NUCLEOLAR DESOXYRIBONUCLEIC ACID IN TETRAHYMENA PYRIFORMIS CHRONOLOGY OF ITS REPLICATION

Electron microscopic autoradiography has been used to study the synthesis of nucleolar DNA in cultures of Tetrahymena pyriformis in logarithmic phase of growth. Under these conditions, the numerous nucleoli are located in a peripheral layer beneath the nuclear envelope and each is shown to contain a small clump of DNA. Following incorporation of 3H-thymidine during periods ranging from 15 min to 6 h, the macronuclei may be either totally labeled or labeled only over the nucleoli. This observation of a differential label confirms other studies showing that the synthesis of nucleolar DNA occurs during a brief fraction of the macronuclear S period. In Tetrahymena, the chronology of DNA synthesis in the micro- and macronuclei provides a basis for establishing the position of nucleolar DNA synthesis in relation to the rest. Results show that the nucleolar DNA is late-replicating: it continues to be synthesized after the rest of the DNA, at the end of the S period. Thus, the cistrons for ribosomal RNA replicate or continue to replicate after those for the other categories of RNA. © 1969.