DNA repair

Question 1

What is Reversal of Damage?

Answer 1

DNA is repaired without new DNA synthesis

Question 2

What is Excision Repair?

Answer 2

Damaged base or a stretch of DNA is removed + new DNA is synthesised to replace the damaged DNA

Question 3

Reversal of Damage Examples

Answer 3

• Photolyases use the energy of light to split pyrimidine dimers - Photoreactivation
• Methyltransferases can remove methyl groups from methylated bases

Question 4

Base Excision Repair

Answer 4

• Excised by a DNA glycosylase which leaves the N-glycosidic bond forming an AP site
• the AP endonuclease system repairs AP site, cleaving DNA at damage site
• DNA polymerase fills gap + nick in DNA is sealed by DNA ligase
• In bacteria - phosphodiesterase removes sugar-phosphate residue

Question 5

Nucleotide Excision Repair (NER)

Answer 5

Repairs a range of DNA damage
- Damaged strand is cleaved on either side at the lesion site
- Single-stranded piece of DNA containing damage is displaced
- DNA polymerase synthesises DNA to fill the gap
- DNA ligase to seal the nick

Question 6

Global NER

Answer 6

XPE and XPC recognise and bind damage

Question 7

Transcription-couple NER

Answer 7

• RNA polymerase stall at the damage
• Recruits CSA and CSB

Question 8

Repairing double strand breaks - Nonhomologous end joining (NHEJ)

Answer 8

• Functions in dividing and non-dividing cells
• Error prone - can result in mutations

Question 9

Repairing double strand breaks - Homologous recombination (HR)

Answer 9

• Uses sequence on the homologous chromosome
• Only functions in dividing cells during the S and G2 phases

Question 10

Nonhomologous end joining in detail

Answer 10

• Both ends of double-strand break is recognised by Ku70 + Ku80
• Leads to recruitment of proteins inc. exonuclease, DNA polymerase and ligase
• DNA ends are processed to produce blunt ends which is ligated by DNA ligase

Question 11

Homologous Recombination in detail

Answer 11

1 of 2 main pathways
- 3’ end of 1 broken strand exchanges equivalent sequence on homologous chromosome forming a D loop
- 3’ end is extended then displaced by DNA helicase, allowing it to anneal to other broken end
- Gaps filled by DNA polymerase + ligase

Question 12

Xeroderma pigmentosum (XP)

Answer 12

Due to mutation in NER system which causes:
- Extreme skin + eye sensitivity to sunlight
- Dry skin (xeroderma)
- Area of hyper- + hypopigmentation (pigmentosum)
- Predisposition to cancer
- Neurological defects
- Accelerated ageing

Question 13

Cockayne Syndrome

Answer 13

Due to defect in the transcription-couple repair pathway of NER:
- Mainly a developmental + neurological disorder
- Inability to repair damage at stalled transcription sites leading to apoptosis
- Results in premature ageing, short stature, deafness & intellectual disability