Chiolo Lecture 4 Flashcards
How are DSBs repaired?
by non-homologous end joining, alternative end joining, or homologous recombination
What are the characteristics of non-homologous end joining (NHEJ)?
- predominant in G1
- fast (30 min-1 h)
- mostly error prone
What are the characteristics of alternative end joining (Alt-EJ)?
- rare, “backup plan”
- slow (few hr)
- error prone
What are the characteristics of homologous recombination (HR)?
- predominant in S/G2
- very slow (8-10 hr)
- mostly error free
Non-homologous End-Joining (NHEJ) does not require?
homology to rejoin the two ends of the DSB and it is usually “error prone”
What is the process of Non-homologous End-Joining (NHEJ)?
- Ku70/80 binds to DSB ends and recruits DNA-PKcs Kinase
- DNA-PKcs activates Artemis exo/endonuclease, which removes mismatched nucleotides and trims overhangs
- polymerization by Polymerase mu or Polymerase lamba/ ligation
- results in point mutations, small deletions
What does Alternative End-Joining (Alt-EJ) use?
microhomology to rejoin the two ends of the DSB and results in deletions
What is the process of Alternative End-Joining (Alt-EJ)?
- requires >4 bp microhomology
- MRN complex and CtIP induces 5’-3’ resection
- microhomology-mediated annealing and ligation
What initiates resection?
by MRN-CtIP endonuclease
What length resection is sufficient for Alt-EJ?
short resection
What length resection is required for homologous recombinatio?
long resection
What does homologous recombination (HR) use to repair DSBs?
long homologous sequences (>100 bp) as templates)
What is the process of homologous recombination?
- requires two sister chromatids or homologous chromosomes
- resection
- strand invasion leads to D-loop (displacement loop)
- holiday junction (HJ) formation
- polymerization restores the missing sequence
- branch migration
- holliday junction (HJ) resolution
? bases are paired in Holliday Junction
all DNA bases
What do homologous recombination between homologous chromosomes involve?
non-identical sequences
