Evidence for semiconservative replication of circular polyoma DNA.

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Abstract
The results reported are compatible with the assumption that polyoma (Py) DNA replicates semiconservatively as do other double-stranded DNA''s. No experimental evidence could be obtained in support of a conservative model of replication. Hybrid Py DNA where thymine deoxyriboside (TdR) is substituted by S-bromo deoxyuridine (BUdR) in one strand and also heavy Py DNA where TdR is replaced in both strands are able to replicate. In contrast, cellular DNA of infected and of dividing, uninfected MK cells, though able to form a hybrid, is unable to undergo a second cycle replication leading to heavy DNA. Sedimentation velocity analyses and melting behavior suggest that the structures of hybrid and heavy Py DNA are analogous to that of normal, light Py DNA. Py DNA I which accounts for most of the viral DNA is a circular helix with separately continuous strands. Semiconservative replication therefore requires the introducti?n of at least one discontinuity into one of the strands. Following duplication, the continuity of the strands of the daughter molecules is restored. The process engaged in strand opening and closing remains unknown.