Chapter 12
The replicon
12.1 Introduction
12.2 Replicons can be linear or circular
12.3 Origins can be mapped by autoradiography and electrophoresis
12.4 The bacterial genome is a single circular replicon
12.5 Each eukaryotic chromosome contains many replicons
12.6 Isolating the origins of yeast replicons
12.7 D loops maintain mitochondrial origins
12.8 The problem of linear replicons
12.9 Rolling circles produce multimers of a replicon
12.10 Rolling circles are used to replicate phage genomes
12.11 The F plasmid is transferred by conjugation between bacteria
12.12 Conjugation transfers single-stranded DNA
12.13 Connecting bacterial replication to the cell cycle
12.14 Cell division and chromosome segregation
12.15 The division apparatus consists of cytoskeletal and regulatory
components
12.16 Partitioning involves membrane attachment and (possibly) a motor
12.17 Multiple systems ensure plasmid survival in bacterial populations
12.18 Plasmid incompatibility is determined by the replicon
12.19 The ColE1 compatibility system is controlled by an RNA regulator
Replicon is a unit of the
genome in which DNA is
replicated; contains an origin
for initiation of replication.
12.1 Introduction
Replicon is a unit of the
genome in which DNA is
replicated; contains an origin
for initiation of replication.
12.1 Introduction
Figure 11.2 Several types of independent genetic
units exist in bacteria.
12.1 Introduction
Replication fork is the point at
which strands of parental
duplex DNA are separated so
that replication can proceed.
12.2 Origins can be mapped by autoradiography
and electrophoresis
Figure 12.1
Replicated
DNA is seen as
a replication
eye flanked by
nonreplicated
DNA.
12.2 Origins can be
mapped by
autoradiography
and electrophoresis
Figure 12.2 Replicons
may be unidirectional
or bidirectional,
depending on whether
one or two replication
forks are formed at the
origin.
12.2 Origins can
be mapped by
autoradiography
and
electrophoresis
Figure 12.3 A replication eye forms a theta
structure in circular DNA.
12.2 Origins can be mapped by autoradiography and
electrophoresis
Figure 12.4 The replication eye
becomes larger as the
replication forks proceed along
the replicon,Note that the
"eye" becomes larger than the
nonreplicated segment,The
two sides of the eye can be
defined because they are both
the same length,Photograph
kindly provided by Bernard
Hirt.
12.2 Origins can be
mapped by
autoradiography and
electrophoresis
Figure 12.5
Different densities
of radioactive
labeling can be used
to distinguish
unidirectional and
bidirectional
replication.
12.2 Origins can be mapped by autoradiography and
electrophoresis
Figure 12.6 The position
of the origin and the
number of replicating
forks determine the shape
of a replicating restriction
fragment,which can be
followed by its
electrophoretic path (solid
line),The dashed line
shows the path for a linear
DNA.
12.2 Origins can be mapped by autoradiography and
electrophoresis
Figure 12.7
Replication termini
in E,coli are located
beyond the point at
which the replication
forks actually meet.
12.3 The
bacterial genome
is a single
circular replicon
Figure 12.7
Replication termini
in E,coli are located
beyond the point at
which the replication
forks actually meet.
12.3 The
bacterial genome
is a single
circular replicon
S phase is the restricted
part of the eukaryotic cell
cycle during which
synthesis of DNA occurs.
12.4 Each eukaryotic chromosome contains
many replicons
Figure 12.5 Different
densities of radioactive
labeling can be used to
distinguish
unidirectional and
bidirectional replication.
12.4 Each eukaryotic
chromosome contains
many replicons
Figure 12.6 The position of
the origin and the number
of replicating forks
determine the shape of a
replicating restriction
fragment,which can be
followed by its
electrophoretic path (solid
line),The dashed line
shows the path for a linear
DNA.
12.4 Each eukaryotic
chromosome contains
many replicons
Figure 12.8
Measuring the size
of the replicon
requires a stretch
of DNA in which
adjacent replicons
are active.
