Chapter 14
Recombination
and repair
14.1 Introduction
14.2 Breakage and reunion involves heteroduplex DNA
14.3 Double-strand breaks initiate recombination
14.4 Double-strand breaks initiate snapsis
14.5 Bacterial recombination involves single-strand assimilation
14.6 Gene conversion accounts for interallelic recombination
14.7 Topological manipulation of DNA
14.8 Specialized recombination involves breakage and reunion at
specific sites
14.9 Repair systems correct damage to DNA
14.10 Excision repair systems in E,coli
14.11 Controlling the direction of mismatch repair
14.12 Retrieval systems in E,coli
14.13 RecA triggers the SOS system
14.14 Eukaryotic repair systems
Bivalent is the structure containing all four chromatids (two representing each
homologue) at the start of meiosis.
Breakage and reunion describes the mode of genetic recombination,in which
two DNA duplex molecules are broken at corresponding points and then rejoined
crosswise (involving formation of a length of heteroduplex DNA around the site
of joining).
Site-specific recombination occurs between two specific (not necessarily
homologous) sequences,as in phage integration/excision or resolution of
cointegrate structures during transposition.
Synapsis describes the association of the two pairs of sister chromatids
representing homologous chromosomes that occurs at the start of meiosis;
resulting structure is called a bivalent.
Synaptonemal complex describes the morphological structure of synapsed
chromosomes.
Transposition refers to the movement of a transposon to a new site in the genome.
14.1 Introduction
Figure 14.1 Recombination
occurs during the first meiotic
prophase,The stages of
prophase are defined by the
appearance of the
chromosomes,each of which
consists of two replicas (sister
chromatids),although the
duplicated state becomes
visible only at the end,The
molecular interactions of any
individual crossing-over event
involve two of the four duplex
DNAs.
14.1
Introduction
Figure 1.22 The ABO blood
group locus codes for a
galactosyltransferase whose
specificity determines the
blood group.
14.1
Introduction
Branch migration describes the ability of a DNA strand
partially paired with its complement in a duplex to extend
its pairing by displacing the resident strand with which it
is homologous.
Hybrid DNA is another term for heteroduplex DNA.
Recombinant joint is the point at which two recombining
molecules of duplex DNA are connected (the edge of the
heteroduplex region).
14.2 Breakage and reunion involves
heteroduplex DNA
Figure 14.1 Recombination occurs
during the first meiotic prophase,
The stages of prophase are defined
by the appearance of the
chromosomes,each of which
consists of two replicas (sister
chromatids),although the
duplicated state becomes visible
only at the end,The molecular
interactions of any individual
crossing-over event involve two of
the four duplex DNAs.
14.2 Breakage and
reunion involves
heteroduplex DNA
Figure 14.2
Recombination between
two paired duplex DNAs
could involve reciprocal
single-strand exchange,
branch migration,and
nicking.
14.2 Breakage and
reunion involves
heteroduplex DNA
Figure 14.3 Branch
migration can occur
in either direction
when an unpaired
single strand
displaces a paired
strand.
14.2 Breakage and
reunion involves
heteroduplex DNA
Figure 14.4 Resolution
of a Holliday junction
can generate parental or
recombinant duplexes,
depending on which
strands are nicked,Both
types of product have a
region of heteroduplex
DNA.
14.2 Breakage and
reunion involves
heteroduplex DNA
Figure 14.5
Recombination is
initiated by a double-
strand break,followed
by formation of single-
stranded 3￠ ends,one
of which migrates to a
homologous duplex.
14.3 Double-strand
breaks initiate
recombination
Figure 14.6 The synaptonemal complex brings chromosomes
into juxtaposition,This example of Neotellia was kindly
provided by M,Westergaard and D,Von Wettstein.
14.4 Double-strand breaks initiate synapsis
Figure 14.7 Double-strand breaks appear when axial elements
form,and disappear during the extension of synaptonemal
complexes,Joint molecules appear and persist until DNA
recombinants are detected at the end of pachytene.
