Mutation

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Mutation

• Point Mutations
• Covered Spontaneous and Induced last time
• Ames Test for Mutagens (because some mutagens
  carcinogens)
• Repair of point mutations
• Transposon as mutagens

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Example Results of the Ames Test
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Repair Processes

• Direct Reversal of DNA lesion
• photolyase repair of UV lesions
• alkyltransferase repair of alkylations
• Homology-dependent repair processes
• Pre-replication repair
• General nucleotide excision repair of bulky
  lesions
• pyrimidine dimers, insertion or deletions loops
• defects cause Xeroderma pigmentosa
• Nucleotide excision repair starting with specific
  glycosidases
• specific base modifications, removal of Uracil
• Post-replication repair
• Mismatch repair
• Defects cause Hereditary Nonpolyposis Colon
  Cancer
• DNA replication at sites of DNA damage
• Recombinational repair (error free)
• SOS system (error prone)

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Base-excision repair
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Base-excision repair
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Mismatch excision repair
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Error-prone SOS-mediated repair
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Transposons as mutagens
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Transposons

• DNA transposons TNs
• Transpose through DNA no RNA intermediate
• Contain inverted repeat sequences at end
• Autonomous TNs encode transposase
• Mechanims of transposion conservative or
  replicative
• 10-20 bps are duplicated at the site of insertion
• Can identify sites of TN insertions even when TN
  is no longer there
• Nonautonomous TNs contain repeats but dont
  encode active transposase
• Retrotransposons
• Transpose through an RNA intermediate (require
  reverse transcriptase)
• Contain direct repeats at ends that contain
  transcriptional signals
• Can be autonomous or nonautonomous
• Major class of TNs in human genome

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Example showing duplication of target site
site of insertion
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Transposons(repetitive DNA elements)
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