Figure 6.

The cdc25-S99A mutation impairs the DNA damage checkpoint. Plasmids containing cdc25⁺ (WT) or cdc25-S99A (S99A) were integrated in cdc25-22 cells. Synchronous cultures of cells in early G₂ phase were produced by centrifugal elutriation and exposed to the radiomimetic drug bleomycin (BL) at a concentration of 5 mU/ml or mock treated. Mock-treated cells underwent mitosis with similar kinetics. Most of the WT cells (∼80%) were unable to divide during the course of the experiment, indicative of a checkpoint arrest. In contrast, all but ∼15% of the S99A mutant cells divided, indicative of a checkpoint failure.