Possible effects of a damaged template on a replication fork. (a) Translesion synthesis occurs when the replisome encounters a lesion and keeps going. (b) If the replisome encounters a lesion that is undergoing repair, a single-strand break can cause the replication fork to collapse, creating a double-strand break. (c) Some lesions prevent progress of the replisome, resulting in a stalled replication fork. (d) In some cases, the lesion is bypassed, leaving a single-strand gap, and replication continues downstream.