Abstract
Electron microscopy was used to identify and quantitate DNA molecules associated with 3H-labeled polyoma minichromosomes which had been fractionated on a sucrose gradient. The percentage of replicating DNA molecules observed in the fractions of the gradient normally designated the replicative intermediate region was up to ninefold higher than in fractions from the mature region. Nevertheless, because of the higher overall concentration of polyoma DNA molecules in the mature region, nearly as many replicating DNA molecules were computed to be in the mature region as in the replicative intermediate region. The replicating molecules in the mature region was predominantly early replicative intermediates. Almost all late replicative intermediates were found in the replicative intermediate region. Under aqueous spreading conditions, a substantial fraction of the replicating DNA structures appeared to be asymmetrical or otherwise unusual, suggesting that extensive single-stranded regions may exist in replicating polyoma minichromosomes.