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Genetic recombination:
1.Homologous Recombination
2. Site-Specific Recombination
3. DNA Transposition
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Homologous Recombination at the Molecular
Level
DNA BREAKS ARE COMMON AND INITIATERECOMBINATION
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MODELS FOR HOMOLOGOUS
RECOMBINATION
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Strand invasion is a key step in homologous recombination
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Resolving Holliday junctions is a
key step to finishing genetic
exchange
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The double-strand break-repair
model describes many
recombination events
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HOMOLOGOUS RECOMBINATION PROTEIN
MACHINES
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RecBCD
RecA
RuvAB
RuvC
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The RecBCD helicase/nuclease processes broken DNA
molecules for recombination
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Structure of RecBCD
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Chi sites control RecBCD
(GCTGGTGG)
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RecA protein assembles on single-stranded DNA and
promotes strand invasion
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Three views of the RecA filament
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Polarity of RecA assembly
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Newly based-paired partners are established within RecA
filament
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RecA homologs are present in all organism
Human Rad51 RecA
Rad A of Archaea
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The RuvAB complex specifically recognizes Holliday
junctions and promote branch migration
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Branch migration can either enlarge heteroduplex regions or
release newly synthesized DNA as a single strand
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Structure of RuvA and model of RuvAB bound to Holliday junction
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Enzyme-catalyzed double branch migration at a Holliday junction.
InE. coli, a tetramer of the RuvA protein (green) and two hexamers of the RuvB
protein (pale gray)bind to the open form of the junction. The RuvB protein uses the
energy of ATP hydrolysis to move the crossover point rapidly along the paired DNA
helices, extending the heteroduplex region as shown. There is evidence that similar
proteins perform this function in vertebrate cells. (Image courtesy of P. Artymiuk;modified from S.C. West,Cell94:699701, 1998.)
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RuvC cleaves specific strands at the Holliday junction to
finish recombination
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Structure of RuvC and model of RuvC dimer bound to Holliday
junction
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RecBCD
RecA
RuvAB
RuvC
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HOMOLOGOUS
RECOMBINATION IN
EUKARYOTESHomologous recombination hasadditional functions in eukaryotes
It is required to pair homologouschromosomes in preparation for the
first nuclear division and for
segregation during meiosis
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Meiotic recombination between
homologous chromatids
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Meiotic recombination also frequently gives rise to crossing over
between genes on the two homologous parental chromosomes
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Programmed generation of double-stranded DNA breaks
occurs during meiosis
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MRX protein processes the
cleaved DNA ends for
assembly of the RecA-likestrand exchange proteins
(DMC1, Rad51)
DMC1 specifically functions
in meiotic recombination
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Many proteins function together to promote meiotic
recombination (Rad51, DMC1, Rad51 paralogs, Rad52,
Rad54)
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MATING-TYPE SWITCHING
Mating-type switching is initiated by a site-specific double-
strand break (synthesis-dependent strand annealing(SDSA))
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Mating-type switching is a gene conversion event and NOT
associate with crossing over (synthesis-dependent strand
annealing )
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GENETIC CONSEQUENCES OF THE
MECHANISM OF HOMOLOGOUS
RECOMBINATION
Hot and cold spots of recombination
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One cause of gene conversion is DNA repair during
recombination
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Site-Specific recombination & Transposition
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Three types of CSSR
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Structures involved in CSSR
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Recombination by a serine
recombinase
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Recombination by a tyrosine
recombinase
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The insertion of a circular bacteriophage
lambda DNA chromosome into the
bacterial chromosome.
Conservative site-specific recombination
was discovered in bacteriophage lambda
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The life cycle of bacteriophage
lambda.
The double-stranded DNA lambda
genome contains 50,000 nucleotide
pairs and encodes 5060 differentproteins. When the lambda DNA
enters the cell, the ends join to
form a circular DNA molecule.
This bacteriophage can multiply in
E. coliby a lytic pathway, which
destroys the cell, or it can enter a
latent prophage state. Damage to acell carrying a lambda prophage
induces the prophage to exit from
the host chromosome and shift to
lytic growth (green arrows). Both
the entrance of the lambda DNA to,
and its exit from, the bacterial
chromosome are accomplished bya conservative site-specific
recombination event, catalyzed by
the lambda integrase enzyme (see
Figure 580).
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DNA inversion by the Hin recombinase of Salmonella
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Transposition
Some genetic elements move to new chromosomal
locations by transposition
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The cut-and-paste
mechanism of transposition
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Replicative transposition
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S i t iti h i t th l
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Some virus use a transposition mechanism to move themselves
into host cell chromosomes
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Reverse transcriptase
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Poly-A retrotransposon
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