Asynchronous bidirectional replication of polyoma virus DNA

The structure of polyoma virus replicative intermediates isolated from infected 3T6 cells was analyzed by two-dimensional agarose gel electrophoresis (Sundin and Varshavsky, Cell 21:103-114, 1980) and quantitative electron microscopy (Krauss and Benbow, J. Virol. 38:815-825, 1981). DNA replication was initiated at a single site (ori) in essentially all of the replicative intermediates. Most of the early replicative intermediates were formed by unidirectional synthesis in the direction of early transcription. Most mid- and late replicative intermediates contained two replication forks which had traveled unequal distances from the origin. Asynchronous initiation of the two growing forks was postulated to account for these observations.