cause changes in the genome and the phenotype of the organism
CHAPTER OUTLINE
I.
Mutations: Their Chemical Basis and Effects
A. Mutation overview
1. A mutation is a stable, heritable change in the genomic nucleotide sequence; this can be a
single base change (point mutation), changes of several bases, or larger insertions, deletions,
inversions, duplications, and translocations
2. Mutations can arise in two ways:
a. Spontaneous mutations arise occasionally in the absence of any added agent
b. Induced mutations are the result of exposure to a mutagen (physical
or chemical agent)
B. Spontaneous mutations
1. Arise occasionally in all cells without exposure to external agents; they are often the result of
errors in replication or lesions to the DNA
2. Errors in replication can be due to tautomeric shifts, which cause base substitutions
a. Transition mutation—substitution of one purine for another, or of one pyrimidine for
another
b. Transversion mutation—substitution of a purine for a pyrimidine or vice versa
3. Lesions in the structure of DNA; the loss of a nitrogenous base creating an apurinic or
apyrimidinic site can cause spontaneous mutations
C. Induced mutations
1. Mutations can be induced by agents that damage DNA, alter its chemistry, or interfere with its
functioning
2. Base analogs are structurally similar to normal nitrogenous bases and can be incorporated into
DNA during replication, but exhibit base-pairing properties different from the bases they
replace
3. Specific mispairing occurs when a mutagen is a DNA-modifying agents that changes a base’s
structure and thereby alters its pairing characteristics (e.g., alkylating agents)
4. Intercalating agents, which become inserted between the stacked bases of the helix, distort the
DNA and thus induce single nucleotide pair insertions or deletions
D. Effects of mutations
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1.
Forward mutation—a conversion from the most prevalent gene form (wild type) to a mutant
form
2. Reversion mutation—a second mutation event that makes the mutant appear to be a wild type
again
a. Back mutation (true reversion)—conversion of the mutant nucleotide sequence back to
the wild-type sequence
b. Suppressor mutation—a reestablishment of the wild-type phenotype by a second
mutation that overcomes the effect of the first mutation; can be in the same gene or a
different gene, but does not restore the original sequence
3. Point mutations affect only one base pair and are more common than large deletions or
insertions
a. Silent mutations are alterations of the base sequence that do not alter the amino acid
sequence of the protein because of code degeneracy
b. Missense mutations are alterations of the base sequence that result in the incorporation of
a different amino acid in the protein; at the level of protein function, the effect may range
from complete loss of activity to no change in activity
c. Nonsense mutations are alterations that produce a translation termination codon; this
results in premature termination of protein synthesis; location of the mutation within the
protein will determine the extent of change in function
d. Frameshift mutations are insertions or deletions of one or two base pairs that thereby alter
the reading frame of the codons
e. Conditional mutations are expressed only under certain environmental conditions
f.
Biochemical mutations result in changes in the metabolic capabilities of a cell;
auxotrophs cannot grow on minimal media because they have lost a biosynthetic
capability and require supplements; prototrophs are wild-type organisms that can grow on
minimal media
g. Resistance mutations result in acquired resistance to some pathogen, chemical, or
antibiotic
4. Mutations can also occur in regulatory sequences and in tRNA and rRNA genes; all can give
observable phenotypes
II. Detection and Isolation of Mutants
A. Mutant detection
1. Visual observation of changes in colony characteristics
2. Auxotrophic mutants can be detected by replica plating on media with and without the growth
factor required; mutants are those growing with the factor but not without it
B. Mutant selection is achieved by finding the environmental condition under which the mutant will
grow but the wild type will not (useful for isolating auxotrophic revertants, resistance mutants, and
substrate utilization mutations)
C. Mutagens and carcinogens
1. Many cancer-causing agents (carcinogens) are also mutagens, therefore tests for mutagenicity
can be used as a screen for carcinogenic potential
2. The Ames test is a widely used mutagenicity test; it detects an increase in reversion of special
strains of Salmonella typhimurium from histidine auxotrophy to prototrophy after exposure to
a potential carcinogen
III. DNA Repair
A. Proofreading by DNA polymerases immediately repairs many replication errors
B. Excision repair
1. Corrects damage that causes distortions of DNA (e.g., thymine dimers, apurinic or
apyrimidinic sites, damaged or unnatural DNA)
2. For nucleotide excision repair, the damaged area is excised, producing a single-stranded gap,
and then the gap is filled in by DNA polymerase I, and DNA ligase joins the new fragment
into the existing DNA strand
3. For base excision repair, DNA glycosylases remove the damaged base, and this signals AP
nucleases to mark the damaged DNA, which is then excised and repaired by DNA polymerase
I and ligase
C. Direct repair of thymine dimers and alkylated bases occurs through photoreactivation or the action
of alkyl- or methyltransferases, respectively
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D.