12.4 Each
eukaryotic
chromosome
contains many
replicons
Figure 12.9 Replication
forks are organized into
foci in the nucleus,Cells
were labeled with BrdU,
The leftmost panel was
stained with propidium
iodide to identify bulk
DNA,The right panel was
stained using an antibody
to BrdU to identify
replicating DNA,
12.4 Each eukaryotic chromosome contains many replicons
12.4 Each eukaryotic chromosome contains many replicons
Figure 12.10 An ARS extends
for ~50 bp and includes a
consensus sequence (A) and
additional elements (B1-B3).
12.5 Isolating the
origins of yeast
replicons
D loop is a region within mitochondrial DNA in
which a short stretch of RNA is paired with one
strand of DNA,displacing the original partner DNA
strand in this region,The same term is used also to
describe the displacement of a region of one strand
of duplex DNA by a single-stranded invader in the
reaction catalyzed by RecA protein.
12.6 D loops maintain mitochondrial
origins
Figure 12.11 The D loop
maintains an opening in
mammalian mitochondrial
DNA,which has separate
origins for the replication of
each strand.
12.6 D loops maintain
mitochondrial origins
Strand displacement is a mode of
replication of some viruses in
which a new DNA strand grows
by displacing the previous
(homologous) strand of the duplex.
12.7 The problem of linear replicons
Figure 12.12 Replication could run off the 3
end of a newly synthesized linear strand,
but could it initiate at a 5 end?
12.7 The problem of linear replicons
Figure 12.13 Adenovirus
DNA replication is
initiated separately at the
two ends of the molecule
and proceeds by strand
displacement.
12.7 The problem of
linear replicons
Figure 12.14 The 5
terminal
phosphate at each
end of adenovirus
DNA is covalently
linked to serine in
the 55 kD Ad-
binding protein.
12.7 The problem of linear replicons
Figure 12.15 Adenovirus
terminal protein binds to
the 5 end of DNA and
provides a C-OH end to
prime synthesis of a new
DNA strand.
12.7 The problem of
linear replicons
Rolling circle is a mode of replication in which a
replication fork proceeds around a circular
template for an indefinite number of revolutions;
the DNA strand newly synthesized in each
revolution displaces the strand synthesized in the
previous revolution,giving a tail containing a
linear series of sequences complementary to the
circular template strand.
12.8 Rolling circles produce multimers
of a replicon
Figure 12.16 The rolling
circle generates a multimeric
single-stranded tail.
12.8 Rolling circles produce
multimers of a replicon
Figure 12.17 A rolling
circle appears as a
circular molecule with
a linear tail by
electron microscopy.
12.8 Rolling circles produce multimers
of a replicon
Figure 12.18 Rolling circles can be used
for varying purposes,depending on the
fate of the displaced tail,Cleavage at
unit length generates monomers,which
can be converted to duplex and circular
forms,Cleavage of multimers generates
a series of tandemly repeated copies of
the original unit,Note that the
conversion to double-stranded form
could occur earlier,before the tail is
cleaved from the rolling circle
12.8 Rolling circles produce
multimers of a replicon
Figure 12.19 fX174 RF DNA is a
template for synthesizing single-
stranded viral circles,The A protein
remains attached to the same genome
through indefinite revolutions,each
time nicking the origin on the viral (+)
strand and transferring to the new 5
end,At the same time,the released
viral strand is circularized.
12.8 Rolling circles produce
multimers of a replicon
Figure 12.20 The tra region of the F plasmid
contains the genes needed for bacterial conjugation.
12.9 Single-stranded genomes are generated
for bacterial conjugation
Figure 12.21 Mating bacteria are initially connected
when donor F pili contact the recipient bacterium.
12.9 Single-stranded genomes are generated
for bacterial conjugation
Figure 12.22 Transfer of DNA occurs
when the F factor is nicked at oriT and
a single strand is led by the 5 end
into the recipient,Only one unit length
is transferred,Complementary strands
are synthesized to the single strand
remaining in the donor and to the
strand transferred into the recipient.
12.9 Single-stranded
genomes are generated for
bacterial conjugation
Figure 12.16 The
rolling circle
generates a
multimeric single-
stranded tail.
12.9 Single-stranded
genomes are generated for
bacterial conjugation
Figure 12.23 Transfer of
chromosomal DNA occurs
when an integrated F factor
is nicked at oriT,Transfer
of DNA starts with a short
sequence of F DNA and
continues until prevented
by loss of contact between
the bacteria.
