Repairable and unrepairable DNA strand breaks induced by decay of3H and125I incorporated into DNA of mammalian cells

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Abstract
Chinese hamster cells (Cl : 1) were labelled with3H-thymidine or125Iododeoxyuridine for 18 h and after 3 h in non-radioactive medium they were stored at 0° C up to 6 h. The number of DNA strand breaks observed after the labelling period (37° C) or after treatment at 0° C was determined using the DNA-unwinding technique.125I-decays in DNA were significantly more efficient than3H-decays in introducing unrepairable DNA strand breaks during the labelling period. 32% of125I-induced and 3% of3H-induced DNA strand breaks were unrepaired after 21 h at 37° C. Comparison between the effects of125I- or 3H-disintegrations in DNA in three different ways shows 7–12 times more pronounced effects for125I-decays. For125I-labelled cells 3–4 DNA strand breaks were found per decay and the corresponding value for3H- labelled cells was 2.