Mechanisms of DNA repair

Question 1

Repair of Single Stranded Sequences: Mismatch repair

Answer 1

-MMR complex differentiates new vs old strand since only old has methyl groups
-MMR complex brings GATC sequence and mismatch together, nicks strand at GATC site and mismatched bases
-MMR degrades new strand between these
-polymerases and DNA ligases fills gap using old strand as template

Question 2

Repair of Single Stranded Sequences: Direct repair

Answer 2

1. photolyase breaks covalent bonds between dimers when activated by light
2. enzyme O6-methylguanine-DNA methyltransferase removes methyl group from O6-methylguanine, restores to guanine

Question 3

Repair of Single Stranded Sequences: Base excision repair

Answer 3

-DNA glycosylases excise damaged base
-leaves apyrimidic/apurinic site
-AP endonuclease cleaves phosphodiester bond
-enzymes remove sugar
-polymerase fills gap
-ligase seals break

Question 4

Repair of Single Stranded Sequences: Nucleotide excision repair

Answer 4

-enzyme detect distortions in helix, separate DNA, SSBs bind
-phosphodiester bonds on both sides of the mutation are cleaved
-helicase removes it
-gap filled and sealed by DNA polymerase and ligase

Question 5

Repair of Double stranded breaks: Homology Dependent Repair

Answer 5

-repairs by using genetic info from sister chromatid
-removal of nucleotides at broken ends
-enzymes 3’ direct strand invasion on unbroken DNA, elongates
-displaced strand forms loop that base pairs with broken DNA
-two strands separated and re-ligated

Question 6

Repair of Double stranded breaks: Nonhomologous End Joining

Answer 6

-independent of homologous template
-during G1 phase
-proteins (Ku) recognize broken ends and bind them together
-more error prone