Relative Radiosensitivity of Different Phases in the Life Cycle of L-P59 Mouse Fibroblasts and Ascites Tumor Cells

Abstract

Cells grown in culture were pulse labeled for 10 minutes with tritiated thymidine and immediately thereafter received 250 rads of Co60 gamma-radiation. The labeled anaphases, which were synthesizing DNA at the time of irradiation (in S period), manifested twice as much chromosomal damage (80% abnormal for irradiated - 26% abnormal for controls) as the unla-beled anaphases (56.5% abnormal for irradiated - 26% abnormal for controls), which were in the G1 period at the time of irradiation. The damage sustained by cells in G2 was slightly greater than that for cells in G1. The mitotic delay of cells irradiated in different phases of the life cycle was 5 to 6 hours for cells in the G2 and S periods and 2 hours for cells in the G1 period. The mitotic delay of cells which showed chromosomal abnormalities, however, was the same as for cells which were cytologically normal. The ultimate fate of the cells was investigated by determining the percentage of labeled metaphases after several divisions. In experiment A cells were irradiated immediately after labeling, and in experiment B cells were irradiated 13 hours after labeling (labeled cells were in the G1 period at the time of irradiation). After five divisions and a 50% inhibition in growth, 38% of the metaphases in experiment A were labeled and 51% of the metaphases in experiment B were labeled with 43% for the controls. Thus, cells in the S period at the time of irradiation not only suffered more chromosomal damage but also did not proliferate as readily.