12.9 Single-stranded genomes are generated
for bacterial conjugation
Multiforked chromosome (in
bacterium) has more than one
replication fork,because a second
initiation has occurred before the
first cycle of replication has been
completed.
12.10 Connecting bacterial replication
to the cell cycle
Figure 12.24 The fixed interval of
60 minutes between initiation of
replication and cell division
produces multiforked chromosomes
in rapidly growing cells,Note that
only the replication forks moving in
one direction are shown; actually
the chromosome is replicated
symmetrically by two sets of forks
moving in opposite directions on
circular chromosomes.
12.10 Connecting bacterial replication
to the cell cycle
Figure 12.24 The fixed interval of
60 minutes between initiation of
replication and cell division
produces multiforked chromosomes
in rapidly growing cells,Note that
only the replication forks moving in
one direction are shown; actually
the chromosome is replicated
symmetrically by two sets of forks
moving in opposite directions on
circular chromosomes.
12.10 Connecting bacterial replication
to the cell cycle
Septum constitutes the material that
forms in the center of a bacterium to
divide it into two daughter cells at the
end of a division cycle.
12.11 Cell division and chromosome segregation
Figure 12.25 Duplication
and displacement of the
periseptal annulus give rise
to the formation of a septum
that divides the cell.
12.11 Cell division and
chromosome
segregation
Figure 12.26 Attachment of
bacterial DNA to the
membrane could provide a
mechanism for segregation.
12.11 Cell division and
chromosome
segregation
Minicell is an anucleate bacterial
(E,coli) cell produced by a
division that generates a
cytoplasm without a nucleus.
12.12 The division apparatus consists of
cytoskeletal and regulatory components
Figure 12.27 Failure of
cell division generates
multinucleated filaments,
Photograph kindly
provided by Sota Hiraga.
12.12 The division apparatus
consists of cytoskeletal and
regulatory components
Figure 12.28 E,coli generate
anucleate cells when
chromosome segregation fails,
Cells with chromosomes stain
blue; daughter cells lacking
chromosomes have no blue stain,
This field shows cells of the
mukB mutant; both normal and
abnormal divisions can be seen.
12.12 The division apparatus
consists of cytoskeletal and
regulatory components
Figure 12.25 Duplication and
displacement of the periseptal
annulus give rise to the
formation of a septum that
divides the cell.
12.12 The division apparatus
consists of cytoskeletal and
regulatory components
Figure 12.29 MinC/D is
a division inhibitor,
whose action is confined
to the polar sites by
MinE.
12.12 The division apparatus
consists of cytoskeletal and
regulatory components
Figure 12.26 Attachment
of bacterial DNA to the
membrane could provide a
mechanism for segregation.
12.13 Partioning
involves membrane
attachment and
(possibly) a motor
Figure 12.30 A common segregation system consists
of genes parA and parB and the target site parS.
12.13 Partioning involves membrane
attachment and (possibly) a motor
Figure 9.23 Sporulation
involves successive changes
in the sigma factors that
control the initiation
specificity of RNA
polymerase,The cascades
in the forespore (left) and
the mother cell (right) are
related by signals passed
across the septum (indicated
by horizontal arrows).
12.13 Partioning involves membrane
attachment and (possibly) a motor
12.14 Multiple systems ensure plasmid
survival in bacterial populations
Figure 12.31
Intermolecular
recombination merges
monomers into dimers,
and intramolecular
recombination releases
individual units from
oligomers.
12.14 Multiple systems ensure
plasmid survival in bacterial
populations
Figure 12.32 Plasmids may
ensure that bacteria cannot
live without them by
synthesizing a long-lived killer
and a short-lived antidote.
12.14 Multiple systems ensure plasmid
survival in bacterial populations
Figure 12.30 A common segregation system consists
of genes parA and parB and the target site parS.
12.14 Multiple systems
ensure plasmid survival in
bacterial populations
Figure 12.26 Attachment of
bacterial DNA to the
membrane could provide a
mechanism for segregation.
Compatibility group of
plasmids contains
members unable to coexist
in the same bacterial cell.
