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FIGURE 10-21 Recombination model for mating-type switching; synthesis-dependent strand annealing (SDSfl). The figure shows the steps leading to gene conversion at the MAT locus. The HMR and MAT regions are shown in green; the region of HMR encoding the a information is represented in dark green whereas the region of MAT encoding the u information is shown irt lime green. Upon completion of process of 5DSA, the o. region originally present ¿t MA? has been replaced by, that is, converted to, the a information present in the HMR region-

Synthesis of two new DNA strands from a information template at HMR a

strand excised

branch migration to disengage duplexes removal of second oki strand at MAT

repair synthesis and sealing of DMA strands a information

HMRa a information

HMRa

a information a information alter the strand Invasion step, this recombination pathway diverges from the DSB-repair mechanism. One hint that the mechanism is different is that the crossover class of recombination products is never observed during mating-type switching. Recall that in the DSB-repair pathway, resolution of the Holliday junction intermediates gives two classes of products: the splice, or crossover class, and the patch, or non-crossover, class (see f igure 10-2). According to the DSli-repair model, these two types of products are predicted to occur at a similar frequency, yet, in mating-type switching, crossover products arc never observed. Therefore, models for recombination that do not involve Holliday junction intermediates better explain mating-type switching,

To explain gene conversion without crossing over, a new recombination model termed synthesis-dependent strand annealing (SDSA) has been proposed. Figure 10-21 shows how mating-type switching can occur using this mechanism. The initiating event is, as described above, the introduction of a DSB at the recombination site (Figure 10-2la). After strand invasion, the invading 3' end serves as the primer to initiate new DNA synthesis (Figure 10-21 c and d). Remarkably, in contrast to what occurs during the DSB-repair pathway, a complete replication fork is assembled at this site. Both leading and lagging strand DNA synthesis occurs. tn contrast to normal DNA replication, however, the newly synthesized strands are displaced from the template. As a result, a new dnuble-stranded DNA segment is synthesized, joined to the DNA site that was originally cut by HO, and resected by MRX, This new segment has the sequence of the DNA segment used as the template (HMffa in Figure 10-21).

Completing recombination requires that the other "old" DNA strand present at MAT (the 3'-ending strand not cleaved by MRX) be removed {the bottom strand in Figure 10-2id). Then, the newly synthesized DNA—an exact copy of the information in the partner DNA molecule—replaces the information that was originally present. This mechanism nicely explains how gene conversion occurs without formation of a Holliday junction. Thus, by this model, the absence of crossover products during mating-type reenmbinatinn is no longer mysterious.

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