Fine structure of interphase nuclei

The initiation and replication sites of DNA synthesis in the plasmodial nuclei of Physarum polycephalum were studied with electron microscopic autoradiography. By using both thin sectioning and whole mount techniques, it was shown that the dense chromatin masses in the nucleus consisted of predominantly elementary chromatin-like fibrils, approximately 300 Å in diameter while the electron transparent region in the nucleus consisted of predominantly finer fibrils, less than 100 Å in diameter. With electron microscopic autoradiography it was found that (1) the initiation sites of DNA synthesis were definitely in the boundary regions between the dense chromatin masses and the electron transparent region, (2) the initiation and replication sites of DNA synthesis were definitely not on the nuclear membrane, (3) within a few minutes, replication sites migrated from the initiation sites to the electron transparent region and (4) in this electron transparent region, almost all of the nuclear DNA was synthesized.