Mismatch repair
1. The mismatch repair system corrects replication errors that result in mismatched base pairs;
newly replicated DNA is detected by a lack of DNA methylation
2. The mismatch is detected by MutS and repaired through excision by MutH
E. Recombinational repair
1. Recombination with an undamaged molecule, if available, is used to restore DNA that has
damage in both strands through the action of RecA protein; an undamaged molecule can be
available in rapidly dividing cells where there is a copy of the chromosome that has not yet
segregated into daughter cells
F. The SOS response
1. SOS repair is a type of recombination repair that depends on the RecA protein; it is used to
repair excessive damage that halts replication; it is an error-prone process that results in many
mutations
2. RecA derepresses the synthesis of a variety of DNA repair genes; very serious damage is
treated by translesion DNA synthesis that is highly error prone
IV. Creating Genetic Variability
A. Recombination is a process by which one or more nucleic acid molecules are rearranged or
combined to produce a new nucleotide sequence; mutant and wild-type alleles (alternate forms of a
gene) can be exchanged
B. Horizontal gene transfer in Bacteria and Archaea
1. Horizontal (or lateral) gene transfer moves genes from one mature, independent organism to
another (compare this to vertical gene transfer—transmission of genes from parents to
offspring)
2. Exogenote—donor DNA that enters the bacterium by one of several mechanisms
a. Conjugation is direct transfer from donor bacterium to recipient while the two are
temporarily in physical contact
b. Transformation is transfer of a naked DNA molecule
c. Transduction is transfer mediated by a bacteriophage
3. Endogenote—the genome of the recipient
a. Merozygote—a recipient cell that is temporarily diploid for a portion of the genome
during the gene transfer process
4. Intracellular fates of exogenote
a. Integration into the host chromosome
b. Independent functioning and replication of the exogenote without integration (a partial
diploid clone develops)
c. Survival without replication (only the one cell is a partial diploid)
d. Degradation by host nucleases (host restriction)
C. Recombination at the molecular level
1. General recombination usually involves a reciprocal exchange in which a pair of homologous
sequences breaks and rejoins (double-stranded break model) in a crossover; nonreciprocal
general recombination involves the incorporation of a single strand into the chromosome to
form a stretch of heteroduplex DNA
2. Site-specific recombination is the nonhomologous insertion of DNA into a chromosome; often
occurs during viral genome integration into the host, a process catalyzed by enzymes specific
for the virus and its host
3. Transposition is a kind of recombination that occurs throughout the genome and does not
depend on sequence homology
V. Transposable Elements
A. Transposition is the movement of pieces of DNA around in the genome; transposons are segments
of DNA that can move about chromosomes, "jumping genes"
B. Insertion sequences (IS elements) contain genes only for those enzymes required for transposition
(e.g., transposase); they are bound on both ends by inverted terminal repeat sequences
C. Some transposons carry other genes in addition to those needed for transposition (e.g., for antibiotic
resistance, toxin production, etc.)
D. Transposition can occur by two mechanisms:
1. Simple transposition is a cut-and-paste process involving transposase-catalyzed excision of a
transposon and insertion into a new target site
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2.
