The Induction and Repair of DNA Breaks in Rat Epidermis Irradiated with Electrons

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Abstract
The dependence of DNA strand breaks (alkali-blue sites) on radiation dose and the rate of repair of such breaks were determined by means of an alkaline unwinding technique. The epidermal DNA was labeled with 3H prior to irradiation by giving i.p. injections of [3H]thymidine. After irradiation the epidermal cells were removed and incubated in an alkaline solution. After the incubation in alkali, S1 nuclease was added to digest the single-stranded portion of the DNA. The amount of undigested double-stranded DNA was determined by using a liquid scintillation counter to count the 3H in the precipitated DNA. Plots of percentage double-stranded DNA vs. time in alkali, referred to as unwinding curves, were utilized to estimate the number of strand breaks (alkali-labile sites) per unit DNA. The strand breaks increased in proportion to radiation dose within the 300-2400 rad dose range employed in the experiment. When the skin was given 1200 rad, the strand breaks were removed with a half-time of about 21 min. This half-time implies a rate of removal much too rapid for single-strand breaks to be the major repairable component of carcinogenic injury, since the latter was found in separate experiments to be removed with a half-time between 110 and 240 min.