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In Saccharomyces cerevisiae, Mec1/ATR plays a primary role in sensing and transducing checkpoint signals in response to different types of DNA lesions, while the role of the Tel1/ATM kinase in DNA damage checkpoints is not as well defined. We found that UV irradiation in G₁ in the absence of Mec1 activates a Tel1/MRX-dependent checkpoint, which specifically inhibits the metaphase-to-anaphase transition. Activation of this checkpoint leads to phosphorylation of the downstream checkpoint kinases Rad53 and Chk1, which are required for Tel1-dependent cell cycle arrest, and their adaptor Rad9. The spindle assembly checkpoint protein Mad2 also partially contributes to the G₂/M arrest of UV-irradiated mec1Δ cells independently of Rad53 phosphorylation and activation. The inability of UV-irradiated mec1Δ cells to undergo anaphase can be relieved by eliminating the anaphase inhibitor Pds1, whose …