Replicative transposition is a mechanism during which a replicated copy of the transposon
inserts at the target site on the DNA, while the original copy remains at the parental site
E. Effects of transposable elements
1. Insertional mutagenesis can cause deletion of genetic material at or near the target site, arrest
of translation or transcription due to stop codons or termination sequences located on the
inserted material, and activation of genes near the point of insertion due to promoters located
on the inserted material
2. Fusion of plasmids and insertion of F plasmids into chromosomes
3. Generation of plasmids with resistance genes
F. Conjugative transposons can move between bacteria through the process of conjugation
VI. Bacterial Plasmids
A. Plasmids are small, circular DNA molecules that replicate independently within host cells; episomes
are plasmids that can exist with or without being integrated into the host chromosomes
B. Conjugative plasmids can transfer copies of themselves to new hosts during conjugation; F factor
plays a key role in bacterial conjugation
VII. Bacterial Conjugation
A. The transfer of genetic information via direct cell-cell contact; this process is mediated by fertility
factors (F plasmids)
B. F+  F– mating
1. In E. coli and other gram-negative bacteria, an F plasmid moves from the donor (F+) to a
recipient (F–) while being replicated
a. Replication is by the rolling circle mechanism where the 3' end is extended from a nick in
one DNA strand, following around the circular genome, and displacing the 5' end
b. The displaced strand is transferred via a sex pilus and then copied to produce doublestranded DNA; the donor retains the other parental DNA strand and its complement; thus
the recipient becomes F+ and the donor remains F+
c. Chromosomal genes are not transferred
C. Hfr conjugation
1. F plasmid integration into the host chromosome results in an Hfr (high frequency of
recombination) strain of bacteria
2. The mechanics of conjugation of Hfr strains are similar to those of F+ strains
3. The initial break for rolling-circle replication is at the integrated plasmid’s origin of transfer
site
a. Part of the plasmid is transferred first
b. Chromosomal genes are transferred next
c. The rest of the plasmid is transferred last
4. Complete transfer of the chromosome takes approximately 100 minutes, but the conjugation
bridge does not usually last that long; therefore, the entire F factor is not usually transferred,
and the recipient remains F–
D. F conjugation
1. When an integrated F plasmid leaves the chromosome incorrectly, it may take with it some
chromosomal genes from one side of the integration site; this results in the formation of an
abnormal plasmid called an F plasmid
2. The F cell (cell harboring an F plasmid) retains its genes, although some of them are in the
chromosome and some are on the plasmid; in conjugation, an F cell behaves as an F+ cell,
mating only with F– cells
3. The chromosomal genes included in the plasmid are transferred with the rest of the plasmid,
but other chromosomal genes usually are not
4. The recipient becomes an F cell, and a partially diploid merozygote
E. Other examples of bacterial conjugation
1. Less is known about conjugative transfer in gram-positive bacteria
2. No sex pilus is formed; however, cells may directly adhere to each other using special
plasmid-encoded proteins
VIII. Bacterial Transformation
A. Transformation—a naked DNA molecule from the environment is taken up by the cell and
incorporated into its chromosome in some heritable form
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B.
A competent cell is one that is capable of taking up DNA and therefore acting as a recipient; only a
limited number of species are naturally competent; the mechanics of the natural transformation
process differ from species to species
C. Species that are not normally competent (such as E. coli) can be made competent by calcium
chloride treatment and other methods that make the cells more permeable to DNA
IX. Transduction
A. Transduction is the transfer of bacterial genes by viruses (bacteriophages); it occurs as the result of
the reproductive cycle of the virus
1. Lytic cycle—a viral reproductive cycle that ends in lysis of the host cell; viruses that use this
cycle are called virulent bacteriophages
2. Lysogeny—a reproductive cycle that involves maintenance of the viral genome (prophage)
within the host cell (usually integrated into the host cell’s chromosome), without immediate
lysis of the host; with each round of cell division, the prophage is replicated and inherited by
daughter cells; bacteriophages reproducing by this mechanism are called temperate phages;
certain stimuli (e.g., UV radiation) can trigger the switch from lysogeny to the lytic cycle
B. Generalized transduction
1. Transfer of any portion of the bacterial genome; occurs during the lytic cycle of virulent and
temperate bacteriophages
2. The phage degrades the host chromosome into randomly sized fragments
3. During assembly, fragments of host DNA of the appropriate size can be mistakenly packaged
into a phage head (generalized transducing particle)
4. When the next host is infected, the bacterial genes are injected and a merozygote is formed
a. Preservation of the transferred genes requires their integration into the host chromosome
b. Much of the transferred DNA does not integrate into the host chromosome, but is often
able to survive and be expressed; the host is called an abortive transductant
C. Specialized transduction
1. Transfer of only specific portions of the bacterial genome; carried out only by temperate
phages that have integrated their DNA into the host chromosome at a specific site in the
chromosome
a. The integrated prophage is sometimes excised incorrectly and contains portions of the
bacterial DNA that was adjacent to the phage’s integration site on the chromosome
b. The excised phage genome is defective because some of its own genes have been
replaced by bacterial genes; therefore, the bacteriophage cannot reproduce
c. When the next host is infected, the donor bacterial genes are injected, leading to the
formation of a merozygote
2. Low-frequency transduction lysates—lysates containing mostly normal phages and just a few
specialized transducing phages
3. High-frequency transduction lysates—lysates containing a relatively large number of
specialized transducing phages; created by coinfecting a host cell with a helper phage (normal
phage) and a transducing phage; the helper phage allows the transducing phage to replicate,
thus increasing the number of transducing phages in the lysate
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TERMS AND DEFINITIONS
Place the letter of each term in the space next to the definition or description that best matches it.
