X-Ray-induced reversible mitotic delay in sea urchin eggs was studied. Detailed cytological analysis showed that sublethal doses of x rays affected the process of chromosome condensation in these eggs. The delay in chromosome condensation corresponded directly to the delay in cell division.nRate and time course of DNA synthesis in these cells were studied over two division cycles by following the incorporation of tritium-labeled Thymidine. The study of radiosensitivity of DNA synthesis showed that ray doses that caused reversible mitotic delay had no effect on the time or rate of DNA synthesis in these cells. Furthermore, the experiments showed that delay of DNA synthesis in the second division was a result of interference with mitosis. An equation to determine the S period was derived and used to compute the S period in these cells.
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