Naturally synchronous populations of developing sea urchin eggs are used in studies of the mechanism of radiation damage. Experiments show the effect on DNA synthesis in the developing eggs of Strongylocentrotus purpuratus fertilized with sperm which have been irradiated with sublethal doses of X-rays or ultraviolet radiation. The profiles of the first DNA synthetic period (S period), as measured by the uptake of3 H- TdR into the acid-insoluble fraction, were identical for developing eggs fertilized with irradiated and nonirradiated sperm. There was a radiation-induced delay in the second S period equal to the division delay. However, the rate and amount of DNA synthesis in this second S period are unaffected by the radiation. The same type of radiation-induced effect was found with gametes of the sea urchin Arbacia punctulata where the sperm are X-irradiated.

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