____ 1.
____ 2.
____ 3.
____ 4.
____ 5.
____ 6.
____ 7.
____ 8.
____ 9.
____10.
____11.
____12.
____13.
____14.
____15.
____16.
____17.
____18.
____19.
____20.
____21.
____22.
____23.
____24.
____25.
____26.
____27.
____28.
Alterations in the base sequence of the genomic
nucleic acid
Mutations that are only expressed under certain
environmental conditions
Physical or chemical agents that can cause mutation
Mutations that result in purine-purine or pyrimidinepyrimidine substitutions
Mutations that result in purine-pyrimidine or
pyrimidine-purine substitutions
A mutational reversion assay that is used to determine
if a compound is carcinogenic
Mutations that result in a change in the reading frame
The most prevalent gene form in a population
A mutation from the most prevalent gene form in the
population
A mutation that restores the wild-type phenotype
A second mutation that overcomes the effect of the
first mutation, but does not restore the wild-type
sequence of nucleotides
A mutation that involves only one base pair
A mutation that does not alter the amino acid sequence
of the resulting protein
A mutation that changes the amino acid sequence of
the resulting protein by substitution of one amino acid
for another
A mutation that causes premature termination of the
synthesis of the protein product
A population of cells that are genetically identical
The process that occurs when genetic material from
two organisms is combined, forming a genotype that
differs from that of either parent
The piece of donor DNA in a recombination event
The recipient DNA in a recombination event
A cell that is temporarily diploid for a portion of the
genome during genetic transfer processes
Process that leads to chromosome exchange during
meiosis
A piece of DNA that can move between chromosomes
or within a single chromosome
Transfer of genetic information via direct cell-cell
contact
Transfer of genetic information by uptake of a naked
DNA molecule from the environment
Transfer of genetic information via viruses
The relationship between a phage and its host in which
the phage genome exists in the host and is replicated
without destroying the host cell
The latent form of the virus genome that remains
within the host without destroying it
Transfer of genes between independent, mature
organisms
146
____29.
____30.
DNA molecule or
sequence that has an
origin of replication
replicated as a single
unit
Bacterial structure that
joins a donor and
recipient together and
may serve as the channel
for DNA transfer during
conjugation
a.
b.
Ames test
back (reversion)
mutation
c. clone
d. conditional
mutations
e. conjugation
f. crossing-over
g. endogenote
h. exogenote
i. forward mutation
j. frameshift
mutations
k. horizontal
gene transfer
l. lysogeny
m. merozygote
n. missense mutation
o. mutagens
p. mutations
q. nonsense mutation
r. point mutation
s. prophage
t. recombination
u. replicon
v. sex pilus
w. silent mutation
x. suppressor mutation
y. transduction
z. transformation
aa. transition mutations
bb. transposable
element
cc. transversion
mutations
dd. wild type
FILL IN THE BLANK
1.
Spontaneous mutations arise without exposure to external agents. They are often the result of replication
errors but can also arise when DNA is damaged. For instance, it is possible for a nucleotide to lose its
nitrogenous base producing either an
site or an
site.
2. Induced mutations arise upon exposure to external agents called
that directly damage DNA,
alter its chemistry, or interfere with repair mechanisms. For instance,
(e.g., 5bromouracil) are similar to normal nitrogenous bases and can be incorporated into a polynucleotide chain
during replication. Some chemicals (e.g., acridine orange) insert themselves between the stacked bases of
a DNA double helix. These are called
. Alkylating agents (e.g.,
nitrosoguanidine) alter the structure of nitrogenous bases and change their base-pairing characteristics,
resulting in
.
