FIGURE 1: Events leading to fork breakage after collision of DNA pol III with transcription elongation complexes (TEC). Diagrammed are events ensuing from codirectional collisions, but can occur similarly with head-on collisions. RNAP is illustrated in red, the fork helicase in green. (A) The fork encounters a TEC, that blocks DNA polymerization. (B) The fork moves on, replication is restarted. (C) A single-strand gap is left behind. (D) Convergence of a second fork (helicase marked with lt green) onto the gap. (E) The resulting one-ended double-strand break.