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A small number of cistrons for ribosomal RNA in the germinal nucleus of a eukaryote, Tetrahymena pyriformis.

The percentage of DNA complementary to 25S and 17S rRNA has been determined for both the macro- and micronucleus of the ciliated protozoan, Tetrahymena pyriformis. Saturation levels obtained by DNA.RNA hybridization indicate that approximately 200 copies of the gene for rRNA per haploid genome were present in macronuclei. The saturation level obtained with DNA extracted from isolated micronuclei was only 5-10% of the level obtained with DNA from macronuclei. After correction for contamination of micronuclear DNA by DNA from macronuclei, only a few copies (possibly only 1) of the gene for rRNA are estimated to be present in micronuclei. Micronuclei are germinal nuclei. Macronuclei serve as somatic nuclei during vegetative growth but are destroyed every sexual generation and are reformed from products of meiosis, fertilization, and division of the micronuclei. Thus, the hybridization data suggest that the gene for rRNA must be amplified during macronuclear formation with each sexual generation. These observations also demonstrate that the multiple copies of a repeated gene in a somatic nucleus of a eukaryote can be generated from a small number of copies of that gene in a germinal nucleus.