Differences in timing of replication and maturation of nascent DNA fragments between satellite and mainband DNA during the S phase of the first cell cycle in cultured carrot root explants

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Considerable portions of cells in cultured carrot root disks replicate DNA in a synchronous fashion, with a maximum peak of synthesis at 48h after excision of the disks. At the early stage of this first replication phase, a satellite DNA component with a higher buoyant density is preferentially synthesized. DNA from the root explants, pulse-labeled with 3H-thymidine at the early (15h) and middle (48h) synthetic stages, sedimented in the 5-9S region in an alkaline sucrose density gradient. After prolonged labeling or chase incubation, the 3H-radioactivity shifted from the 5-9S components to longer intermediate molecules of 10-11S, 12-13S, 18-20S, and 24-26S with the bulk DNA synthesized at the middle stage, but this shift appeared to be delayed with the satellite DNA synthesized at the early stage. The results of chromatographic fractionation of EcoRI-digests of 2h-labeled DNA, and the EcoRI-digests treated additionally with S1-nuclease over benzoylated naphthoylated DEAE-cellulose, indicated that the relative ratio of DNA molecules containing substantial single-stranded regions to predominantly double-stranded molecules was clearly higher in the early replicating DNA.