14.4 Double-strand breaks initiate synapsis
Figure 14.8 Spo11 is
covalently joined to
the 5￠ ends of
double-strand breaks.
14.4 Double-strand
breaks initiate
synapsis
Figure 14.9 RecBCD
nuclease approaches a chi
sequence from one side,
degrading DNA as it
proceeds; at the chi site,it
makes an endonucleolytic
cut,loses RecD,and retains
only the helicase activity.
14.5 The bacterial
RecBCD system is
stimulated by chi
sequences
Paranemic joint describes a region in which two
complementary sequences of DNA are associated side
by side instead of being intertwined in a double
helical structure.
Single-strand assimilation describes the ability of
RecA protein to cause a single strand of DNA to
displace its homologous strand in a duplex; that is,the
single strand is assimilated into the duplex.
14.6 RecA catalyzes single-
strand assimilation
Figure 14.10 RecA promotes the assimilation of invading single strands into
duplex DNA so long as one of the reacting strands has a free end.
14.6 RecA catalyzes single-strand assimilation
Figure 14.2
Recombination between
two paired duplex
DNAs could involve
reciprocal single-strand
exchange,branch
migration,and nicking.
14.6 RecA
catalyzes single-
strand
assimilation
Figure 14.11 RecA-
mediated strand exchange
between partially duplex
and entirely duplex DNA
generates a joint molecule
with the same structure as
a recombination
intermediate.
14.6 RecA
catalyzes single-
strand
assimilation
Figure 14.12
RuvAB is an
asymmetric
complex that
promotes
branch
migration of
a Holliday
junction.
14.6 RecA catalyzes single-strand assimilation
Figure 14.13 Bacterial enzymes
can catalyze all stages of
recombination in the repair
pathway following the
production of suitable substrate
DNA molecules.
14.6 RecA catalyzes
single-strand
assimilation
Gene conversion is the alteration of one strand of a
heteroduplex DNA to make it complementary with
the other strand at any position(s) where there were
mispaired bases.
Postmeiotic segregation describes the segregation of
two strands of a duplex DNA that bear different
information (created by heteroduplex formation
during meiosis) when a subsequent replication allows
the strands to separate.
14.8 Gene conversion accounts for
interallelic recombination
Figure 14.14 Spore
formation in the
Ascomycetes
allows
determination of
the genetic
constitution of each
of the DNA strands
involved in meiosis.
14.8 Gene conversion accounts for
interallelic recombination
Supercoiling describes the coiling of a closed duplex
DNA in space so that it crosses over its own axis.
Topological isomers are molecules of DNA that are
identical except for a difference in linking number.
Twisting number of a DNA is the number of base
pairs divided by the number of base pairs per turn of
the double helix.
Writhing number is the number of times a duplex axis
crosses over itself in space.
14.9 Topological manipulation of DNA
Figure 14.14 Spore
formation in the
Ascomycetes allows
determination of the
genetic constitution of
each of the DNA
strands involved in
meiosis.
14.9 Topological manipulation of DNA
Figure 14.15 Separation
of the strands of a DNA
double helix could be
achieved by several
means.
14.9 Topological
manipulation of DNA
Figure 9.18 E,coli sigma factors recognize
promoters with different consensus sequences,
(Numbers in the name of a factor indicate its mass.)
14.9 Topological manipulation of DNA
Figure 14.16 Bacterial type I
topoisomerases recognize
partially unwound segments
of DNA and pass one strand
through a break made in the
other.
14.9 Topological
manipulation of DNA
Figure 14.17 Type II
topoisomerases can pass a
duplex DNA through a
double-strand break in
another duplex.
14.9 Topological
manipulation of DNA
Figure 14.18 DNA gyrase
may introduce negative
supercoils in duplex
DNA by inverting a
positive supercoil.
