This ribosomal RNA makes 80 per cent of cellular RNA and is synthesized on ribosomal genes, which could be isolated. Isolation of ribosomal genes was considered convenient due to their following three characteristic features:

(i) availability of homogeneous rRNA;

(ii) differences between ribosomal RNA genes and other genes, due to relatively high G + C content in rRNA (rRNA has 45-60% G +c, while the remaining RNA has only 40% G +C) ;

(iii) ribosomal genes are present in multiple copies, their number within a cell sometimes approaching several thousands.

In view of the above, ribosomal RNA genes were isolated for the first time (in 1965) in. an amphibian named Xenopus, by Hugh Wallace and Max L. Birnstiel, working at the University of Edinburgh (U.K.). Following steps were involved in this isolation;

(i) rRNA was isolated from ribosomes of Xenopus and made radioactively labelled, due to its replication in a medium containing tritiated uridine;

(ii) ribosomal DNA was isolated by density gradient centrifugation followed by its denaturation(G+C content of rDNA differs from that of bulk DNA and helps in its separation by centrifugation);

Isolation of ribosomal DNA allowed the first characterization of ribosomal genes, which was followed by the isolation of 5S genes by Donald Brown. For a detailed account of isolation of ribosomal genes in Xenopus as originally done, readers are referred to an article Isolation of Genes in Scientific American (August, 1973).