FIG. 3.

Most cells accumulating in G₂/M after IR come from S phase. (A) Schematic representation of cell cycle progression after IR. Characterized checkpoints are labeled with asterisks. ①, G₁ checkpoint, which is p53 dependent and occurs at restriction point; ②, ATM-dependent S-phase checkpoint, which occurs throughout S phase; ③, G₂/M checkpoint, which is ATM dependent and occurs about 30 min prior to chromosome condensation. The early G₂/M checkpoint measures the inhibition of cells that are in G₂ at the time of IR from entering mitosis. G₂/M accumulation, which is ATM independent, measures the accumulation of cells that were in S phase (or G₁) at the time of irradiation. (B) HeLa [ATM(+/+)] and GM9607 [ATM(−/−)] cells were labeled with 30 μM BrdUrd for 30 min and then irradiated with 6 Gy. The top row shows flow-cytometric dot plots of BrdUrd labeling on the ordinate and DNA content on the abscissa before IR and 24 h after IR. The BrdUrd-positive cells (labeled R1) were gated and displayed as DNA content versus cell counts in the bottom row.