COMPARISON BETWEEN THE TIMING OF MICRONUCLEAR AND MACRONUCLEAR DNA SYNTHESIS IN EUPLOTES EURYSTOMUS

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Abstract
These studies demonstrate that two different kinds of nuclei in a common cytoplasm may undergo DNA synthesis at totally different parts of the cell cycle. This asynchrony between macronucleus and micronucleus contrasts with the synchrony of DNA synthesis in the two macronuclei of double Euplotes. Although the latter observation shows that the cytoplasm exerts some control over the initiation of DNA synthesis in this organism, the position of this synthesis within the cell cycle is a characteristic of the nucleus. Only nuclei of a certain kind can respond to a particular set of cytoplasmic conditions at a given point of the cycle. Tetrahymena pyriformis and Paramecium aurelia demonstrate two additional temporal relations between macronuclear and micronuclear DNA synthesis. In T. pyriformis micronuclear synthesis occurs between late anaphase and early inter-phase as in Euplotes, but macronuclear S occupies the early part of the interdivision interval. In P. aurelia, micronuclear S normally occurs midway in the interdivision interval and nearly coincides with the beginning of macronuclear S. Under special conditions micronuclear S is delayed, whereas the beginning macronuclear S is not. It is obvious that there are no set rules in ciliates either for the relation between the timing of DNA synthesis in the two nuclear types or for the relation between the cell cycle and DNA synthesis in either nuclear type.

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