Abstract
Mature SV40 DNA synthesized for different periods of time either in isolated nuclei or in intact cells was highly purified and then digested with restriction endonucleases in order to relate the time of synthesis of newly replicated viral DNA to its location in the genome. Replication in nuclei supplemented with a cytosol fraction from uninfected cells was a faithful continuation of the bidirectional process observed in intact cells, but did not exhibit significant initiation of new replicons. SV40 DNA replication in cells at 37°C proceeded at about 145 nucleotides/min per replication fork. In the absence of cytosol, when DNA synthesis was limited and joining of Okazaki fragments was retarded, bidirectional SV40 DNA replication continued into the normal region where separation yielded circular duplex DNA molecules containing one or more interruptions in the nascent DNA strands. In the presence of cytosol, this type of viral DNA was shown to be a precursor of covalently closed, superhelical SV40 DNA, the mature from of viral DNA.