Repairable and unrepairable DNA strand breaks induced by decay of 3H and 125I incorporated into DNA of mammalian cells.

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