TIMING OF MICRONUCLEAR MITOSIS AND ITS RELATION TO COMMITMENT TO DIVISION IN PARAMECIUM-TETRAURELIA

This study examines the timing of micronuclear mitosis during the vegetative cell cycle and shows that mitosis begins early in the division process and coincides approximately with the earliest stages of oral morphogenesis (about 0.6 in the cell cycle in synchronous cell samples). The cc1 mutation blocks cell cycle progression prior to the point of commitment to division. Although the cc1 mutation blocks macronuclear DNA synthesis under restrictive conditions, it does not block micronuclear DNA synthesis. However, absence of functional cc1 gene product leads to blockage of micronuclear mitosis prior to completion of anaphase. This point coincides with commitment to division and is also the point at which oral morphogenesis is blocked in cc1 cells. The timings of the transition points for micronuclear mitosis and oral morphogenesis in cc1 cells are closely associated in both synchronous cell samples and in asynchronous cultures.