9 - Mechanisms of repair

Question 1

Consequences of DNA damage in frequently dividing cells

Answer 1

Gives rise to mutations which can allow cells to proliferate without control and/or evade apoptosis = cause of cancer

Question 2

Consequences of DNA damage in infrequently dividing cells

Answer 2

DNA damage accumulates over time and likely contributes to aging

Question 3

6 common DNA repair mechanisms

Answer 3

• Mismatch repair
• Direct reversal of damage
• Base excision
• Nucleotide excision
• Homologous recombination
• Non homologous end joining

Question 4

Mismatch repair template

Answer 4

Complementary DNA strand

Question 5

Direct reversal of damage template

Answer 5

No template

Question 6

Base excision template

Answer 6

Complementary DNA strand

Question 7

Nucleotide excision template

Answer 7

Complementary DNA strand

Question 8

Homologous recombination template

Answer 8

Sister chromatid

Question 9

Non homologous end joining template

Answer 9

No template

Question 10

Type of DNA error repaired by mismatch

Answer 10

Mutation: replication errors, mispaired bases, strand slippage

Question 11

Type of DNA error repaired by direct reversal of damage

Answer 11

Single strand damage. Pyrimidine dimers, alkylation

Question 12

Type of DNA error repaired by base excision

Answer 12

Single strand damage. Abnormal bases, modified bases, pyrimidine dimers

Question 13

Type of DNA error repaired by nucleotide excision

Answer 13

Single strand damage. Abnormal bases, modified bases, pyrimidine dimers

Question 14

Type of DNA error repaired by homologous recombination

Answer 14

double stranded breaks

Question 15

Type of DNA error repaired by non homologous end joining

Answer 15

double stranded breaks

Question 16

Steps and proteins involved in mismatch repair

Answer 16

1. MutS recognises mismatched base and binds
2. MutH searches DNA for hemimethylated GATC sites, nicks DNA
3. MutL recruits a helicase, DNA is an wound between the nick and the mismatch the daughter strand containing the mismatch is degraded
4. New strand synthesised by DNA polymerase and gap sealed by ligase

Question 17

Protein involved in direct reversal of damage

Answer 17

• Photolyases (not in humans): Light-dependent enzymes that reverse UV induced pyrimidine-dimers
• Alkyl transferases (in humans): Remove alkyl groups (a functional group of carbon and hydrogen)

Question 18

Steps in base excision repair

Answer 18

• For lesions that do not distort double helix (small)
• Initiated by DNA glycosylases (recognise and remove damaged bases without breaking the sugar phosphate backbone)
• AP endonuclease) makes a single-strand break which is then repaired by: Replacing a single nucleotide (short-patch), Replacing 2 – 10 nucleotides (long-patch)

Question 19

Steps in nucleotide excision repair (NER)

Answer 19

• For bulky lesions that distort double helix
  1. A) GG-NER: “Damage sensing” proteins constantly scan the genome and recognise distortions in the DNA helix
  1.B) TC-NER: is initiated when DNA damage causes RNA pol to stall
  2. Assembly of multi-protein complex at site to stabilise the single strands
  3. On affected strand, DNA around the lesion is excised by cleaving the sugar-phosphate backbone
  4. DNA polymerase fills the gap using undamaged strand as template & DNA ligase seals the backbone

Question 20

Two sub pathways of Nucleotide excision repair (NER)

Answer 20

• Global genomic NER (GG-NER)
• Transcription coupled NER (TC-NER)

Question 21

Two pathways of homologous recombination

Answer 21

• the synthesis-dependent strand annealing (SDSA) pathway (common pathway)
• the double-strand break repair (DSBR) pathway

Question 22

Steps to non homologous end joining

Answer 22

1. Recognising DNA damage
2. Binding of broken ends by KU70 & KU80
3. Trimming the ends to get blunt 5’-P & 3’-OH
4. Ligation by DNA ligase IV