Timing and Regulation of Nuclear and Cortical Events in the Cell Cycle of Tetrahymena pyriformis*

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Abstract
Using continuous flow cultures based on the chemostat principle, cell generation times of the ciliate T. pyriformis strain GL varied from 4.9-22.2 h and various parameters of the cell cycle at 28 C were studied. These included the duration of the periods required for oral morphogenesis, macronuclear division, cell division, G1, S and G2. The size of individual cells was measured. Independent of growth rate, the period of oral morphogenesis occurred during the last 90 min of the cell cycle. In all cases macronuclear and cell divisions took place during the last part of these 90 min, and the final macronuclear separation occurred just before final cell separation. The S-period increased slightly; the G1 and G2 increased in roughly the same relative proportion to the increasing generation times. Slowly growing cells (generation time 20.5 h) were shorter but broader and somewhat larger in volume than quickly growing cells (generation time 4.9 h).