DNA replication in Physarum polycephalum. Analysis of replicating nuclear DNA using the electron microscope

DNA has been isolated from Physarum polycephalum nuclei obtained from macroplasmodia at different stages in the mitotic cycle, and examined using the electron microscope. Putative replicating structures were identified, the majority of which contained clusters of 2--37 'microbubbles', each microbubble corresponding to a segment of DNA 100--5000 nucleotides long. The microbubble-containing structures are unstable in the formamide hyperphase used to prepare specimens for electron microscopy, possibly due to dissociation of newly replicated nascent DNA fragments from the parental DNA template during manipulation. The microbubble clusters present in early S-phase DNA extent over segments averaging 16400 nucleotide residues, and are separated by non-replicated regions of DNA varying in length from 10000 to 50000 nucleotides. It is suggested that each microbubble cluster may represent a 'replicon', and that many 'replicons' in Physarum DNA may contain several sites for the initiation of DNA synthesis that are active during S-phase.