Induction and repair of DNA strand breaks in CHO-cells irradiated in various phases of the cycle.

Induction and repair of DNA strand breaks was investigated in synchronized CHO cells by using the alkaline DNA-unwinding technique followed by chromatography on hydroxyapatite. The velocity of DNA denaturation in an alkaline solution (pH 12.1) was found to be independent of the position in the cycle. In unirradiated cells the number of DNA strand breaks per 3.6 x 10(12) dalton was found to vary with age in cell cycle amounting to about 250 breaks per cell for G1, 400 for S, and 500 for G2 + M cells. The number of induced DNA strand breaks per unit dose was found to be the same in all phases; an average of 340 strand breaks were induced per Gy and per 3.6 x 10(12) dalton. Within the first hour after irradiation about 90 per cent of all radiation-induced strand breaks are rejoined.