Ultraviolet light (UV) and ionizing radiation stimulate two types of unscheduled DNA synthesis in human lymphocytes: a fast process completed in about 1 hour after irradiation and a slow process which lasts for at least 5 hours. A study of the temperature dependence of unscheduled and of normal DNA synthesis shows that the rate for the fast process peaks between 42° and 44°C when stimulated by UV and near 42°C when stimulated by ionizing radiation. Normal DNA synthesis and DNA synthesis stimulated by phytohemagglutinin reach maximum rates between 37° and 40°C. Over the temperature interval 22°-37°C, the range of the activation energy for the fast process is 18-25 kcal/mole when induced by UV, 16-18 kcal/mole when induced by ionizing radiation, and 21-24 kcal/mole for normal DNA synthesis. The upper limit for the activation energy of the slow process is 13 kcal/mole.

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