Three Classes of DNA Strand Breaks Induced by X-irradiation and Internal β-rays

Abstract

Repair kinetics of DNA strand breaks were investigated after exposing exponentially growing CHO cells to X-radiation or to internal β-rays from incorporated tritium, respectively. DNA strand breaks were analysed by the alkaline unwinding technique followed by chromatography on hydroxyapatite. For either type of radiation, the repair kinetics are statistically best described by a sum of three exponential components. The half-times determined are τ_I ∼ 2 min, τ_II ∼ 20 min and τ_III ∼ 170 min; they are identical for both types of radiation. But the initial fractions of the components are different for X- and internal β-rays; X-rays: f_I = 0·70, f_II = 0·25, f_III = 0·05; internal β-rays: f_I = 0·40, f_II = 0·40, f_III = 0·20. Components I and II are considered to represent the repair of two different classes of single-strand breaks and component III the repair of double-strand breaks. Two alternative interpretations for the occurrence of the two classes of single-strand breaks are discussed.