| Abstract: | Paramecium cells were selected which received the entire parental macronucleus at fission and thus started the cell cycle with twice the normal post-fission DNA content. During each of the subsequent two cell cycles the cells synthesized approximately as much DNA as did control cells. The amount of excess macronuclear DNA was consequently halved during each cell cycle. The minimum pre-fission DNA content was just larger than the mean post-replication DNA amount, confirming that a similar amount of DNA, approximately equal to the mean post-fission DNA content of the non-selected population, was synthesized in macronuclei, regardless of the post-fission DNA content. These observations confirm a model for DNA content regulation previously devised for Paramecium and are inconsistent with DNA content regulation schemes proposed for other ciliates. The increased DNA content has no effect either on the subsequent total protein content of pre-fission cells, or on the rate of cell growth. This suggests that the rate of cell growth is limited by the size of the cell when the macronuclear gene-dosage is equal to or greater than that in normal cells. The results also suggest that the amount of DNA synthesized within an interfission period is also limited by the size of the cell and is proportional to the cell mass. Paramecium does not require a fixed nucleocy oplasmic ratio as a pre-condition either for cell division, or, by inference, for initiation of DNA synthesis. |
