Download Genetic recombination 2012-05-09

2012-05-09
Genetic recombination
Although genetic stability is essential for survival of individuals,
genetic variation is essential for long term survival in a changing environment
Genetic recombination by breaking and joining of DNA sequences
Genetic recombination occurs upon formation of germ cells, during DNA repair
and transposition and integration of DNA sequences
Two main variants
* General (homologous) recombination occurs between
homologous sequences, e.g. during meiosis, DNA repair
RecA dependent or similar protein
Driven by DNA homology
* Site-specific recombination occurs between non-homologous
sequences of limitited homology, specific 15 bp sequences
recognized by specific enzymes, e.g. integrase
Driven by DNA-protein interactions
Transposition requires special sequence on the donor DNA and
transposase
Illegitimate recombination (very rare) no specific sequences or
enzymes required
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General recombination
An important event ocurring in viruses,
bacteria and eukaryotes
Exchange can occur anywhere in
the homologous region
Strand exchange resulting in a
cross-over
No nucleotide sequence loss/gain
at the site of cross-over. Exact!
Recombination events occur at heteroduplex
joints
At the joint some mismatches can
happen
Sequences are not necessarily exactly
the same on either side of the joint
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Recombination is initiated by a nick/break
event
RecBCD protein of E.coli, chemical agents
and irradiation can produce nicks
A random search for basepairing allows
for rapid zippering and formation of a
double helix
slow
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Enzymes required for homologous
recombination
RecA (RAD51) forms triple strands by pairing ssDNA with homologous dsDNA
RecBCD unwinds and rewinds DNA at specific recognition sequence
RUV protein forms the four-stranded Holliday structure
DNA polymerase copies DNA
SSB keeps ssDNA straight
DNA ligase seals DNA
1. In E. coli RecA protein binds to single-stranded
DNA and can hold a single strand and a double
helix together
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2. RecA directs branch migration in one
direction
Cross-strand exchange
A. Nick and exchange of one strand
B, C. Molecules are rotated
(isomerized)
D. Cut
Robin Holliday, 1964
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Gene conversion can result in an unequal
inheritance of gene parts
Gene conversion is caused by mismatch repair
on heteroduplex during recombination
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Mismatch proof-reading can prevent
promiscuous recombination
Site-specific recombination
Guided by recombination enzyme
No need for base-pairing (homology) between molecules
Heteroduplex joint is only a few basepairs
Integration and excision of bacteriophage lambda in the E. coli chromosome
by the enzyme lambda integrase
Transposition
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Site-specific recombination by λ phage in E.coli
continued
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attP
attB
Int protein for integration into bacterial genome
Xis protein for excision from bacterial genome
Gene rearrangements in immunoglobulins and T-cell
receptors accounts for high diversity of antibodies
300 V
4J
numerous D
8C
Somatic
hypermutation
due to
Deaminations
3000 L x
5000 H
15 million comb.
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Transposable genetic elements (transposons) insert
randomly in genome by recombination
independent of RecA
One contributing factor for spread of antibiotic resistance
Transposition mechanism
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Effects of transposition
• Target site (3-12 basepairs) is duplicated upon insertion of the transposon
• Can induce homologous recombination between duplicated sequences
causing restructuring of chromosome
• Can inactivate genes upon insertion
• Can cause deletions and inversion
• Can activate neighbouring genes
• Can transfer antibiotic resistance by transferring antibiotic resistance passenger gene
• Retroviruses have similarities to transposons
Alu sequences (300bp) are similar to transposable
elements and common in the mammalian genome
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Retrotransposon
Do not encode protein
10 % of the human genome
In non-coding regions
Associated to some diseases
Human chromosomes stained
with Alu-probe (green)
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Position of Alu sequences is conserved in
mammals and are found outside coding regions
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