14.9 Topological
manipulation of DNA
att sites are the loci on a phage and the
bacterial chromosome at which
recombination integrates the phage into,
or excises it from,the bacterial
chromosome.
14.10 Specialized recombination involves
breakage and reunion at specific sites
Figure 14.19 Circular phage DNA
is converted to an integrated
prophage by a reciprocal
recombination between attP and
attB; the prophage is excised by
reciprocal recombination between
attL and attR.
14.10 Specialized
recombination involves
breakage and reunion at
specific sites
Figure 14.20 Does
recombination
between attP and
attB proceed by
sequential exchange
or concerted cutting?
14.10 Specialized
recombination
involves breakage
and reunion at
specific sites
Figure 14.21 Staggered cleavages in the common core
sequence of attP and attB allow crosswise reunion to
generate reciprocal recombinant junctions.
14.10 Specialized recombination involves
breakage and reunion at specific sites
Figure 14.2
Recombination
between two paired
duplex DNAs could
involve reciprocal
single-strand exchange,
branch migration,and
nicking.
14.10 Specialized
recombination
involves breakage
and reunion at
specific sites
Figure 14.22 Int and IHF bind to different sites in
attP,The Int recognition sequences in the core
region include the sites of cutting.
14.10 Specialized recombination involves
breakage and reunion at specific sites
Figure 14.23 The Int binding sites
in the core lie on one face of
DNA,The large circles indicate
positions at which methylation is
influenced by Int binding; the
large arrows indicate the sites of
cutting,Photograph kindly
provided by A,Landy.
14.10 Specialized recombination
involves breakage and reunion at
specific sites
Figure 14.24 Multiple
copies of Int protein may
organize attP into an
intasome,which initiates
site-specific recombination
by recognizing attB on free
DNA.
14.10 Specialized
recombination involves
breakage and reunion at
specific sites
Figure 14.25
Substitutions of
individual bases
create mismatched
pairs that may be
corrected by
replacing one base;
if uncorrected they
cause a mutation in
one daughter duplex.
14.11 Repair systems correct damage to DNA
Figure 14.26
Modifications or
removal of bases may
cause structural defects
that prevent replication
or induce mutations in
each replication cycle
until they are removed.
14.11 Repair
systems correct
damage to DNA
Figure 14.14 Spore formation in the Ascomycetes
allows determination of the genetic constitution of
each of the DNA strands involved in meiosis.
14.11 Repair systems correct damage to DNA
Excision of phage or episome or
other sequence describes its
release from the host chromosome
as an autonomous DNA molecule.
14.12 Excision repair systems in E,coli
Figure 14.27 Excision-
repair removes and
replaces a stretch of DNA
that includes the damaged
base(s).
14.12 Excision repair
systems in E,coli
Figure 14.28 The Uvr
system operates in stages
in which UvrAB
recognizes damage,UvrBC
nicks the DNA,and UvrD
unwinds the marked region.
14.12 Excision repair
systems in E,coli
Figure 14.28 The Uvr
system operates in stages
in which UvrAB
recognizes damage,UvrBC
nicks the DNA,and UvrD
unwinds the marked region.
14.12 Excision repair
systems in E,coli
Figure 14.37 A helicase
unwinds DNA at a damaged
site,endonucleases cut on
either side of the lesion,and
new DNA is synthesized to
replace the excised stretch.
14.12 Excision repair
systems in E,coli
Figure 14.29 A
glycosylase
removes a base
from DNA by
cleaving the bond
to the deoxyribose.
14.13 Base flipping is
used by methylases
and glycosylases
Figure 14.30 A
glycosylase hydrolyzes the
bond between base and
deoxyribose (using H20),
but a lyase takes the
reaction further by pening
the sugar ring (using NH2).
14.13 Base flipping is
used by methylases
and glycosylases
Figure 14.31 A
methylase "flips" the
target cytosine out of
the double helix in
order to modify it,
Photograph kindly
provided by Rich
Roberts.
