Figure 2.

Repair of gaps generated by P element excision. (A) Blue lines represent two strands of a DNA duplex from which a P element has excised, leaving a gap relative to the sister chromatid (red lines). Resection, Rad-51-mediated strand exchange, and repair synthesis occur as in most models of DSB repair by HR. In SDSA, the nascent strand is dissociated from the template. (B) For a gap, a single cycle of repair synthesis is unlikely to have spanned to gap to produce sequence complementary to the other end of the break (McVey et al. 2004). This end can then reinvade the template, and be extended by additional synthesis. The other end can also invade and be extended. The two ends can even invade different templates—an observation that was important in development of the SDSA model (Nassif et al. 1994). (C) Multiple such cycles of strand exchange, synthesis, and dissociation can produce single-stranded DNA that overlaps in a region of complementarity. (D) SDSA can then be completed by annealing these regions, resulting in restoration of the excised sequences. (E) In some cases, however, repair is completed by EJ, especially TMEJ (Adams et al. 2003; Chan et al. 2010). The result is partial gap filling, commonly seen as “internal deletions” in P elements.