Figure 2.

A portal to the modern era: the dependence of ribosomal RNA synthesis on the nucleolus. Anucleolate (left) or wild-type (right) Xenopus laevis embryos at the neurula stage were incubated with C¹⁴-labeled carbon dioxide and RNA was extracted 20 hours later, when both groups of embryos were still morphologically and physiologically indistinguishable. Shown are sucrose gradient sedimentation profiles of 28S and 18S rRNA and transfer RNA. This finding was a keystone in establishing the role of the nucleolus in the biosynthesis of ribosomes. (Reproduced from Brown DC and Gurdon JB 1964. Proc Natl Acad Sci USA 51: 139–146, by kind permission of Donald D. Brown, Carnegie Institution for Science, Washington, D.C.)