The role of the nucleolus in ribonucleic acid- and protein synthesis. I. Incorporation of cytidine into normal and nucleolar inactivated hela cells

Nucleoli of HeLa cells growing in tissue culture were selectively irradiated with an u.v. microbeam and the subsequent incorporation of [3H]cytidine into the ribonucleic acid of the nucleolus, extra-nucleolar parts of the nucleus, and the cytoplasm, was studied by autoradiography. Comparison with cells containing normal nucleoli revealed that approximately 2 3 of the cytoplasmic incorporation is directly dependent on the nucleolus, but that less than 1 3 of the incorporation into extra-nucleolar parts of the nucleus is dependent on the nucleolus. These results when combined with quantitative kinetic studies of cytidine incorporation strongly support the hypothesis that a major part of the ribonucleic acid of the cytoplasm is synthesized in the nucleolus. © 1961.SCOPUS: ar.j