DNA replication in macronuclei of Tetrahymena pyriformis GL

• Published in: Experimental cell research Vol. 93; no. 2; pp. 253 - 260
• Main Authors: Vorob'ev, V.I., Borchsenius, S.N., Belozerskaya, N.A., Merkulova, N.A., Irlina, I.S.
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 01.07.1975
• Subjects: Animals; Cell Division; Cell Nucleus - metabolism; Cell Nucleus - ultrastructure; DNA - analysis; DNA Replication; Genes; Models, Biological; Tetrahymena pyriformis - growth & development; Tetrahymena pyriformis - metabolism; Tetrahymena pyriformis - ultrastructure
• ISSN: 0014-4827; 1090-2422
• DOI: 10.1016/0014-4827(75)90448-6

DNA replication in macronuclei of Tetrahymena pyriformis GL has been studied to discriminate between hypotheses developed for the interpretation of intraclonal differentiation in ciliated protozoa (the diploid subnuclear, and the ‘master’-‘slave’ hypotheses). Tetrahymena cells were grown in a heavy 15N- 3H medium and then transferred to a light 14N- 14C medium. DNA was isolated after various periods following this transfer and studied in equilibrium CsCl density gradient centrifugation. Time-related changes in the DNA buoyant density pattern were investigated. The data obtained are interpreted to mean that all DNA in macronuclei of asynchronously growing Tetrahymena at exponential phase replicates semiconservatively once in a cell cycle. These data are in good agreement with the findings of Andersen & Zeuthen obtained on synchronous Tetrahymena cultures in the presence of BUdR. These results are not consistent with the ‘master’-‘slave’ hypothesis. The diploid subnuclear hypothesis is not in accord with other experimental evidence. An alternative hypothesis has been proposed concerning the nature of the macronuclear units and the process of determination. The two main points of this hypothesis are: ( a) macronuclear units are diploid genome fragments (‘nucleosomes’); ( b) determination is a process of haploidization by ‘allelic splitting’ at a definite macronuclear fission. Consistency with experimental data is discussed and some predictions of the hypothesis are given.