Rapidly labeled, i.e., nascent, DNA from HeLa cells was separated from the bulk DNA by ultracentrifugation. Further characterization of the rapidly labeled component revealed that its sedimentation coefficient is in the range of 4S and that it exists in a single- and double-stranded conformation. Moreover, analysis by nitrocellulose chromatography and CsSO4 density sedimentation of the nascent DNA labeled with 3H-uridine revealed that it is covalently linked to short chains of RNA, indicating that in HeLa cells RNA primer is involved in DNA replication.

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