Figure 1.

 DNA intermediates in DSB-mediated meiotic recombination. Many other genes are also required for one or more of the steps shown (Shinohara and Ogawa 1995; Kupiec et al. 1997); however, only those genes relevant to the work in this paper are listed. (A) Meiosis-specific DSBs are generated by the product of the SPO11 gene. RAD50, MRE11, and MER2 are also required. (B) The ends at the sites of DSB are processed resulting in resection of the 5′ single strands. RAD50S refers to the activity that is absent in rad50S mutants. (C) ssDNA ends at DSBs invade homologous target duplex. Invasion leads to strand exchange with the “like” strand of a target duplex yielding a heteroduplex joint. All of the genes listed have been shown by mutant analysis to be required for optimal efficiency of this process. Mutant analysis has not shown directly that the genes that encode RPA are required for this step; however, several observations, including some presented in this paper, make it likely that RPA is involved at this step. (D) DNA repair synthesis generates two complete duplexes joined by a double Holliday junction, which are then resolved into recombination products.