DNA Repair and Recombiantion

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DNA Repair and Recombiantion
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Methyl-directed mismatch repair (1)

• If any mismatch escapes the proof reading
  mechanisms it will cause distortion of the helix.
• This can be detected and repaired but it is
  important that the repair enzyme can distinguish
  the new strand from the old.
• This is possible in E. coli because there is an
  enzyme which methylates the A in a sequence GATC.
  This methylation does not occur immediately after
  synthesis and until it does the two strands are
  distinguishable.

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Methyl-directed mismatch repair (2)
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Repair of DNA damage in E. coli (1)

• DNA damage occurs continually for a range of
  reasons including depurination, depyrimidation
  (to a lesser extent), ionizing radiation, free
  radicals, carcinogens, UV light.
• It is statistically unlikely for most of these
  changes, which affect one strand, that the second
  strand will be hit at the same time.
• This means the other strand can be used as
  template to repair the damaged strand.

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Repair of DNA damage in E. coli (2)

• UV light causes the formation of thymine dimer
  where adjacent Ts occur on the same strand. There
  is an enzyme which returns the thymines to their
  original form.
• Alkylation of bases can be reversed by the alkyl
  group being transferred to a protein, which is
  then destroyed.
• If the helix is distorted, for instance by T
  dimers, the distortion can be removed and
  replaced using the other strand as template.

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Repair of double-strand breaks

• One mechanism involves the ligation of breaks.
  This is likely to change the sequence but it is
  statistically unlikely that the site will be
  critical.
• The second requires homologous recombination with
  the other chromosome in a diploid organism.

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DNA damage repair in eukaryotes

• The dealkylating proteins found in bacteria are
  also found in humans, as are enzymes repairing
  thymidine dimers and genes corresponding to Mut S
  and Mut L.
• Xeroderma pigmentosum is a genetic disease in
  which the capacity to repair thymine dimers is
  lost. People with this disease suffer skin cancer
  at high rates on exposure to sunlight.
• Mismatch repair proteins have also been found in
  humans. Mutations in these genes are a cause of
  colorectal cancer.

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Homologous recombination

• Homologous recombination is illustrated in Fig.
  23.25.
• Recombination occurs naturally in cells. General
  recombination is the commonest type. In addition
  site specific recombination also occurs.
• Homologous recombination allows reassortment of
  related but slightly different genes in an
  organism, thus generating genetic diversity.
• This genetic diversity allows recombination of
  genes, generates genetic diversity and
  facilitates the operation of natural selection.

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Mechanism of homologous recombination in E. coli

• The process is best understood in this organism.
• The first step requires single strand invasion,
  where a single strand inserts itself into the DNA
  duplex and displaces its homologous strand via a
  triple stranded intermediate. It is catalysed by
  the enzyme RecA.
• A D loop is formed (Fig. 23.26) and the process
  uses ATP.

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Formation of cross-over junctions by
single-stranded invasion

• One strand of each DNA duplex is nicked to
  generate free 3 ends.
• These invade the homologous duplexes and result
  in displacement of the original partner.
• The nicks are sealed to form Holliday junctions.

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Figure 14.05
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Chi site GCTGGTGG
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Recombination in eukaryotes

• Meiosis is accompanied by strand crossovers or
  chiasmata. Following DNA replication chromatids
  pair and recombination occurs. There are two
  rounds of cell division.