The Mechanism of Gene Amplification in Xenopus Laevis Oöcytes

Abstract

In somatic cells the DNA coding for 28S and 18S ribosomal RNA (rRNA) is confined to the chromosomal nucleolus organizer (Birnstiel et al., 1966). However, in oocytes from a wide variety of animals, this nucleolar DNA is selectively amplified and accumulated in the nucleus free of the chromosomes (Gall, 1969). In Xenopus laevis, the best studied example, almost three-quarters of the nuclear DNA in a diplotene oocyte codes exclusively for ribosomal RNA (Brown and Dawid, 1968). The extra ribosomal DNA (rDNA) is synthesized during a three-week period in early meiotic prophase (Gall, 1968; Macgregor, 1968; Bird and Birnstiel, 1971; Watson-Coggins and Gall, 1971) and during diplotene it forms hundreds of extrachromosomal nucleoli, which actively engage in rRNA synthesis (Miller and Beatty, 1969). This paper will summarize our recent studies of the amplification mechanism by autoradiography of isolated rDNA using both light and electron microscopy.