3. One repair mechanism,
, is able to repair damaged DNA for which there is no
remaining template. The
protein is important in this type of repair. One example of this type of
repair system is
repair, which is induced when DNA damage is so great that DNA synthesis is
stopped. Although this repair mechanism may allow the bacteria to survive, it is error prone and produces
.
4. Mismatched pairs that are not detected during replication by the
activity of DNA polymerase
are usually subsequently corrected by
repair system. This involves excising
nucleotides from one strand and replacing them; thus, this postreplication repair mechanism is also a type
of
repair. In order for this repair system to work, it must be able to distinguish old DNA strands
from new. This distinction is possible because old strands have methyl groups on the bases as the result of
a process called
.
5. Mutations can alter phenotype in several different ways. ____________ mutations change the cellular or
colonial characteristics. ____________ mutations, when expressed, result in the death of the organism. In
diploid organisms, these are usually only recovered if they are recessive; in haploid organisms the
mutation must be ____________ if it is to be recovered.
6. Mutations that inactivate a metabolic pathway are called ____________ mutations. A microorganism
with this kind of mutation is often unable to grow on minimal medium, and for growth it requires an
adequate supply of the pathway’s end product. Such mutants are called ____________, while microbial
strains that can grow on minimal medium are called ____________.
7.
mutations are those that cause a shift in the
frame of a gene. When the
mutation occurs early in a gene, virtually the entire
region is altered resulting in the
synthesis of a truncated or nonfunctional protein. A second
mutation shortly downstream
from the first may restore the
frame and thereby minimize the phenotypic effect. This
second mutation is a good example of a ____________ mutation.
8. Thymine dimers can be split apart into separate thymines with the help of visible light in a photochemical
reaction catalyzed by the enzyme photolyase. This is called ____________, and since it does not remove
and replace nucleotides, it is relatively free of ____________.
9. Mutations that appear to have been chosen by the organism so that it is better adapted to its
environment are called _______________ or _______________ mutations. It is hypothesized that
they may be the result of _______________ followed by selection of favorable mutants.
10. The most common form of recombination is ____________ recombination, which usually involves a
reciprocal exchange between a pair of homologous DNA sequences. Integration of viral genomes into
bacterial chromosomes can occur by another type of recombination known as ____________
recombination, in which the viral genetic material is not homologous with the host DNA. In
recombination, recombination accompanies replication of genetic material and does not depend on
sequence homology.
11. Bacterial recombination normally takes place when a piece of donor DNA, the ____________, enters the
cell and becomes a stable part of the recipient’s genome, the ____________. During replacement of host
genetic material, the recipient becomes a diploid for a portion of the genome and is referred to as a
____________.
12. There are two types of transposable elements. The simplest are
, which contain only the
genes needed for transposition bounded at both ends by inverted repeats. One of these genes codes for the
147
13.
14.
15.
16.
17.
18.
19.
20.
enzyme
. The second type of transposable element is more complex and contains genes
other than those required for transposition. Some transposons bear transfer genes and can move between
bacteria through the process of conjugation. These are called
.
Proteins produced by bacteria that destroy other bacteria are called
.
The F (fertility) factor carries genes for synthesis of a
and for plasmid transfer. During
mating of donor and recipient strains, a process called ____________, the F factor replicates by the
rolling-circle mechanism and one copy moves to the recipient.
Transfer of genetic information by uptake of a naked DNA molecule from the environment is called
____________. In order to take up a naked DNA molecule, a cell must be ____________, which may
only occur at certain stages in the life cycle of the organism.
The transfer of bacterial genes by viruses is called
. When any part of the bacterial
chromosome is transferred, it is called unrestricted, or _________
, and the phages that
transfer the DNA are called
phages. Some temperate phages can incorrectly excise
from the host chromosome when switching from lysogeny to the lytic cycle, and these may carry genes
that were adjacent to the integration site. Since the genes that may be carried are restricted to those
located near specific integration sites, this is called restricted, or ____________,
.
