Polyoma DNA synthesis in isolated nuclei: evidence for defective replication forks

Nuclei, isolated from polyoma virus-infected mouse 3T6 cells, were incubated under condtions suitable for polyoma DNA synthesis. By using electrom microscopy and standard regression statistics, it was shown that replication is mainly unidirectional in a large number of molecules, indicating the presence of inactive replication forks. The replication forks were inactivated randomly, and the defect seemed to be present from the beginning of the in vitro incubation.