The rates of syntheses of 18S and 28S rRNA, 5S RNA, capped mRNA and 4S RNA were determined in isolated cells from pre‐ and post‐gastrular embryos of Xenopus laevis. The rate of rRNA synthesis per nucleolated cell Mas about 0.2 pg/hr, or about 5.5 × 104 molecules/hr at the blastula stage, and this value remained constant in later stages. At the blastula stage, about 30 molecules of 5s RNA, 10 molecules of capped mRNA and 900 molecules of 4S RNA were synthesized per molecule of 18S or 28S rRNA. These values were all greatly reduced during the gastrula stage, and at the neurula stage, one molecule each of 5S RNA and capped mRNA and 10 molecules of 4S RNA were synthesized per molecule of 18S or 28S rRNA.
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