Figure 4.

CHK1^−/− cells are defective for the G₂/M DNA damage checkpoint. (A) FACS (DNA content) analysis of irradiated GFP- or Cre-transfected CHK1^+/+, CHK1^flox/+, and CHK1^flox/− cells. Twenty-four hours after transfection, cells were subjected to 10 Gy IR, harvested at 0, 12, 18, and 24 hr and stained with PI. The percentage of G₁–S and G₂ population (mitotic cells are included in the G₂ counts but are generally a small proportion) is shown above each FACS sample. (B) FACS (DNA content) analysis of IR and nocodazole-treated cells. Cells of the indicated genotypes were transfected and irradiated as in A and incubated in nocodazole (0.2 μg/ml) containing media 30 min after irradiation. (C) Images (1000×) of the DAPI-stained 12-hr samples in B. Arrows indicate mitotic cells with condensed chromosomes and no nuclear membrane. (D) A mitotic index graph of IR and nocodazole-treated cells described in B and C. Cells (250–300) were counted for each sample. GFP or Cre transfections are indicated by open or closed circles. f5 and f6 represent two independent CHK1^flox/− cell lines and f7 is a control CHK1^flox/+ cell line.