14.13 Base flipping is
used by methylases and
glycosylases
Figure 14.32
Preferential removal
of bases in pairs that
have oxidized
guanine is designed to
minimize mutations.
14.15 Controlling the direction of mismatch repair
Figure 13.30 Replication
of methylated DNA
gives hemimethylated
DNA,which maintains
its state at GATC sites
until the Dam methylase
restores the fully
methylated condition.
14.15 Controlling the direction of mismatch repair
Figure 14.33 GATC sequences
are targets for the Dam
methylase after replication,
During the period before this
methylation occurs,the
nonmethylated strand is the
target for repair of mismatched
bases.
14.15 Controlling the
direction of mismatch repair
Figure 14.34 MutS
recognizes a mismatch and
translocates to a GATC site,
MutH cleaves the
unmethylated strand at the
GATC,Endonucleases
degrade the strand from the
GATC to the mismatch site.
14.15 Controlling the
direction of mismatch
repair
Figure 14.31 An E,coli
retrieval system uses a normal
strand of DNA replace the gap
left in a newly synthesized
strand opposite a site of
unrepaired damage.
14.16
Retrival
systems in
E,coli
Figure 14.32 The
LexA protein represses
many genes,including
repair functions,recA
and lexA,,Activation
of RecA leads to
proteolytic cleavage of
LexA and induces all
of these genes.
14.17 RecA
triggers the
SOS system
Figure 14.32 The
LexA protein represses
many genes,including
repair functions,recA
and lexA,,Activation
of RecA leads to
proteolytic cleavage of
LexA and induces all
of these genes.
14.17 RecA
triggers the
SOS system
Figure 14.37 A helicase
unwinds DNA at a damaged
site,endonucleases cut on
either side of the lesion,and
new DNA is synthesized to
replace the excised stretch.
14.18 Eukaryotic
repair systems
Summary
Recombination
and repair
14.1 Introduction
14.2 Breakage and reunion involves heteroduplex DNA
14.3 Double-strand breaks initiate recombination
14.4 Double-strand breaks initiate snapsis
14.5 Bacterial recombination involves single-strand assimilation
14.6 Gene conversion accounts for interallelic recombination
14.7 Topological manipulation of DNA
14.8 Specialized recombination involves breakage and reunion at
specific sites
14.9 Repair systems correct damage to DNA
14.10 Excision repair systems in E,coli
14.11 Controlling the direction of mismatch repair
14.12 Retrieval systems in E,coli
14.13 RecA triggers the SOS system
14.14 Eukaryotic repair systems
Bivalent is the structure containing all four chromatids (two representing each
homologue) at the start of meiosis.
Breakage and reunion describes the mode of genetic recombination,in which
two DNA duplex molecules are broken at corresponding points and then rejoined
crosswise (involving formation of a length of heteroduplex DNA around the site
of joining).
Site-specific recombination occurs between two specific (not necessarily
homologous) sequences,as in phage integration/excision or resolution of
cointegrate structures during transposition.
Synapsis describes the association of the two pairs of sister chromatids
representing homologous chromosomes that occurs at the start of meiosis;
resulting structure is called a bivalent.
Synaptonemal complex describes the morphological structure of synapsed
chromosomes.
Transposition refers to the movement of a transposon to a new site in the genome.
14.1 Introduction
Figure 14.1 Recombination
occurs during the first meiotic
prophase,The stages of
prophase are defined by the
appearance of the
chromosomes,each of which
consists of two replicas (sister
chromatids),although the
duplicated state becomes
visible only at the end,The
molecular interactions of any
individual crossing-over event
involve two of the four duplex
DNAs.
14.1
Introduction
Figure 1.22 The ABO blood
group locus codes for a
galactosyltransferase whose
specificity determines the
blood group.
