Rad18p is essential for cell cycle arrest after DNA damage. (A) Spores from rad18+/rad18::ura4 were germinated until the appearance of polar growth (10 h) at 30°C in medium lacking uracil to select only rad18::ura4 cells, with rad18+ spores as a control. MMS or 4-NQ was then added, and cultures continued incubation for a further 3 h. Samples were taken, fixed, and stained with DAPI. Note the presence of cells with septa bisecting rad18::ura4 nuclei in the presence of both 4-NQ and MMS. Bar, 10 μm. (B) Kinetics of aberrant mitoses in spore germination experiments. Spores were germinated as above for wild-type (ura4+/ura4-D18), rad18::ura4(rad18+/rad18::ura4), and chk1::ura4 (chk1+/ chk1::ura4) strains and either left untreated (□), or MMS (♦) or 4-NQ (●) was added at time 0. Samples were then taken hourly, fixed, and stained with DAPI, and the mean percent of cells with aberrant mitoses (either cut or DNA stretched along the division plane; see text) was recorded from three samples of 100–150 cells. SE ≤ 5%.