Chinese hamster cells, synchronized by selecting mitotic cells either with or without a 2-hour pretreatment with Colcemid, were x-irradiated during different phases in the cell cycle and then analyzed both for lethality (loss of colony formation) and for chromosomal aberrations. For the chromosomal analysis, each population of irradiated cells was sampled in metaphase at different time intervals after irradiation. To kill a certain fraction of the cells synchronized following Colcemid pretreatment or to produce a certain number of aberrations, cells in G1 required doses 1.5-1.8 (dose modifying factors) times greater than cells irradiated in metaphase, and cells in mid to late S required doses 2.8-4.2 times greater than those irradiated in metaphase. The dose modifying factors for S phase decreased as comparisons were made at levels of greater damage because the dose-effect response had a quasi-threshold or shoulder for S phase compared with a more linear relationship for metaphase and G1 cells. The dose modifying factors based on lethality agreed within 15% of those based on chromosomal aberrations. Cells synchronized without Colcemid pretreatment gave the same qualitative results. However, Colcemid pretreatment prior to selecting the mitotic cells sensitized mitotic and G1 cells in terms of both lethality and chromosomal damage. As suggested by lethality studies, cells irradiated in early S phase sustained a 2-3-fold increase in the number of chromosomal aberrations compared with the number sustained in cells irradiated in late G1. These positive correlations between lethality and chromosomal aberrations support the hypothesis that damage in the chromatin structures is the main cause of x-ray induced mammalian cell killing.

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