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

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Abstract
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, 1967 a). 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 syn-thesis 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.