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Synthesis of ribosomal RNA during the mitotic cycle on the slime mould Physarum polycephalum.

1. An isotope dilution technique has been used to analyze the synthesis of metabolically stable nucleic acids during the mitotic cycle in surface plasmodia of the slime mould Physarum polycephalum. Microplasmodia that had been labelled with [3H]uridine were used to prepare a surface culture, after a period of growth long enough to ensure that radioactivity was present only in tRNA, rRNA and DNA. The synthesis of rRNA or nuclear DNA during the growth of the surface plasmodium was then followed by measuring the specific activity of the nucleic acid. 2. Synthesis of rRNA during the mitotic cycle shows the following characteristics: (a) it is low during the immediate period of nuclear division, (b) synthesis is then continuous throughout interphase and (c)the rate of synthesis increases 5-6-fold between the beginning and end of interphase. These results are discussed in relation to the pattern of replication of the genes for rRNA. 3. Approximately 80% of the nuclear DNA replicates during the first 90 min of the mitotic cycle; completion of replication, however, occupies the remainder of interphase.