The relative rates of synthesis of DNA, sRNA and rRNA in the endodermal region and other parts of Xenopus laevis embryos.

The onset and rates of synthesis of the major classes of nucleic acids have been extensively studied during the development of whole frog embryos (reviews by Brown, 1965; Gurdon, 1967a). Such information is of interest because nucleic acids are the immediate products of genes, and their rates of synthesis therefore provide a direct measure of changes in gene activity. To date nucleic acid synthesis in parts of frog embryos has been analysed mainly by methods which do not distinguish different classes of RNA (e.g. Bachvarova & Davidson, 1966; Flickinger, Miyagi, Moser & Rollins, 1967). Since embryos consist of many different cell types, it is important to know to what extent the pattern of nucleic acid synthesis observed in the whole embryo is true for its individual regions, and in particular for one differentiating cell type. Any differences between parts of early embryos in respect of nucleic acid synthesis are of further interest, since they are likely to be related to unequally distributed components of the egg cytoplasm. Such a relationship may eventually lead to the identification of cytoplasmic components which regulate the activity of genes during cell differentiation. In this article we compare three types of gene activity, the synthesis of soluble RNA (sRNA), ribosomal RNA (rRNA) and DNA, in the endoderm and in the rest of the embryo. These three classes of nucleic acid have been studied because they are easy to extract quantitatively, separate and characterize. sRNA (believed for reasons given below to consist primarily of transfer RNA) and rRNA are not homogeneous populations of molecules, but the different kinds of molecules within each class are functionally related. DNA is quite another kind of gene product, but like different kinds of RNA its rate of synthesis changes markedly and consistently during early development. DNA synthesis is also related to the final differentiation of a number of specialized cells (e.g. Stockdale & Topper, 1966). The endoderm region of embryos was chosen be-