Complex sentence: PARP1 is a protein that is important for repairing single-strand breaks,'nicks' in the DNA, and if these persist unrepaired until DNA is replicated (which it must do before the cells divide) this causes double strand breaks to form.
Split into simpler sentences: PARP1 is a protein that is important for repairing single-strand breaks ('nicks' in the DNA). If such nicks persist unrepaired until DNA is replicated (which must precede cell divison), the flaws cause double strand breaks to form.