12.15 Plasmid incompatibility is
connected with copy number
12.15 Plasmid incompatibility is
connected with copy number
Figure 12.33 Two
plasmids are
incompatible (they
belong to the same
compatibility group) if
their origins cannot be
distinguished at the
stage of initiation,The
same model could
apply to segregation.
12.15 Plasmid
incompatibility is
connected with
copy number
Figure 12.34 Replication
of ColE1 DNA is initiated
by cleaving the primer
RNA to generate a 3 -
OH end,The primer
forms a persistent hybrid
in the origin region.
12.15 Plasmid incompatibility is
connected with copy number
Figure 12.35 The sequence of RNA I is
complementary to the 5 region of primer RNA.
12.15 Plasmid incompatibility is
connected with copy number
Figure 12.36 Base pairing
with RNA I may change the
secondary structure of the
primer RNA sequence and
thus prevent cleavage from
generating a 3 -OH end.
12.15 Plasmid incompatibility is
connected with copy number
Figure 12.37 Mutations
in the region coding for
RNA I and the primer
precursor need not
affect their ability to
pair; but they may
prevent pairing with
the complementary
RNA coded by a
different plasmid.
1,The entire chromosome is replicated once for every
cell division cycle,
2,Eukaryotic replication is (at least) an order of
magnitude slower than bacterial replication,
3,The minimal E,coli origin consists of ~245 bp and
initiates bidirectional replication,
4,The rolling circle is an alternative form of replication
for circular DNA molecules in which an origin is nicked
to provide a priming end,
5,Rolling circles are used to replicate some phages.
Summary
6,Rolling circles also are involved in bacterial
conjugation,when an F plasmid is transferred from a
donor to a recipient cell,following the initiation of
contact between the cells by means of the F-pili,
7,A fixed time of 40 minutes is required to replicate the
E,coli chromosome and a further 20 minutes is required
before the cell can divide,
8,Segregation involves additional sequences that have
not yet been characterized,
9,Plasmids have a variety of systems that ensure or
assist partition,and an individual plasmid may carry
systems of several types.
Summary
The replicon
12.1 Introduction
12.2 Replicons can be linear or circular
12.3 Origins can be mapped by autoradiography and electrophoresis
12.4 The bacterial genome is a single circular replicon
12.5 Each eukaryotic chromosome contains many replicons
12.6 Isolating the origins of yeast replicons
12.7 D loops maintain mitochondrial origins
12.8 The problem of linear replicons
12.9 Rolling circles produce multimers of a replicon
12.10 Rolling circles are used to replicate phage genomes
12.11 The F plasmid is transferred by conjugation between bacteria
12.12 Conjugation transfers single-stranded DNA
12.13 Connecting bacterial replication to the cell cycle
12.14 Cell division and chromosome segregation
12.15 The division apparatus consists of cytoskeletal and regulatory
components
12.16 Partitioning involves membrane attachment and (possibly) a motor
12.17 Multiple systems ensure plasmid survival in bacterial populations
12.18 Plasmid incompatibility is determined by the replicon
12.19 The ColE1 compatibility system is controlled by an RNA regulator
Replicon is a unit of the
genome in which DNA is
replicated; contains an origin
for initiation of replication.
12.1 Introduction
Replicon is a unit of the
genome in which DNA is
replicated; contains an origin
for initiation of replication.
12.1 Introduction
Figure 11.2 Several types of independent genetic
units exist in bacteria.
12.1 Introduction
Replication fork is the point at
which strands of parental
duplex DNA are separated so
that replication can proceed.
12.2 Origins can be mapped by autoradiography
and electrophoresis
Figure 12.1
Replicated
DNA is seen as
a replication
eye flanked by
nonreplicated
DNA.
12.2 Origins can be
mapped by
autoradiography
and electrophoresis
Figure 12.2 Replicons
may be unidirectional
or bidirectional,
depending on whether
one or two replication
forks are formed at the
origin.
12.2 Origins can
be mapped by
autoradiography
and
electrophoresis
Figure 12.3 A replication eye forms a theta
structure in circular DNA.