14.1
Introduction
Branch migration describes the ability of a DNA strand
partially paired with its complement in a duplex to extend
its pairing by displacing the resident strand with which it
is homologous.
Hybrid DNA is another term for heteroduplex DNA.
Recombinant joint is the point at which two recombining
molecules of duplex DNA are connected (the edge of the
heteroduplex region).
14.2 Breakage and reunion involves
heteroduplex DNA
Figure 14.1 Recombination occurs
during the first meiotic prophase,
The stages of prophase are defined
by the appearance of the
chromosomes,each of which
consists of two replicas (sister
chromatids),although the
duplicated state becomes visible
only at the end,The molecular
interactions of any individual
crossing-over event involve two of
the four duplex DNAs.
14.2 Breakage and
reunion involves
heteroduplex DNA
Figure 14.2
Recombination between
two paired duplex DNAs
could involve reciprocal
single-strand exchange,
branch migration,and
nicking.
14.2 Breakage and
reunion involves
heteroduplex DNA
Figure 14.3 Branch
migration can occur
in either direction
when an unpaired
single strand
displaces a paired
strand.
14.2 Breakage and
reunion involves
heteroduplex DNA
Figure 14.4 Resolution
of a Holliday junction
can generate parental or
recombinant duplexes,
depending on which
strands are nicked,Both
types of product have a
region of heteroduplex
DNA.
14.2 Breakage and
reunion involves
heteroduplex DNA
Figure 14.5
Recombination is
initiated by a double-
strand break,followed
by formation of single-
stranded 3￠ ends,one
of which migrates to a
homologous duplex.
14.3 Double-strand
breaks initiate
recombination
Figure 14.6 The synaptonemal complex brings chromosomes
into juxtaposition,This example of Neotellia was kindly
provided by M,Westergaard and D,Von Wettstein.
14.4 Double-strand breaks initiate synapsis
Figure 14.7 Double-strand breaks appear when axial elements
form,and disappear during the extension of synaptonemal
complexes,Joint molecules appear and persist until DNA
recombinants are detected at the end of pachytene.
14.4 Double-strand breaks initiate synapsis
Figure 14.8 Spo11 is
covalently joined to
the 5￠ ends of
double-strand breaks.
14.4 Double-strand
breaks initiate
synapsis
Figure 14.9 RecBCD
nuclease approaches a chi
sequence from one side,
degrading DNA as it
proceeds; at the chi site,it
makes an endonucleolytic
cut,loses RecD,and retains
only the helicase activity.
14.5 The bacterial
RecBCD system is
stimulated by chi
sequences
Paranemic joint describes a region in which two
complementary sequences of DNA are associated side
by side instead of being intertwined in a double
helical structure.
Single-strand assimilation describes the ability of
RecA protein to cause a single strand of DNA to
displace its homologous strand in a duplex; that is,the
single strand is assimilated into the duplex.
14.6 RecA catalyzes single-
strand assimilation
Figure 14.10 RecA promotes the assimilation of invading single strands into
duplex DNA so long as one of the reacting strands has a free end.
14.6 RecA catalyzes single-strand assimilation
Figure 14.2
Recombination between
two paired duplex
DNAs could involve
reciprocal single-strand
exchange,branch
migration,and nicking.
14.6 RecA
catalyzes single-
strand
assimilation
Figure 14.11 RecA-
mediated strand exchange
between partially duplex
and entirely duplex DNA
generates a joint molecule
with the same structure as
a recombination
intermediate.
14.6 RecA
catalyzes single-
strand
assimilation
Figure 14.12
RuvAB is an
asymmetric
complex that
promotes
branch
migration of
a Holliday
junction.
14.6 RecA catalyzes single-strand assimilation
Figure 14.13 Bacterial enzymes
can catalyze all stages of
recombination in the repair
pathway following the
production of suitable substrate
DNA molecules.
