Delay in entry into S phase after heat shock

Synchronized cells of the Harding Passey melanoma grown in culture were given a heat shock treatment of 44 degrees C for 36 min. Thymidine incorporation was measured at frequent intervals after heat shock to determine the time of onset of the next DNA synthetic period. If the heat shock was given at the end of G1, the following S was delayed by 20 hr. Heating at other times in the cell cycle resulted in an even longer interval before the onset of S. The end of G1 was also the most resistant to hyperthermic killing and to the effect of heat on the magnitude of thymidine incorporation in the following S. Heating the cells a second time did not repeat the effect of the first treatment unless the second heat shock treatment was at a considerably higher temperature. Thus thermotolerance to heat shock killing also applies to cell-cycle delay.