A/ The broken pieces of DNA usually end up by getting knitted together by specific enzymes, but before that many steps occur :
- the break has to be "sensed" by special protein complexes (MRN, Ku 70/Ku 80) - key repair inducers are activated by these sensors (ATM, ATR, DNA-PK) - foci regrouping all these enzymes around the break have to be built
B/ It does not always happen that way :
- PARP (poly ADP-ribose polymerase) is a very special enzyme that detects interruptions of DNA structures, and participates in BER (base excision repair), and repair of DNA breaks induced by alkylating agents, such as temozolomide. The PARP pathway seems overefficient in patients with BRCA mutations.
- AGT : Alkylating agents induce breaks by adding a methyl group to the oxygen in position 6 of guanine. Adenyl guanine transferase (AGT) can correct this.
- HOMOLOGOUS RECOMBINATION which requires first the Rad 51 complex to search the genome for DNA sequence homology used for the repair.