DNA Replication, Repair, and Recombination

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• DNA Replication, Repair, and Recombination

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• Mutations are substitutions, deletions or
  additions in a DNA sequence
• Many mutations are bad and eliminated by natural
  selection
• Life requires that the mutation rate be low.
• DNA replication must ensure that the mutation
  rate is low

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• DNA polymerase is responsible for copying and
  repairing DNA
• DNA replication is asymmetrical
• One strand of DNA is always retained during
  replication
• DNA polymerase moves along the DNA in the 5 to
  3 direction always

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• As DNA polymerase moves along the DNA template
  the two strands of DNA separate forming a
  replication fork.
• The replication fork has a leading strand and a
  lagging strand. Replication along the leading
  strand is continuous while it is discontinuous
  along the lagging strand

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• The lagging strand consists of short DNA
  sequences known as Okazaki fragments.
• The Okazaki fragments are joined together by DNA
  ligase

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• The mutation rate of DNA replication is one nt in
  nine billion nts
• DNA polymerase has a proof reading activity to
  ensure that the correct nt is added
• Before DNA polymerase can catalyze the covalent
  bond between nts it must undergo a conformational
  change.

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• Exonucleolytic proofreading occurs after an
  incorrect nt has been covalently bonded to a DNA
  strand
• DNA polymerase requires a base paired 3 OH to
  add nts.
• If a mismatch has occurred then the DNA
  polymerase cannot move along the DNA template
• DNA polymerase has a requirement for a base
  paired primer to proceed.

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• The 3 to 5 proofreading exonuclease removes
  unpaired nts from the growing DNA strand.

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