Variations in the rates of DNA and RNA synthesis during the DNA synthetic ( S ) period of the cell cycle have been determined in explants of early Rana pipiens embryos.
Cells of dorsal axial ectoderm-mesoderm regions and belly endoderm regions of the embryo were partially synchronized using 5-fluorodeoxyuridine. The partially synchronized explants were incubated continuously or at intervals during the S period with radioactive nucleic acid precursors. Autoradiographic and biochemical evidence indicates that at the gastrula, neurula and tailbud stages the DNA is replicated discontinuously in two maxima with a slowing in the synthetic rate in the middle of the S period. There is an increase in the proportion of the DNA which replicates late as development proceeds from gastrulation. At the neurula and tailbud stages the proportion of late replicating DNA is greater in the belly endoderm than in the dorsal axial ectoderm-mesoderm.
Experiments utilizing H3-5-uridine indicate that RNA is synthesized discontinuously in two maxima during the S period in both dorsal axial and belly regions at the earlier neurula stage. By the tailbud stage, however, a significant decrease in the second maximum of RNA transcription occurs. RNA extraction experiments indicate that these changes can be attributed, in part, to changes in the synthesis of DNA-like RNA. These findings are discussed in relation to cell determination.