DNA single-strand-breakage efficiency was measured in frozen Chinese hamster cells (V79-S171) after they accumulated disintegrations from incorporated tritium. The results are compared to the single-strand-breakage efficiency of X-rays at dry ice or liquid nitrogen temperatures, which is reduced by a factor of 4 from the strand-breakage efficiency found at 37°C.3 H decays in cells that incorporated [5-3 H]uridine into RNA after a 30-min pulse were 20% as effective as those in [3 H]thymidine-labeled cells in causing single-strand breaks in DNA. This difference in DNA strand-breakage efficiencies for [3 H]uridine and [3 H]thymidine is almost the same as the difference in efficiencies of cell killing from the two macromolecular locations of tritium within the nucleus of the cell. In cells labeled in one or both strands of DNA with [3 H]thymidine the breakage efficiencies were 2.2-2.3 breaks per3 H decay, equivalent to 0.5 rads per decay-a value very close to that predicted for the dosimetry of3 H beta particles. Irradiation by3 H decays from tritiated water added to the medium before freezing was intermediate between [3 H] uridine and [3 H]thymidine in producing single-strand breaks. Isolation of3 H-labeled DNA and freezing in solution at a concentration less than that found in the cell nucleus produced a lower strand-breakage efficiency because of a reduction in the dose to the DNA by the short-ranged beta particles.

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