Figure 4.

DSBs occur in transcriptionally active or replicating cells due to reactive oxygen species or ionizing radiation or during replication process. The choice between HR or NHEJ is directed by BRCA1 and 53bp1. Moreover, PARP1 promotes more extensive end-resection via exonuclease activity of EXO1 and BLM to reveal ssDNA and promote HR. RPA binds to ssDNA, while the BRCA2 mediates replacement of RPA with RAD51. Finally, the RAD51 nucleoprotein filament invades a homologous sequence, acting as donor and synthesis extends the invading 3′ end, which then anneals with the resected end to lead to accurate repair.