In the myxomycete, Physarum polycephalum, nuclear DNA synthesis commences immediately upon completion of mitosis. While the synthesis of extranucleolar DNA is completed within a few hours, nucleolar DNA synthesis occurs during most of the S-phase and the entire G2 phase of the intermitotic period. When large (polyploid), late-interphase nuclei were allowed to bypass mitosis by transplantation into recipient plasmodia which were at early interphase and which belonged to a strain having smaller nuclei, the nucleolar DNA of the transplanted nuclei continued to be labelled (autoradiographs) after incubation of the host plasmodia with [3H]thymidine until they entered prophase along with the nuclei of the host plasmodium, approximately one intermitotic period later. This labelling was DNase-sensitive and RNase-resistant. When late-interphase nuclei were labelled with [3H]thymidine just prior to transplantation, there was no decrease of label after transplantation during the additional intermitotic period. We conclude from these experiments that there is no obligatory alternation between nucleolar DNA duplication and mitosis in Physarum polycephalum and that nucleolar DNA replication might exhibit amplification during an experimentally prolonged intermitotic period.

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