Fig. 7: Single-ended double-strand breaks result from head-to-tail collisions of S forks with unterminated G1 forks.

a S-phase replication forks are most likely to collide with G1 replication forks in a head-to-tail-orientation. Several factors determine where these collisions occur. First, G1 replication will usually initiate at early firing origins that are preferentially located close to chromosome centers. G1 replication forks going inward will often be terminated by G1 or S replication forks that also initiate from early firing origins. In contrast, G1 replication forks going outward are less likely to be terminated by a G1 or S-phase replication fork emanating from a late-firing origin reaches and therefore will be more frequently subject of head-to-tail collisions. b For simplification, a single replication fork moving to the right telomere is shown. A replication fork moving to the left telomere will encounter the same events just with opposite strand directionality. Unterminated forks persist after unscheduled replication in G1 due to incomplete duplication/missing termination. c,d Head-to-tail collision of an S-phase replication fork with an unterminated G1 replication fork. A single-ended double-strand break (seDSB) occurs independent of whether the S-phase replication fork travels on the parental reverse strand (c) or the reverse strand synthesized by the G1 replication fork (d). Single-stranded DNA (ssDNA) could either be directly exposed during seDSB generation (due to incomplete lagging strand replication) or afterwards by resection. Independent of its generation, the ssDNA will be bound by RPA and gives rise to a strand-biased pattern of reads in RPA-ChIP-seq experiments.