Homologous Recombination at the Molecular Level

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Chapter 10
Homologous Recombination at the Molecular Level
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OUTLINE Models for the H.M
H.M Protein Machines H.M in
Eukaryotes Mating-Type Switching
Genetic Consequences of the Mechanism of
H.M
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Key steps of homologous combination shared by
these models include
1.Alignment of two homologous DNA
molecules. 2.Introduction of breaks in the
DNA. 3.Formation of initial short regions of
base pairing between the two
recombination DNA molecules
4.Movement of the Holliday junction 5.Cleavage
of the junction.
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Holliday-------Models for the Homologous
Recombination
A simple and historically important model for
homologous recombination is the Holliday model.
Although it is now clear that most recombination
events involve some new DNA synthesis ---a
feature absent from this model ---the Holliday
model very well illustrates the DNA strand
invasion, branch migration and Holliday junction
resolution processes central to homologous
recombination.
BUT how the Holliday Model illustrates
Key Steps in Homologous
Recombination?
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----Aligned homologous duplexes
NICK
----single stranded breaks in each duplex
DNA strands neat the nick site can
then be peeled away from their complementary
strands, freeing these strands to invade and
ultimately base pair with the homologous duplex.
----strand invasion
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Holliday junction
----Branch migration
A Holliday junction can move along the DNA by the
repeated melting and formation of base pairs
In fact ,there are two ways of Holliday junction
cleavage
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Two alternative pairs of DNA sites can be cut
during resolution. Different cleavage would
make the products different.
How about two or more Holliday junction?
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Cleavage of both junction at site 2 will
give a patch product . Cleavage at
both junctions at site 1 also will give a
patch product. Cleavage of one junction
at site 1, but the other at site 2
therefore generates crossover products.
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There is another important model of the
Homologous Recombination The Double strand
Break Repair Model
The Double-strand Break Repair Pathway starts
with aligned homologous chromosomes. Compare to
the Holliday Model, the DSB is in one of the
two DNA molecules, not nicks on the two DNA
molecules. As a matter of fact, the DBS is
more common in the cells , so Homologous
Recombinations most the important function is
to repair the DSBs.
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Sometimes damage in the DNA template
can lead to DSB formation during DNA
replication . If there are nicks in the DNA
templates, it would take the fork collapse
to form a DSB. And there are DNA lesions in the
DNA template , the two newly formed DNA strands
can base pair and the fork can regress
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The proteins that promote the recombination in
E.coli via a major DBS repair way , known as the
RecBCD pathway
The steps of the RecBCD pathway
1.
This strand usually has a DSB in one DNA molecule.
2.
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3.
The RecB moves more slower than RecD, so the DNA
molecules accumulate a single
strand loop on the top strand during the
unwinding
4.
Chi sites
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• Chi sites are recombination hot spots in the
  genomes of enteric bacteria.
• Chi is the DNA sequence 5 GCTGGTGG 3.
  
• When RecBCD encounters a Chi sequence, its
  function changes from an
• exonuclease to a helicase, producing a single
  strand of DNA that can
• invade homologous double-stranded DNA mediated
  by RecA

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The RecA protein
Pairing homologous DNAs and strand invasion
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The structure of the RecA
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RuvAB complexes catalyzes the branch migration
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RuvC complexes catalyze the Holliday junction
resolution
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Homologous Recombination in Eukarytoes
Programmed Generation of the DSBs during Meiosis
Homologous Recombination in Eukarytoes is
required for Chromosome Segregation during Meiosis
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Mechanism of Meiotic Recombination
Proteins in need
Rad51 and Dmc1 as RecA MRX protein as
RecBCD Mus81 protein as RuvC
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Mating-Type Switching
Mating Type Switching is initiated by a
site-specific DSB. This reaction is Performed by
a specialized DNA-cleaving Enzyme. Called the HO
endonuclease.
Mating Type Switching is a gene conversion
event, not associated with crossing over
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Genetic Consequences of the Mechanism of H.M
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A corollary of the fact that recombination is
generally independent of sequence is that the
frequency of recombination between any two genes
is generally protentional to the distances
between those genes. So,we can use a geneic map
to display the order and spacing of genges along
a chromosome.
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THE SUMMARY

• Two models of the Homologous Recombination
  Holliday junction and the
• Double strands breaking repair pathway , make a
  compare of them

2. Describe RecBCD pathway and protein involved
in bacteria, and the function of the eukaryotic
homologue Spo11,MBX and Dmc1
3. Make a clear definition of the Mating-Type
Switching and Gene conversation