Coupled Increases in Nuclear and Nucleolar Sizes with the Cell Phase Transition in the Cell Cycle of Physarum polycephalum

The dependence of nuclear growth on cell stages in the mitotic cycle was investigated in plasmodia of Physarum polycephalum. Nuclei were photographed and the areas of nuclei and nucleoli were measured. Parallel changes in the increase curves of the nuclear and nucleolar areas were observed.Both sizes increased considerably after the S to G2 and the G2 to M phase transitions. Curves for increases in both sizes were also obtained from shortened mitotic cycles after UV-irradiation during incubation with or without nutrients. These curves were similar to those of corresponding controls. Increases in nuclear and nucleolar size seem to be highly coordinated with the progress of the cell stage rather than with increases in synthetic cellular activities. Furthermore, the protein and RNA contents were determined for nucleoli isolated during the cell cycle. The ratio of RNA to protein increased with the progress of age in the cell cycle.