The temporal replication of ribosomal genes in synchronized Chinese hamster cells.

Abstract The time of ribosomal gene replication has been examined in Chinese hamster cells synchronized by selective detachment of mitotic cells. DNA of synchronized cells was density labeled with bromodeoxyuridine for successive two-hour intervals spanning the S phase of the cell cycle, and the DNA replicated during each time interval was separated from the remaining DNA by centrifugation and fractionation in cesium chloride. Ribosomal genes in newly replicated DNA and bulk DNA from each interval were detected by hybridization of the DNA in each fraction with a very high specific activity in vitro transcript (complementary RNA) of purified Xenopus laevis ribosomal DNA. This heterologous probe hybridizes with high buoyant density Chinese hamster DNA in cesium chloride and is competed with unlabeled Chinese hamster ribosomal RNA. During the first two-hour interval (3 to 5 h after mitosis) about half of the ribosomal genes replicate as well as 27% of the non-ribosomal DNA. Most of the remaining ribosomal DNA replicates during the next time interval (5 to 7 h), indicating that in Chinese hamster cells the multiple ribosomal gene copies (about 300) initiate and complete their replication during an early portion of the S phase.

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