On nucleolar DNA replication in Tetrahymena pyriformis GL-8: An electron microscope autoradiographic study of heat synchronized and exponentially multiplying cells

Abstract

Replication of nucleolar DNA in Tetrahymena was studied, after a 10-min labelling with tritiated thymidine, in synchronized cells («one heat shock per generation») around the synchronous cell division, and in cells from an exponentially multiplying culture. The purpose was to investigate the timing of replication of the ribosomal RNA genes, since there is a controversy in the literature on this point. Replication of the ribosomal RNA genes was determined in electron microscope autoradiograms as the presence of silver grains above the peripherally located nucleoli in the macronucleus; the degree of labelling within each nucleolus cannot be determined since the size of an activated silver bromide crystal corresponds to the diameter of a nucleolus. Our results, expressed as the percentage of cells showing labelling of the nucleolar region, revealed a significant fall during the synchronous cell division, thus indicating that replication of nucleolar DNA does not take place during cell division. Another finding was that cells active in DNA synthesis (S-phase) had a higher percentage of labelled nucleoli than that found in cells at any other stage. No difference in replication of the nucleolar DNA was found in heat synchronized cells and in cells from exponentially multiplying populations, apart from the absence of a G1-stage in the former cells. Our conclusion is that the ribosomal RNA genes are replicated throughout the cell cycle except during division and that cells in the S-phase may have an intensified rate of replication of these genes.