Conjugation involving ____________ strains is frequently used to map the relative locations of bacterial
genes. The technique involves disruption of the conjugation bridge in what is called an ____________
____________ experiment.
During bacterial transformation,
is formed. This is a short stretch of DNA for
which one strand is from the donor and the other is from the recipient.
Some phage-infected bacteria only produce phages under certain environmental conditions. These
bacteria are said to be
or
. The phages infecting these bacteria are called
phages, and the latent form of the phage genome that remains in the host without destroying it is called
the
.
The lysate produced following induction of lysogenized bacteria is called a
lysate,
because it contains only a few transducing phages. Each transducing phage is defective and can only
integrate into a new host genome if a normal phage, called the
phage is in the same cell. The
lysate produced following induction of bacteria lysogenized by both a defective phage and a normal
phage is called a
lysate, because it contains a roughly equal number of normal
phages and transducing phages.
MULTIPLE CHOICE
1.
2.
3.
Which of the following repair mechanisms
corrects damage that causes distortions in the
DNA double helix (e.g., thymine dimers,
apurinic sites, apyrimidinic sites) by
removing and replacing a short stretch of
nucleotides in the damaged strand?
a. photoreactivation
b. mismatch repair
c. excision repair
d. recombination repair
Resistance mutations can confer resistance to
which of the following?
a. pathogens
b. chemicals
c. antibiotics
d. All of the above are correct.
A particular mutation results in the
substitution of cytosine for thymine in one
strand of the DNA. Upon subsequent DNA
replication, one of the daughter cells receives
4.
148
a GC pair in this position instead of an AT
pair. What is this type of mutation called?
a. transversion
b. transition
c. frameshift
d. insertion
Which of the following can cause transition
and transversion mutations?
a. incorporation of a base analog that
exhibits different base-pairing
properties from those of the base it
replaces
b. chemical modification of an existing
base in the DNA so that during the next
round of replication it will base pair
differently from the unmodified base
c. Both of the above are correct.
d. None of the above is correct.
5.
6.
7.
8.
9.
b. F–- plasmid
c. Hfr plasmid
d. F′ plasmid
10. Which of the following has NOT been used
to map chromosomal locations of bacterial
genes?
a. Hfr  F– conjugation
b. F  F– conjugation
c. F+  F– conjugation
d. All of the above have been used to map
bacterial genes.
11. Which of the following represents the best
description of host restriction?
a. the inability to take up an exogenote
during transformation
b. the inability to integrate an exogenote
into the host chromosome
c. the degradation of an exogenote by host
nucleases
d. the inability to express the genes
located on an exogenote
12. Which of the following is NOT a possible
fate for an exogenote?
a. integration into the host chromosome
b. expression of the genes and replication
of the exogenote without integration
into the host chromosome
c. survival of the exogenote without
integration or replication
d. All of the above are possible fates for
an exogenote.
Back mutations that restore the wild-type
phenotype can occur by which of the
following mechanisms?
a. true reversion back to the wild-type
nucleotide sequence
b. mutation that results in a different
nucleotide sequence from that of the
wild type, but that restores the amino
acid sequence in the protein to the wildtype sequence
c. a second mutation that overcomes the
effect of the first mutation; the first
mutation is not changed, but the
function of the protein is restored
d. All of the above can restore the wildtype phenotype.
In which way do transposable elements differ
from temperate bacteriophages or from
plasmids?
a. Transposable elements lack a viral life
cycle.
b. Transposable elements are unable to
reproduce autonomously.
c. Transposable elements are unable to
exist apart from the chromosome.
d. All of the above are ways that
transposable elements differ from
bacteriophages or plasmids.
What impact can transposons have on the
host cell?
a. They can cause mutations.
b. They can block transcription or
translation.
c. They can turn genes on or off.
d. All of the above are possible impacts of
transposable elements.
Which of the following is NOT true about
Hfr  F– matings?
a. The recipient can become F+ (or Hfr) if
the mating lasts long enough for the
entire bacterial chromosome to be
transferred.
b. The recipient usually remains F–
because the connection usually breaks
before the entire bacterial chromosome
can be transferred.
c. The recipient may become F if more
than half of the plasmid is transferred.
d. All of the above are true about Hfr  F–
matings.
When an F factor leaves an Hfr chromosome,
it occasionally picks up some bacterial genes.