12.2 Origins can be mapped by autoradiography and
electrophoresis
Figure 12.4 The replication eye
becomes larger as the
replication forks proceed along
the replicon,Note that the
"eye" becomes larger than the
nonreplicated segment,The
two sides of the eye can be
defined because they are both
the same length,Photograph
kindly provided by Bernard
Hirt.
12.2 Origins can be
mapped by
autoradiography and
electrophoresis
Figure 12.5
Different densities
of radioactive
labeling can be used
to distinguish
unidirectional and
bidirectional
replication.
12.2 Origins can be mapped by autoradiography and
electrophoresis
Figure 12.6 The position
of the origin and the
number of replicating
forks determine the shape
of a replicating restriction
fragment,which can be
followed by its
electrophoretic path (solid
line),The dashed line
shows the path for a linear
DNA.
12.2 Origins can be mapped by autoradiography and
electrophoresis
Figure 12.7
Replication termini
in E,coli are located
beyond the point at
which the replication
forks actually meet.
12.3 The
bacterial genome
is a single
circular replicon
Figure 12.7
Replication termini
in E,coli are located
beyond the point at
which the replication
forks actually meet.
12.3 The
bacterial genome
is a single
circular replicon
S phase is the restricted
part of the eukaryotic cell
cycle during which
synthesis of DNA occurs.
12.4 Each eukaryotic chromosome contains
many replicons
Figure 12.5 Different
densities of radioactive
labeling can be used to
distinguish
unidirectional and
bidirectional replication.
12.4 Each eukaryotic
chromosome contains
many replicons
Figure 12.6 The position of
the origin and the number
of replicating forks
determine the shape of a
replicating restriction
fragment,which can be
followed by its
electrophoretic path (solid
line),The dashed line
shows the path for a linear
DNA.
12.4 Each eukaryotic
chromosome contains
many replicons
Figure 12.8
Measuring the size
of the replicon
requires a stretch
of DNA in which
adjacent replicons
are active.
12.4 Each
eukaryotic
chromosome
contains many
replicons
Figure 12.9 Replication
forks are organized into
foci in the nucleus,Cells
were labeled with BrdU,
The leftmost panel was
stained with propidium
iodide to identify bulk
DNA,The right panel was
stained using an antibody
to BrdU to identify
replicating DNA,
12.4 Each eukaryotic chromosome contains many replicons
12.4 Each eukaryotic chromosome contains many replicons
Figure 12.10 An ARS extends
for ~50 bp and includes a
consensus sequence (A) and
additional elements (B1-B3).
12.5 Isolating the
origins of yeast
replicons
D loop is a region within mitochondrial DNA in
which a short stretch of RNA is paired with one
strand of DNA,displacing the original partner DNA
strand in this region,The same term is used also to
describe the displacement of a region of one strand
of duplex DNA by a single-stranded invader in the
reaction catalyzed by RecA protein.
12.6 D loops maintain mitochondrial
origins
Figure 12.11 The D loop
maintains an opening in
mammalian mitochondrial
DNA,which has separate
origins for the replication of
each strand.
12.6 D loops maintain
mitochondrial origins
Strand displacement is a mode of
replication of some viruses in
which a new DNA strand grows
by displacing the previous
(homologous) strand of the duplex.
12.7 The problem of linear replicons
Figure 12.12 Replication could run off the 3
end of a newly synthesized linear strand,
but could it initiate at a 5 end?
12.7 The problem of linear replicons
Figure 12.13 Adenovirus
DNA replication is
initiated separately at the
two ends of the molecule
and proceeds by strand
displacement.
12.7 The problem of
linear replicons
Figure 12.14 The 5
terminal
phosphate at each
end of adenovirus
DNA is covalently
linked to serine in
the 55 kD Ad-
binding protein.
12.7 The problem of linear replicons
Figure 12.15 Adenovirus
terminal protein binds to
the 5 end of DNA and
provides a C-OH end to
prime synthesis of a new
DNA strand.
12.7 The problem of
linear replicons
Rolling circle is a mode of replication in which a
replication fork proceeds around a circular
template for an indefinite number of revolutions;
the DNA strand newly synthesized in each
revolution displaces the strand synthesized in the
previous revolution,giving a tail containing a
linear series of sequences complementary to the
circular template strand.
