DNA damage and repair

Question 1

What mechanism corrects errors immediately post replication?

Answer 1

strand directed mismatch repair

Question 2

How much does strand directed mismatch repair increase fidelity of DNA?

Answer 2

100 fold

Question 3

What does strand directed mismatch repair correct?

Answer 3

mismatches of bases, small insertions or deletions

Question 4

What proteins recognise the mismatch ?

bacteria and human different

Answer 4

bacteria = MutS
human = MSH2-6

Question 5

What are the repair enxymes for mismatch repair?

Answer 5

bacteria = MutL and MutH
human = MLH1

Question 6

What are 2 examples of damage fixed by Direct Reversal?

Answer 6

UV light (causes pyramidine dimers)
Methylation of G bases

Question 7

How does Direct Reversal repair damage (pyramidine dimers)?

Answer 7

Photoreactivation, DNA photolyase use light energy to break bond

Question 8

How does direct reversal repair damage

(methylation of G bases)?

Answer 8

Mehtyl guanyl methyl transferases reverse it

Question 9

What repair mechanism are DNA Glycosylases a part of?

Answer 9

Base Excision Repair

Question 10

How does base excision repair work?

Answer 10

Glycosylase removes base 
produces abasic site
AP endonuclease Cleaves site 
AP lyase removes Deoxyribose phosphate
Gap filled by DNA pol+lyage

Question 11

When is the nucleotide excison repair mechanism used? (NER)

Answer 11

when there are helix distortions

Question 12

What is the mechanism of NER?

Answer 12

Unwinding and cleavage of damaged site

Gap filled by DNA pol+Ligase

Question 13

What is the name of the NER system in E.Coli?

Answer 13

UvrABCD

Question 14

Describe the mechanism of homologous recombination

Answer 14

• 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

Question 15

Describe the mechanism of non homologous end joining?

Answer 15

• Ku protein detects and binds to the DNA strand breaks
• Ku-DNA complex dimerises and ligns fragments
• nuclease, polymerase and ligase join fragments

Question 16

When is Translesion synthesis repair used?

Answer 16

When there is a lesion on the DNA causing synthesis to stall

Question 17

Describe the mechanism of Translesion synthesis

Answer 17

• 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

Question 18

what’s the problem with TLS?

Answer 18

low fidelity

Question 19

How does DNA pol 1 increase fidelity?

Answer 19

3’ to 5’ exonuclease activity
proof reading
100 fold increased fidelity

Question 20

How to single stranded binding proteins increase accuracy of DNA?

Answer 20

Stop secondary structure forming

Question 21

How do topoisomerases increase DNA accuracy?

Answer 21

prevent the helix overwinding

Question 22

How much does base pairing specificity increase DNA fidelity?

Answer 22

100 fold

correct geometry at active site