Figure 1.

The idea of replication fork collapse at a single-strand interruption in template DNA. (A) A DNA segment with a long-lived nick. (B) A replication fork is approaching the nick. (C) The replication fork has reached the nick and collapsed; another replication fork is approaching the remaining single-strand interruption from the opposite direction. (D) The second replication fork has reached the interruption and collapsed; as a result, one of the daughter chromosomes has a double-strand break. (E) An alternative scenario of double-strand breakage at replication forks: replication fork “explosion” at the nick.