The genes for rRNA are preferentially replicated during oogenesis in amphibia. In Xenopus laevis, for example, there are 2,000 rDNA repeats integrated into the chromosomes of the oocyte. However, there are 2 million DNA repeats distributed among about 1,000 extrachromosomal nucleoli that lie near the periphery of the nucleus of each diplotene oocyte. These amplified genes arose from single copies of the chromosomal rDNA, and during pachynema they replicated extrachromosomally by a rolling circle (q.v.) mechanism. These extrachromosomal nucleoli function to transcribe the rRNAs stored in the growing oocyte. Amplification of rDNA also occurs commonly in insects with panoistic ovaries and in the macronuclei of protozoa, such as Tetrahymena. See Chronology, 1968, Gall, Brown and Dawid; insect ovary types, Miller trees, nucleolus, nucleolus organizer.
Subjects: Genetics and Genomics.