14.6 RecA catalyzes
single-strand
assimilation
Gene conversion is the alteration of one strand of a
heteroduplex DNA to make it complementary with
the other strand at any position(s) where there were
mispaired bases.
Postmeiotic segregation describes the segregation of
two strands of a duplex DNA that bear different
information (created by heteroduplex formation
during meiosis) when a subsequent replication allows
the strands to separate.
14.8 Gene conversion accounts for
interallelic recombination
Figure 14.14 Spore
formation in the
Ascomycetes
allows
determination of
the genetic
constitution of each
of the DNA strands
involved in meiosis.
14.8 Gene conversion accounts for
interallelic recombination
Supercoiling describes the coiling of a closed duplex
DNA in space so that it crosses over its own axis.
Topological isomers are molecules of DNA that are
identical except for a difference in linking number.
Twisting number of a DNA is the number of base
pairs divided by the number of base pairs per turn of
the double helix.
Writhing number is the number of times a duplex axis
crosses over itself in space.
14.9 Topological manipulation of DNA
Figure 14.14 Spore
formation in the
Ascomycetes allows
determination of the
genetic constitution of
each of the DNA
strands involved in
meiosis.
14.9 Topological manipulation of DNA
Figure 14.15 Separation
of the strands of a DNA
double helix could be
achieved by several
means.
14.9 Topological
manipulation of DNA
Figure 9.18 E,coli sigma factors recognize
promoters with different consensus sequences,
(Numbers in the name of a factor indicate its mass.)
14.9 Topological manipulation of DNA
Figure 14.16 Bacterial type I
topoisomerases recognize
partially unwound segments
of DNA and pass one strand
through a break made in the
other.
14.9 Topological
manipulation of DNA
Figure 14.17 Type II
topoisomerases can pass a
duplex DNA through a
double-strand break in
another duplex.
14.9 Topological
manipulation of DNA
Figure 14.18 DNA gyrase
may introduce negative
supercoils in duplex
DNA by inverting a
positive supercoil.
14.9 Topological
manipulation of DNA
att sites are the loci on a phage and the
bacterial chromosome at which
recombination integrates the phage into,
or excises it from,the bacterial
chromosome.
14.10 Specialized recombination involves
breakage and reunion at specific sites
Figure 14.19 Circular phage DNA
is converted to an integrated
prophage by a reciprocal
recombination between attP and
attB; the prophage is excised by
reciprocal recombination between
attL and attR.
14.10 Specialized
recombination involves
breakage and reunion at
specific sites
Figure 14.20 Does
recombination
between attP and
attB proceed by
sequential exchange
or concerted cutting?
14.10 Specialized
recombination
involves breakage
and reunion at
specific sites
Figure 14.21 Staggered cleavages in the common core
sequence of attP and attB allow crosswise reunion to
generate reciprocal recombinant junctions.
14.10 Specialized recombination involves
breakage and reunion at specific sites
Figure 14.2
Recombination
between two paired
duplex DNAs could
involve reciprocal
single-strand exchange,
branch migration,and
nicking.
14.10 Specialized
recombination
involves breakage
and reunion at
specific sites
Figure 14.22 Int and IHF bind to different sites in
attP,The Int recognition sequences in the core
region include the sites of cutting.
14.10 Specialized recombination involves
breakage and reunion at specific sites
Figure 14.23 The Int binding sites
in the core lie on one face of
DNA,The large circles indicate
positions at which methylation is
influenced by Int binding; the
large arrows indicate the sites of
cutting,Photograph kindly
provided by A,Landy.
14.10 Specialized recombination
involves breakage and reunion at
specific sites
Figure 14.24 Multiple
copies of Int protein may
organize attP into an
intasome,which initiates
site-specific recombination
by recognizing attB on free
DNA.
14.10 Specialized
recombination involves
breakage and reunion at
specific sites
Figure 14.25
Substitutions of
individual bases
create mismatched
pairs that may be
corrected by
replacing one base;
if uncorrected they
cause a mutation in
one daughter duplex.
