FIG. 9.

Models for Mre11 function. In meiotic cells, the 5′ ends at DSB sites remain bound by the Spo11 protein, preventing access of exonucleases. The Mre11-Rad50-Xrs2 complex binds to break sites and unwinds the ends to reveal single-stranded DNA for cleavage by Mre11. This cleavage removes Spo11 and the 5′ strand, resulting in the formation of a 3′ single-stranded tail. When DSBs are produced by ionizing radiation or by the HO endonuclease, the 5′ ends are generally free and accessible to other nucleases in addition to the Mre11-Rad50-Xrs2 complex.