Nuclear and Cytoplasmic DNA Synthesis during the Mitotic Cycle of HeLa Cells

Abstract

In HeLa S 3 cells grown in suspension and synchronized with thymidine, while nuclear DNA is replicated during the S‐phase, cytoplasmic DNA is synthesized with two peaks in the S‐ and G 2 ‐phases: the first has its maximum 1–2 h before that of nuclear DNA; the second takes place after that of nuclear DNA, approximately at the 9th hour from the removal of synchronizing thymidine from the culture medium. Newly synthesized nuclear and G 2 ‐phase‐cytoplasmic DNAs in CsCl gradients have buoyant density of 1.698 and 1.688 g/cm 3 , respectively. The buoyant density of S‐phase‐cytoplasmic DNA is 1.701 g/cm 3 , probably corresponding to the closed circular duplex form of mitochondrial DNA. The kinetics of synthesis of nuclear DNA is exponential, differing from that of S‐ and G 2 ‐phase‐cytoplasmic DNAs which show an asymptotic shape of [ 3 H]thymidine labelling. Ethidium bromide inhibits the synthesis in vivo of both S and G 2 peaks of cytoplasmic DNA, but does not appreciably influence the labelling of nuclear DNA.