chapter 35: DNA repair and recombination

when do we use mismatch repair?

incorrect base pairs

explain mismatch repair mechanism

1. MutS scans DNA and recognizes mismatch, binding to the mismatched base pair.

2. MutL binds to form complex.

3. MutH (nuclease) cleaves phosphodiester bond near mismatch.

4. exonuclease removes section of DNA containing mismatch

5. DNA polymerase fills gap

6. DNA ligase seals the nick

when do we use base excision repair?

fixes small DNA lesions caused by oxidative damage, deamination, and alkylation

explain base excision repair mechanism

1. Enzyme AlkA acts as glycosylase, cleaving the glycosidic bond to release the damaged base

2. Leaves an AP site because it is apurinic (missing A or G) or apyrimidinic (missing C or T)

3. AP endonuclease cleaves phosphodiester backbone

4. Deoxyribose phosphodiesterase removes deoxyribose phosphate unit

5. DNA pol I inserts undamaged nucleotide

6. DNA ligase seals strand

when is nucleotide excision repair used

distortions in DNA double helix due to damaged base

explain nucleotide excision repair mechanism

1. UvrABC exinuclease cuts the damaged DNA strand at two sites: 8 nt away from damaged site on 5’ and 4 nt away on 3’ side

2. DNA pol I fills gap

3. DNA ligase seals strand

explain repair mechanism for cytosine deamination

if uracil is detected in DNA, it is removed by uracil DNA glycosylase. AP endonuclease cuts DNA next to AP site. DNA Pol adds correct base and DNA ligase seals the strand.