FIGURE 9.

Double-strand break formation in dam mutant strains exposed to MNNG. A. Replication of an O⁶meG base in the parental strand can result in the insertion of a T or G in the complementary strand. The mismatch repair system treats both types of base pairs as substrates, and a futile cycle of repair ensues. Repair can occur on either strand, and if it occurs on both strands simultaneously, a DSB may result due to overlapping repair tracts or by MutH (arrowheads) nicking at the same or closely spaced GATCs (black rectangles). B. During futile cycling a second replication fork encounters the gap present during mismatch repair excision, which results in replication fork collapse.