DNA synthesis and repair: Revision session topics

Question 1

Define Damethalation

Answer 1

Tells enzymes what the new strand is

Question 2

What is the role of Dammethylase?

Answer 2

GATC sequence are targets fro the Dam methylase after replication. During the period before this methylation occurs, the non-methylated strand is the target for repair mismatched bases

Question 3

Explain the process of repair of mismatched places

Answer 3

Replication error generates (A and T converted to A and C)

Non-methylated adenine targets strand for repair

Non-methylated strand is degraded (C)

DNA synthesis corrects error

Question 4

What is the role of MutS?

Answer 4

Binds Mismatches on un-methylated DNA strands

Question 5

How does MutS complete its role?

Answer 5

Checks methylation to work out which is the wrong or newly made strand

Recognises a mismatch and translocates to a GATC site

Cleaves the unmethylated strand at the GATC

Endonucleases degrades the strand from the GATC to the mismatch site

Question 6

What is the role of MutSL

Answer 6

Binds to mismatches on unmethylated DNA strands

MutS dimer binds to mismatch

MutL dimer binds to MutS

MutS translocates to GATC

MutH joins complex at GATC sequence

Question 7

Explain MutS/MutC repair replication slippages

Answer 7

The MutS/MutL system initiates repair of mismatches
produced by replication Slippage

Replication Slippage generates a single-stranded loop

Question 8

Explain MutS/MutC repair replication slippages: How is a single-stranded loop formed?

Answer 8

MutS binds to the mismatch

MutC binds

The mismatch is removed by exonuclease, helicase, DNA polymerase and ligase

Question 9

Excision repair systems: What does the LevR System do?

Answer 9

Makes incisions, 12 base pairs apart on both sides of damaged DNA, removes the DNA between them and re-synthesises new DNA

Question 10

What is the general role of an Excision repair system?

Answer 10

Each system removes mispaired or damaged bases from DNA and then synthesizes an new stretch of DNA to replace them

Question 11

Excision repair system: Explain Incision

Answer 11

An endonuclease recognises the damaged area in the DNA and isolates it by cutting the DNA strand on both sides of the damage

Question 12

Excision repair system: Explain Excision

Answer 12

A 5-3 endonuclease removes a stretch of the damaged strand

Question 13

Excision repair system: Explain synthesis

Answer 13

Polymerase synthesizes replacement DNA

Question 14

Excision repair system: Explain Ligase

Answer 14

Seal nick

Excision-repair removes and replaces a stretch of DNA that includes the damaged bases

Question 15

How does the Uvr system remove damaged materials?

Answer 15

Deformation of DNA

UvrA recognises damage and binds with UvrB

UvrA released, UvrC binds

UvrC nicks DNA or both sides of the damage

UvrD unwinds regions, releasing damaged strands

Question 16

How is Mammalian excision repair triggered?

Answer 16

Directly removing of damage base from DNA

Triggers the removal and replacement of stretch and polynucleotides

The nature of the base removal reaction determines which of two pathways for excision repair is activated

Question 17

What does the PolS/6 pathway do?

Answer 17

Replaces a long polynucelotide stretch

Question 18

What does the Pol Beta pathway do?

Answer 18

replaces a short stretch

Question 19

Mammalian excision repair: What are the enzymes that remove DNA called?

Answer 19

Glycosylases and lyases

Question 20

Mammalian excision repair: How is uracil removed from DNA?

Answer 20

By Glycosylates, cleaving the bond to deoxyribose bising H2O but lyase takes the reaction further by opening the sugar ring using NH2

Question 21

explain non-homologous end-joining

Answer 21

Ligates blunt ends

Major mechanism to repair the double strand breaks

Non-homologous end joining requires recognition of the broken ends, trimming of overhanging ends and/or filling , followed by ligation