DNA synthesis and cell cycle progression of synchronized L-cells after irradiation in various phases of the cell cycle.

The varying sensitivity to radiation in the different phases of the cell cycle was investigated using L-929 cells of the mouse. The cells were synchronized by mechanical selection of mitotic cells. The synchronous populations were X-irradiated with a single dose of 10 Gy in the middle of the G1-phase, at the G1/S-transition or in the middle of the S-phase, respectively. The radiation effect was determined in 2 h intervals a) by 14C-TdR incorporation (IT) into the DNA, b) by autoradiography (AR), c) by flow cytometry (FCM). The incorporation rate decreased in all three cases, but the reasons appeared to be different, as can be derived from FCM and AR data: After irradiation in G1, a fraction of cells was prevented from entering S-phase, after irradiation at G1/S a proportion of cells was blocked in the S-phase, and after irradiation in S, DNA synthesis rate was reduced. As a consequence of these effects, the mean transition time through S-phase increased. The G2 blocks, obtained after irradiation at the three stages of the cycle were also different: Cells irradiated in G1 are partly released from the block after 10 h. Irradiation at G1/S caused a persisting accumulation of 50% of the cells in G2, and for irradiation in S more than 80% of the cells were arrested in G2.