In view of the possible utilization of the method of autoradiographic DNA repair as a prescreening test for chemical carcinogens, an estimate of the combined biological and photographical variability of the results was carried out. The unscheduled DNA synthesis was measured in mouse fibroblast-like cells within the first 6 hr of repair following 1 hr damage by N-diazoacetylglycine amide. In 14 out of 17 separate experiments performed at different times over a period of about one year, the mean number of grains/nucleus ranged from 24.2 to 37.7; in the remaining 3 experiments this value was of 48.7, 57.6, and 78.1. Such a variability is considered satisfactory for the proposed use of this method. An attempt to quantitatively estimate the absolute amount of unscheduled DNA synthesis gave the value of 1.83 × 106 molecules of the four bases/grain, approximately 8.3 × 10-5 of the total cell DNA.
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