12.8 Rolling circles produce multimers
of a replicon
Figure 12.16 The rolling
circle generates a multimeric
single-stranded tail.
12.8 Rolling circles produce
multimers of a replicon
Figure 12.17 A rolling
circle appears as a
circular molecule with
a linear tail by
electron microscopy.
12.8 Rolling circles produce multimers
of a replicon
Figure 12.18 Rolling circles can be used
for varying purposes,depending on the
fate of the displaced tail,Cleavage at
unit length generates monomers,which
can be converted to duplex and circular
forms,Cleavage of multimers generates
a series of tandemly repeated copies of
the original unit,Note that the
conversion to double-stranded form
could occur earlier,before the tail is
cleaved from the rolling circle
12.8 Rolling circles produce
multimers of a replicon
Figure 12.19 fX174 RF DNA is a
template for synthesizing single-
stranded viral circles,The A protein
remains attached to the same genome
through indefinite revolutions,each
time nicking the origin on the viral (+)
strand and transferring to the new 5
end,At the same time,the released
viral strand is circularized.
12.8 Rolling circles produce
multimers of a replicon
Figure 12.20 The tra region of the F plasmid
contains the genes needed for bacterial conjugation.
12.9 Single-stranded genomes are generated
for bacterial conjugation
Figure 12.21 Mating bacteria are initially connected
when donor F pili contact the recipient bacterium.
12.9 Single-stranded genomes are generated
for bacterial conjugation
Figure 12.22 Transfer of DNA occurs
when the F factor is nicked at oriT and
a single strand is led by the 5 end
into the recipient,Only one unit length
is transferred,Complementary strands
are synthesized to the single strand
remaining in the donor and to the
strand transferred into the recipient.
12.9 Single-stranded
genomes are generated for
bacterial conjugation
Figure 12.16 The
rolling circle
generates a
multimeric single-
stranded tail.
12.9 Single-stranded
genomes are generated for
bacterial conjugation
Figure 12.23 Transfer of
chromosomal DNA occurs
when an integrated F factor
is nicked at oriT,Transfer
of DNA starts with a short
sequence of F DNA and
continues until prevented
by loss of contact between
the bacteria.
12.9 Single-stranded genomes are generated
for bacterial conjugation
Multiforked chromosome (in
bacterium) has more than one
replication fork,because a second
initiation has occurred before the
first cycle of replication has been
completed.
12.10 Connecting bacterial replication
to the cell cycle
Figure 12.24 The fixed interval of
60 minutes between initiation of
replication and cell division
produces multiforked chromosomes
in rapidly growing cells,Note that
only the replication forks moving in
one direction are shown; actually
the chromosome is replicated
symmetrically by two sets of forks
moving in opposite directions on
circular chromosomes.
12.10 Connecting bacterial replication
to the cell cycle
Figure 12.24 The fixed interval of
60 minutes between initiation of
replication and cell division
produces multiforked chromosomes
in rapidly growing cells,Note that
only the replication forks moving in
one direction are shown; actually
the chromosome is replicated
symmetrically by two sets of forks
moving in opposite directions on
circular chromosomes.
12.10 Connecting bacterial replication
to the cell cycle
Septum constitutes the material that
forms in the center of a bacterium to
divide it into two daughter cells at the
end of a division cycle.
12.11 Cell division and chromosome segregation
Figure 12.25 Duplication
and displacement of the
periseptal annulus give rise
to the formation of a septum
that divides the cell.
12.11 Cell division and
chromosome
segregation
Figure 12.26 Attachment of
bacterial DNA to the
membrane could provide a
mechanism for segregation.
12.11 Cell division and
chromosome
segregation
Minicell is an anucleate bacterial
(E,coli) cell produced by a
division that generates a
cytoplasm without a nucleus.
12.12 The division apparatus consists of
cytoskeletal and regulatory components
Figure 12.27 Failure of
cell division generates
multinucleated filaments,
Photograph kindly
provided by Sota Hiraga.
12.12 The division apparatus
consists of cytoskeletal and
regulatory components
Figure 12.28 E,coli generate
anucleate cells when
chromosome segregation fails,
Cells with chromosomes stain
blue; daughter cells lacking
chromosomes have no blue stain,
This field shows cells of the
mukB mutant; both normal and
abnormal divisions can be seen.