14.11 Repair systems correct damage to DNA
Figure 14.26
Modifications or
removal of bases may
cause structural defects
that prevent replication
or induce mutations in
each replication cycle
until they are removed.
14.11 Repair
systems correct
damage to DNA
Figure 14.14 Spore formation in the Ascomycetes
allows determination of the genetic constitution of
each of the DNA strands involved in meiosis.
14.11 Repair systems correct damage to DNA
Excision of phage or episome or
other sequence describes its
release from the host chromosome
as an autonomous DNA molecule.
14.12 Excision repair systems in E,coli
Figure 14.27 Excision-
repair removes and
replaces a stretch of DNA
that includes the damaged
base(s).
14.12 Excision repair
systems in E,coli
Figure 14.28 The Uvr
system operates in stages
in which UvrAB
recognizes damage,UvrBC
nicks the DNA,and UvrD
unwinds the marked region.
14.12 Excision repair
systems in E,coli
Figure 14.28 The Uvr
system operates in stages
in which UvrAB
recognizes damage,UvrBC
nicks the DNA,and UvrD
unwinds the marked region.
14.12 Excision repair
systems in E,coli
Figure 14.37 A helicase
unwinds DNA at a damaged
site,endonucleases cut on
either side of the lesion,and
new DNA is synthesized to
replace the excised stretch.
14.12 Excision repair
systems in E,coli
Figure 14.29 A
glycosylase
removes a base
from DNA by
cleaving the bond
to the deoxyribose.
14.13 Base flipping is
used by methylases
and glycosylases
Figure 14.30 A
glycosylase hydrolyzes the
bond between base and
deoxyribose (using H20),
but a lyase takes the
reaction further by pening
the sugar ring (using NH2).
14.13 Base flipping is
used by methylases
and glycosylases
Figure 14.31 A
methylase "flips" the
target cytosine out of
the double helix in
order to modify it,
Photograph kindly
provided by Rich
Roberts.
14.13 Base flipping is
used by methylases and
glycosylases
Figure 14.32
Preferential removal
of bases in pairs that
have oxidized
guanine is designed to
minimize mutations.
14.15 Controlling the direction of mismatch repair
Figure 13.30 Replication
of methylated DNA
gives hemimethylated
DNA,which maintains
its state at GATC sites
until the Dam methylase
restores the fully
methylated condition.
14.15 Controlling the direction of mismatch repair
Figure 14.33 GATC sequences
are targets for the Dam
methylase after replication,
During the period before this
methylation occurs,the
nonmethylated strand is the
target for repair of mismatched
bases.
14.15 Controlling the
direction of mismatch repair
Figure 14.34 MutS
recognizes a mismatch and
translocates to a GATC site,
MutH cleaves the
unmethylated strand at the
GATC,Endonucleases
degrade the strand from the
GATC to the mismatch site.
14.15 Controlling the
direction of mismatch
repair
Figure 14.31 An E,coli
retrieval system uses a normal
strand of DNA replace the gap
left in a newly synthesized
strand opposite a site of
unrepaired damage.
14.16
Retrival
systems in
E,coli
Figure 14.32 The
LexA protein represses
many genes,including
repair functions,recA
and lexA,,Activation
of RecA leads to
proteolytic cleavage of
LexA and induces all
of these genes.
14.17 RecA
triggers the
SOS system
Figure 14.32 The
LexA protein represses
many genes,including
repair functions,recA
and lexA,,Activation
of RecA leads to
proteolytic cleavage of
LexA and induces all
of these genes.
14.17 RecA
triggers the
SOS system
Figure 14.37 A helicase
unwinds DNA at a damaged
site,endonucleases cut on
either side of the lesion,and
new DNA is synthesized to
replace the excised stretch.
14.18 Eukaryotic
repair systems
Summary
