9 - Mechanisms of repair Flashcards
Consequences of DNA damage in frequently dividing cells
Gives rise to mutations which can allow cells to proliferate without control and/or evade apoptosis = cause of cancer
Consequences of DNA damage in infrequently dividing cells
DNA damage accumulates over time and likely contributes to aging
6 common DNA repair mechanisms
- Mismatch repair
- Direct reversal of damage
- Base excision
- Nucleotide excision
- Homologous recombination
- Non homologous end joining
Mismatch repair template
Complementary DNA strand
Direct reversal of damage template
No template
Base excision template
Complementary DNA strand
Nucleotide excision template
Complementary DNA strand
Homologous recombination template
Sister chromatid
Non homologous end joining template
No template
Type of DNA error repaired by mismatch
Mutation: replication errors, mispaired bases, strand slippage
Type of DNA error repaired by direct reversal of damage
Single strand damage. Pyrimidine dimers, alkylation
Type of DNA error repaired by base excision
Single strand damage. Abnormal bases, modified bases, pyrimidine dimers
Type of DNA error repaired by nucleotide excision
Single strand damage. Abnormal bases, modified bases, pyrimidine dimers
Type of DNA error repaired by homologous recombination
double stranded breaks
Type of DNA error repaired by non homologous end joining
double stranded breaks
Steps and proteins involved in mismatch repair
- MutS recognises mismatched base and binds
- MutH searches DNA for hemimethylated GATC sites, nicks DNA
- MutL recruits a helicase, DNA is an wound between the nick and the mismatch the daughter strand containing the mismatch is degraded
- New strand synthesised by DNA polymerase and gap sealed by ligase
Protein involved in direct reversal of damage
- 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)
Steps in base excision repair
- 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)
Steps in nucleotide excision repair (NER)
- 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
Two sub pathways of Nucleotide excision repair (NER)
- Global genomic NER (GG-NER)
- Transcription coupled NER (TC-NER)
Two pathways of homologous recombination
- the synthesis-dependent strand annealing (SDSA) pathway (common pathway)
- the double-strand break repair (DSBR) pathway
Steps to non homologous end joining
- Recognising DNA damage
- Binding of broken ends by KU70 & KU80
- Trimming the ends to get blunt 5’-P & 3’-OH
- Ligation by DNA ligase IV
