The involvement of RNA in the initiation of DNA synthesis in mammalian cells. Artifacts arising during caesium-salt density-gradient centrifugation which simulate a covalent attachment of RNA to newly synthesized DNA.

Artifacts were encountered during caesium salt density gradient centrifugation which simulated results expected if newly synthesized DNA is covalently attached to RNA. Newly synthesized DNA (in baby hamster kidney cells, BHK-21/C13), pulse-labelled with [3H]thymidine for 10 min at temperatures below 25 degrees C, banded at a greater buoyant density than mature [14C]DNA (heat-denatured nucleic acids) when centrifuged to equilibrium in caesium chloride gradients. Some of the newly synthesized RNA, labelled with [3H]uridine, banded at a buoyant density slightly greater than DNA in caesium sulphate gradients.