05. DNA Repair — Systems

Question 1

Describe the proofreading ability of the replisome.

Answer 1

• DNA polymerase has 3’-5’ exonuclease activity.
• Removes mismatched bases via hydrolysis

Question 2

Describe the mismatch repair system (in prokaryotes).

Answer 2

• MutS detects and binds to mismatch in DNA
• The complex recruits MutL and translocates until GATC
• A loop forms, where MutSL activates the endonuclease MutH
• MutH nicks the unmethylated strand
• Strand is progressively degraded until mismatch
• DNA polymerase III fills in the gaps, sealed by DNA ligase

Question 3

Describe the direct reversal of thymine dimers.

Answer 3

• Photolyase is activated by light
• Photoreactivation breaks the covalent bond between thymine dimers

Question 4

Describe base excision repair.

Answer 4

• Removes the mismatched nitrogenous base
• DNA glycosylase cleaves glycosidic bond
• APE1 removes abasic sugar
• PCNA set DNA polymerase to fill in the gap
• DNA ligase repairs phosphodiester bonds

Question 5

Describe nucleotide excision repair.

Answer 5

• Also for thymine dimer repair
• UvrAB scans along DNA
  
  • A detects distortion
  • B melts DNA and recruits UvrC
• UvrC makes incisions on either side of the mismatch
• UvrD binds + unwinds DNA
• DNA polymerase replaces damaged strand