SUMMARY: The measurement of the time at which normal colony-forming ability returns in irradiated cultures of tsI held at 30 °C can be used to estimate the time of completion of recombination repair. By comparing the times to complete such repair in populations given increasing radiation doses it is possible to calculate the rate of recombination repair. The rate was independent of the radiation dose; recombination could repair in one minute the damage caused either by 1.2 krad gamma radiation or 4 x 10 J mm u.v. radiation.

The time taken for the normal rate of DNA synthesis to return in irradiated tsI was measured under conditions identical to those used to measure recombination repair. The delay in DNA synthesis was 1.0 min per 1.2 krad gamma radiation and 1.0 min per 5.6 x 10 J mm u.v. radiation. The data suggest that the normal rate of DNA synthesis resumes immediately after the completion of recombination repair of gamma-induced damage, but before the completion of recombination repair of u.v.-induced damage. It is postulated that cell death at the lethal dose of u.v. radiation is caused by a second round of replication of DNA which is still being repaired by recombination.


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