12.12 The division apparatus
consists of cytoskeletal and
regulatory components
Figure 12.25 Duplication and
displacement of the periseptal
annulus give rise to the
formation of a septum that
divides the cell.
12.12 The division apparatus
consists of cytoskeletal and
regulatory components
Figure 12.29 MinC/D is
a division inhibitor,
whose action is confined
to the polar sites by
MinE.
12.12 The division apparatus
consists of cytoskeletal and
regulatory components
Figure 12.26 Attachment
of bacterial DNA to the
membrane could provide a
mechanism for segregation.
12.13 Partioning
involves membrane
attachment and
(possibly) a motor
Figure 12.30 A common segregation system consists
of genes parA and parB and the target site parS.
12.13 Partioning involves membrane
attachment and (possibly) a motor
Figure 9.23 Sporulation
involves successive changes
in the sigma factors that
control the initiation
specificity of RNA
polymerase,The cascades
in the forespore (left) and
the mother cell (right) are
related by signals passed
across the septum (indicated
by horizontal arrows).
12.13 Partioning involves membrane
attachment and (possibly) a motor
12.14 Multiple systems ensure plasmid
survival in bacterial populations
Figure 12.31
Intermolecular
recombination merges
monomers into dimers,
and intramolecular
recombination releases
individual units from
oligomers.
12.14 Multiple systems ensure
plasmid survival in bacterial
populations
Figure 12.32 Plasmids may
ensure that bacteria cannot
live without them by
synthesizing a long-lived killer
and a short-lived antidote.
12.14 Multiple systems ensure plasmid
survival in bacterial populations
Figure 12.30 A common segregation system consists
of genes parA and parB and the target site parS.
12.14 Multiple systems
ensure plasmid survival in
bacterial populations
Figure 12.26 Attachment of
bacterial DNA to the
membrane could provide a
mechanism for segregation.
Compatibility group of
plasmids contains
members unable to coexist
in the same bacterial cell.
12.15 Plasmid incompatibility is
connected with copy number
12.15 Plasmid incompatibility is
connected with copy number
Figure 12.33 Two
plasmids are
incompatible (they
belong to the same
compatibility group) if
their origins cannot be
distinguished at the
stage of initiation,The
same model could
apply to segregation.
12.15 Plasmid
incompatibility is
connected with
copy number
Figure 12.34 Replication
of ColE1 DNA is initiated
by cleaving the primer
RNA to generate a 3 -
OH end,The primer
forms a persistent hybrid
in the origin region.
12.15 Plasmid incompatibility is
connected with copy number
Figure 12.35 The sequence of RNA I is
complementary to the 5 region of primer RNA.
12.15 Plasmid incompatibility is
connected with copy number
Figure 12.36 Base pairing
with RNA I may change the
secondary structure of the
primer RNA sequence and
thus prevent cleavage from
generating a 3 -OH end.
12.15 Plasmid incompatibility is
connected with copy number
Figure 12.37 Mutations
in the region coding for
RNA I and the primer
precursor need not
affect their ability to
pair; but they may
prevent pairing with
the complementary
RNA coded by a
different plasmid.
1,The entire chromosome is replicated once for every
cell division cycle,
2,Eukaryotic replication is (at least) an order of
magnitude slower than bacterial replication,
3,The minimal E,coli origin consists of ~245 bp and
initiates bidirectional replication,
4,The rolling circle is an alternative form of replication
for circular DNA molecules in which an origin is nicked
to provide a priming end,
5,Rolling circles are used to replicate some phages.
Summary
6,Rolling circles also are involved in bacterial
conjugation,when an F plasmid is transferred from a
donor to a recipient cell,following the initiation of
contact between the cells by means of the F-pili,
7,A fixed time of 40 minutes is required to replicate the
E,coli chromosome and a further 20 minutes is required
before the cell can divide,
8,Segregation involves additional sequences that have
not yet been characterized,
9,Plasmids have a variety of systems that ensure or
assist partition,and an individual plasmid may carry
systems of several types.
Summary
