Effect of Irradiation and Theory of Role of Mitotic Delay on the Time Course of Labeling of HeLa S3 Cells with Tritiated Thymidine

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Abstract
The percentage of HeLa S3 cells in DNA synthesis at various times after 500 r of X-irradiation was determined by means of auto-radiography with H3-thymidine. The percentage rises during the period of mitotic delay so that at 4 to 8 hours after irradiation almost twice the proportion of cells are synthesizing DNA as are doing so in controls. To explain this on the basis of the build-up of cells in DNA synthesis occurring as a result of mitotic delay, the kinetics of the effects of irradiation on the numbers of cells in the various stages of the mitotic cycle were determined. These kinetics are discussed with respect to their importance in the interpretation of data on the effects of irradiation of DNA synthesis, but they fail to account completely for the results reported here. Alternatively the interpretation is that many cells in DNA synthesis at time of X-irradiation remain in this stage for an abnormally long time and there is no effect on the rate at which cells come into DNA synthesis.