Microbubbles in replicating nuclear deoxyribonucleic acid from Physarum polycephalum.

Clusters of microbubbles, represent probable sites of newly initiated DNA synthesis, were identified in nuclear DNA from Physarum polycephalum by using the electron microscope. Their presence is associated specifically with S-phase. Each microbubble corresponds in size to a replicating segment of DNA about 100-5000 nucleotide residues in length. The DNA structures containing microbubbles are metastable, and revert to native DNA in the presence of moderate concentrations of formamide used to prepare samples for electron microscopy. It is suggested that each cluster of microbubbles may correspond to a unit of replication (a replicon) in Physarum DNA.