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Fig. 5. Inactivation of chk1-ts1 following the accumulation of DNA damage results in a rapid increase in aberrant mitoses. (A) Asynchronous wild-type (chk1:ep), chk1{Delta} and chk1-ts1 cells were treated with either 0 Gy or 250 Gy ionizing radiation (0.7 Gy minute–1, 6 hour exposure) or 0.0075% MMS (16 hour exposure) at 25°C. Cultures were then shifted to 36°C to inactivate the chk1-ts1 allele. For each strain, the number of aberrant mitoses was counted every 15 minutes for 1 hour after temperature shift for irradiated ({bullet}), MMS-treated ({square}) and untreated ({blacksquare}) cells. (B) DAPI-stained wild-type (chk1:ep), chk1{Delta} and chk1-ts1 cells are shown at 25°C, at 25°C following 250 Gy irradiation and following a temperature shift after irradiation to 36°C for 60 minutes. Cells showing aberrant mitotic figures are indicated by the arrows.