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The division of synchronized Tetrahymena pyriformis (strain W) irradiated with low doses of ultraviolet light is delayed if the irradiation is administered during the first 30 min after the end of the synchronizing treatment. After a short period of transition, division in most cells is not delayed even by relatively high doses. The delay induced by a particular low dose administered at any time up to the transition to insensitivity is essentially constant. The division of synchronized cells is delayed by heat-shocks (34 ± 0.1 °C, for 20 min) applied at any time up to approximately 60 min after the end of the synchronizing treatment. After this time, for any single cell in a sample, there is an instantaneous transition to heat-shock insensitivity. The delays in cell division induced by heat shocks increase with time from the end of the synchronizing treatment, attaining a maximum just before the transition to insensitivity. The two different transition times in the responses of synchronized cells to UV on the one hand and to heat shocks on the other are interpreted as representing the completion of two different processes in the final preparation of the cells for division. The qualitative and quantitative differences between the responses to UV and to heat shock suggest that two different division-essential substances are involved, one sensitive to UV and one to heat. Some evidence is presented that suggests that the completion of the heat-sensitive process depends upon the UV-sensitive process. |
