Differential Replication of Ribosomal RNA Genes in Eukaryotes

Abstract

General Occurrence of Differential rDNA Replication
Several lines of evidence have demonstrated that the amount of DNA in the cell nucleus, and hence the nuclear information content of the cell, remains constant in development (Alfert 1954). From this fact, it has been concluded that cellular differentiation takes place by selective expression of the genes rather than by the selective loss or multiplication of specific genes. While DNA constancy has repeatably been demonstrated to be a sound general rule, it has some striking exceptions. The most carefully studied of these exceptions is the variability in the number of copies of the genes for the ribosomal RNAs.

In the oocytes of a great variety of organisms, especially the Amphibia, there is a selective extrachromosomal replication of the rDNA, the DNA sequences that code for 28S and 18S rRNA (Brown and Dawid 1968; Gall 1968, 1969). The rDNA amplification is generally accompanied by an increase in the number of nucleoli in the germinal vesicle. The amount of extrachromosomal rDNA varies from a few times to several thousand times the amount of rDNA in the somatic nucleolus organizer. The subject of ribosomal gene amplification in oocytes is treated more completely by Reeder in this volume.

There have also been several reports of the differential replication of ribosomal RNA genes in cells other than those of the germ line. Koch and Cruceanu (1971) examined DNA from cultured human liver cells which had been treated with a hormone, triiodothyronine. This hormone is known to induce the...