Module 8: Causes of DNA mutation and DNA repair

• mutation = an alternative in the nucleotide sequence of a DNA molecule
• 2 ways a mutation can occur:

  1. errors in DNA replication
  2. caused by mutagens
• Errors in DNA replication (spontaneous mutations)

  *caused by DNA polymerase making a mistake

  *most mistakes caused by base tautomerism

  -->TAUTOMERS: isomers with slightly different chemical structures
• Mutagens = chemical/ environmental agents that cause changes in DNA molecules
• Mutagens cause mutations by:

  1. 5-bromouracil (5bU) is an analog of Thymine

  -->Keto-5bU base pairs with adenosine and gets incorporated

  into DNA during replication- the enrol tautomer is very

  common (enol-5bU pairs G not A )

  2.Deaminating agents change the structures of some nucleotides
• *Deamination of adenosine = hypoxanthine pairs with C not T

  *Deamination of cytosine = uracil, pairs with A not G

  *Deamination of guanine = xanthine, blocks DNA replication

  *Thymine is not deaminated- no amine group
• Other agents causing structural changes to nucleotides:

  1. Alkylating agents- add alkyl groups which causes transition mutations
  2. Intercalating agents- insert between base pairs, causes insertion mutations
  3. Ultraviolet radiation- causes base dimerisation
  4. Heat causes detachment of bases- gives rise to an AP site
• Repair of DNA mutations- direct repair

  *an enzyme corrects a nucleotide alteration caused by a mutagen (uncommon)

  *alkylating agents and base dimers formed by UV radiation can be directly repaired
• Repair of DNA mutations- excision repair

  *the damaged nucleotide is removed and the gas filled by DNA synthesis

  1. Base excision repair- a single altered base is removed
  2. Nucleotide excision repair- a longer piece of DNA containing the altered bases is removed
• Base excision repair in E.coli

  1. Begins with the removal of the damaged base by a DNA glycosylase enzyme
  2. Heat causes detachment of bases (gives rise to an AP site), the resulting AP site is then filled in
  3. Nucleotide excision repair is carried out by the UvrABC endonuclease
  4. The region containing the damaged nucleotide is excised and resynthesized
• Repair of DNA mutations- mismatch repair

  *corrects errors in DNA replication

  *parent strand contains the correct nucleotide; the daughter strand contains the mismatch
• Mismatch repair in E.coli

  1. the parent strand is methylated
  2. mismatch us recognised by the MutH and MutS enzymes
  3. The mismatch is excised and the DNA resynthesised
• Mismatch repair in humans

  *methylation is not involved

  *base-base mismatches recognised by MutS like proteins

  *don't have MutH but another protein acts as an endonuclease

  *Human mismatch repair enzymes tightly associated with the replication fork so can recognise the daughter strand as its being synthesised
• Repair of DNA mutations- non homologous end joining

  *corrects DNA breaks

  *distinguisg real breaks from the natural ends of chromosomes- telomeres mark the natural ends

  *DNA ligase comes in and joins the two ends together