DNA damage and repair Flashcards
(22 cards)
What mechanism corrects errors immediately post replication?
strand directed mismatch repair
How much does strand directed mismatch repair increase fidelity of DNA?
100 fold
What does strand directed mismatch repair correct?
mismatches of bases, small insertions or deletions
What proteins recognise the mismatch ?
bacteria and human different
bacteria = MutS human = MSH2-6
What are the repair enxymes for mismatch repair?
bacteria = MutL and MutH human = MLH1
What are 2 examples of damage fixed by Direct Reversal?
UV light (causes pyramidine dimers) Methylation of G bases
How does Direct Reversal repair damage (pyramidine dimers)?
Photoreactivation, DNA photolyase use light energy to break bond
How does direct reversal repair damage
(methylation of G bases)?
Mehtyl guanyl methyl transferases reverse it
What repair mechanism are DNA Glycosylases a part of?
Base Excision Repair
How does base excision repair work?
Glycosylase removes base produces abasic site AP endonuclease Cleaves site AP lyase removes Deoxyribose phosphate Gap filled by DNA pol+lyage
When is the nucleotide excison repair mechanism used? (NER)
when there are helix distortions
What is the mechanism of NER?
Unwinding and cleavage of damaged site
Gap filled by DNA pol+Ligase
What is the name of the NER system in E.Coli?
UvrABCD
Describe the mechanism of homologous recombination
- 5’ endonuclease digests ends leaving single strand 3’
- single strand invasion of 3’ ends into homologous DNA duplex
- D loop formed
- DNA pol elongates ends
- Ends Ligated to form Holliday junction
- Junctions resolved
Describe the mechanism of non homologous end joining?
- Ku protein detects and binds to the DNA strand breaks
- Ku-DNA complex dimerises and ligns fragments
- nuclease, polymerase and ligase join fragments
When is Translesion synthesis repair used?
When there is a lesion on the DNA causing synthesis to stall
Describe the mechanism of Translesion synthesis
- DNA pol 3 stalls at lesion (T dimer)
- TLS polymerases replace DNA pol 3 and cross the lesion
- DNA pol 3 carries on after lesion
what’s the problem with TLS?
low fidelity
How does DNA pol 1 increase fidelity?
3’ to 5’ exonuclease activity
proof reading
100 fold increased fidelity
How to single stranded binding proteins increase accuracy of DNA?
Stop secondary structure forming
How do topoisomerases increase DNA accuracy?
prevent the helix overwinding
How much does base pairing specificity increase DNA fidelity?
100 fold
correct geometry at active site
