To explain gene conversion without crossing over, a new recombination model termed synthesis-dependent strand annealing (SDSA) has been proposed.Figure 11-26 shows how mating-type switching can occur using this mechanism.The initiating event is, as we saw above, the introduction of a DSB at the recombination site.After 5’-to-3’ resection and strand invasion (Fig. 11-26b,c), the invading 3’ end serves as the primer to initiate new DNA synthesis at a region of homology flanking Ya and continue copying the Ya sequence (Fig. 11-26c,d).Remarkably, in contrast to what occurs during the DSB-repair pathway, a complete replicationfork is assembled at this site.In contrast to normal DNA replication, however, the newly synthesized strand is displaced from its template and anneals with the second resected 5’ end.Once this annealing step occurs, the corresponding long 3’ tail (bottom strand in Fig. 11-26d) is clipped off by an endonuclease, and the new 3’ end is used to prime and extend and copy the second strand of Ya sequences(Fig. 11-26e,f).