Cytological Studies on Mitosis of Plasmodium of Physarum polycephalum

In the multinucleated plasmodium of Physarum polycephalum all nuclei are completely synchronized in mitotic cycle. The duration of a mitotic cycle was 9 hours at 25°C and those of M, G1, S and G2-phase were 0.5 hours, less than 1 minute, 2.5 hours and 6.0 hours, respectively. M-phase was further divided into 15 minutes of prophase, 10 minutes of prometaphase, 5 minutes of metaphase, less than 1 minute of anaphase and 4 minutes of telophase.In the first, incorporation of arginine-3H into total protein of plasmodium was investigated. There were three main peaks: the first one in early S-phase, the second in early G2-phase, and the third in late G2-phase.From the inhibitory effect of cycloheximide on protein syntheses just before and after the start of mitosis, it is considered that protein required for initiation of mitosis is synthesized until before prophase, in the other words, at a phase just before prophase or a little more earlier phase, and another protein that required for shift of mitotic phase from metaphase to anaphase is synthesized probably in prophase and in prometaphase in a certain synthetic rate. Besides, from the inhibitory effect of actinomycin D on mitosis, the RNA required for this mitotic shift is synthesized in premitotic phase about 40 minutes before the start of prophase.