Figure 3.

Cellular responses to a DSB in S. cerevisiae. A DSB is rapidly recognized by MRX that recruits Tel1. In G1 phase of the cell cycle, the DSB is repaired mainly by NHEJ (not represented). In S/G2 phases of the cell cycle, the DSB is processed for repair by HR. The 5′–3′ resection is achieved by the concerted action of nucleases and helicases including Sae2, Sgs1, Dna2, and Exo1. The resulting ssDNA is then coated by RPA. In yeast, Rad52 then catalyzes the displacement of RPA by Rad51. The resulting Rad51 filament can then initiate homology search and strand invasion and prime DNA synthesis. Resolution of DNA structure is then required to conclude the gene conversion. The DNA checkpoint pathway starts by Tel1 recruitment, which modifies nearby nucleosomes (not represented) to form a first platform to recruit additional checkpoint components. The RPA-coated ssDNA forms a second platform that recruits Ddc2-Mec1, Dpb11, and the 9-1-1 checkpoint clamp (Ddc1-Rad17-Mec3), assisted by the checkpoint clamp loader (Rad24-Rfc2-5, not represented). Rad9 is then recruited and activated. Collectively, this edifice contributes to the efficient activation of Rad53 and Chk1 that will phosphorylate downstream targets.