What is the resulting plasmid called?
a. F+ plasmid
149
TRUE/FALSE
____ 1.
____ 2.
____ 3.
____ 4.
____ 5.
____ 6.
____ 7.
____ 8.
____ 9.
____ 10.
____ 11.
____ 12.
____ 13.
____ 14.
____ 15.
____ 16.
____ 17.
Missense mutations may play an important role in providing new variability to drive evolution
because they are often not lethal and, therefore, they remain in the gene pool.
Nonsense mutations that cause premature termination of translation always severely affect the
phenotypic expression of the gene by resulting in the production of a nonfunctional gene product.
Point mutations are more common than large deletions or insertions.
Bacteria that are partial diploids, containing nonintegrated, transduced DNA, are called abortive
transductants.
A plasmid that can exist independent of the host chromosome but that cannot be integrated into the
host chromosome is called an episome.
The F factor is a conjugative plasmid that is particularly efficient at initiating conjugation with
appropriate recipient cells.
Nonconjugative plasmids can move between bacteria during conjugation if conjugation is initiated
by another plasmid that is conjugative.
Multiple-drug-resistant plasmids are usually produced when a single plasmid accumulates several
transposons, each carrying one or more antibiotic resistance genes.
Transposable elements have only been found in prokaryotes and do not appear to play a major role
in eukaryotic genetics.
In an F+  F- mating, the recipient becomes F+ after mating has been completed.
In an F+  F- mating, the donor becomes F- after mating has been completed.
In an F+ F- mating, chromosomal genes are frequently transferred.
All R factor plasmids are nonconjugative.
Host restriction refers to the ability of some organisms to degrade exogenotes that enter the cell.
During transposition, the original transposon is replicated and remains at the parental site in the
chromosome, while the copy moves to a new site.
Even if two viruses simultaneously enter a host cell, no recombination can occur between the two
viral genomes.
Bacterial recombination is a two-way process in which DNA is exchanged between the two cells
involved.
CRITICAL THINKING
1.
A strain of bacteria is protrophic. How would you isolate from this strain one that requires the amino acid
leucine (i.e., it is a leucine auxotroph)?
2.
You have a bacterial strain that is a tryptophan auxotroph and sensitive to the antibiotic streptomycin.
You expose this strain to a mutagen. How would you isolate mutants that no longer require tryptophan
(i.e., strains that have reverted to prototrophy)? How would you isolate mutants that are resistant to
streptomycin? How would you isolate mutants that no longer require leucine and that are resistant to
streptomycin?
ANSWER KEY
Terms and Definitions
1. p, 2. d, 3. o, 4. aa, 5. cc, 6. a, 7. j, 8. dd, 9. i, 10. b, 11. x, 12. r, 13. w, 14. n, 15. q, 16. c, 17. t, 1 8. h, 19. g,
20. m, 21. f, 22. bb, 23. e, 24. z, 25. y, 26. l, 27. s, 28. k, 29. u, 30. v
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Fill in the Blank
1. apurinic; apyrimidinic 2. mutagens; base analogs; intercalating agents; specific mispairing 3. recombination
repair; recA; SOS; mutations 4. proofreading; mismatch; excision; DNA methylation 5. Morphological; Lethal;
conditional 6. biochemical; auxotrophs; prototrophs 7. Frameshift; reading; coding; frameshift; reading;
suppressor 8. photoreactivation; error 9. directed; adaptive; hypermutation 10. general; site-specific;
replicative 11. exogenote; endogenote; merozygote 12. insertion sequences; transposase; conjugative
transposons 13. bacteriocins 14. sex pilus; conjugation 15. transformation; competent 16. transduction;
generalized transduction; generalized transducing; specialized transduction 17. Hfr; interrupted mating 18.
heteroduplex DNA 19. lysogens; lysogenic; temperate; prophage 20. low-frequency transduction; helper; highfrequency transduction
Multiple Choice
1. c, 2. d, 3. b, 4. c, 5. d, 6. c, 7. d, 8. c, 9. d, 10. c, 11. c, 12. d
True/False
1. T, 2. F, 3. T, 4. T, 5. F, 6. T, 7. T, 8. T, 9. F, 10. T, 11. F, 12. F, 13. F, 14. T, 15. T, 16. F, 17. F
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