FIG. 5.

Cell cycle checkpoints induced by ionizing radiation. (A) Representative histographs for the analysis of the G₁/S checkpoint exposed to either 0 or 500 rad. BrdU-labeled cells appear in the top three boxes of each histograph, which are outlined by one box in bold. The average percentage (± standard deviation) of cells labeled with BrdU is shown at the top of each histograph. (B) Representative histographs for the analysis of the G₂/M checkpoint exposed to either 0 or 500 rad. BrdU-labeled cells in G₁ are shown in the box in bold. The average percentage (± standard deviation) of G₁ cells labeled with BrdU is at the top of the histograph. (C) Graph depicting the G₁/S (open bar) and G₂/M (closed bar) checkpoints. For the G₁/S checkpoint, the fraction of irradiated BrdU-labeled cells is divided by the fraction of unirradiated BrdU-labeled cells. For the G₂/M checkpoint, the fraction of irradiated BrdU-labeled cells in G₁ is divided by the fraction of unirradiated BrdU-labeled cells in G₁. The averages of clones are shown for control (n = 2), p53^−/− (n = 5), ku80^−/− (n = 5), and p53^−/− ku80^−/− (n = 2) cells.