Quantitation of Human Ribosomal DNA: Hybridization of Human DNA with Ribosomal RNA for Quantitation and Fractionation

Abstract

Extract: Quantitation of human genes for ribosomal RNA (rRNA) was possible by molecular DNA/RNA hybridization on nitrocellulose discs. Labeled ribosomal RNA hybridizes with an average of 14 × 10^-5 of the total human DNA. If we assume that the diploid cell contains 5.5 × 10¹² daltons of DNA, the ribosomal RNA hybridizes with 7.7 × 10⁸ daltons of ribosomal DNA (rDNA), which is equal to about 320 genes. Hybridization of rRNA with DNA of three different diploid sources shows a rRNA/DNA ratio which varies from 12–15.4 × 10^-5. The rRNA/DNA ratio of DNA derived from trisomy 21 cells is not significantly different from the normal cells. The genes for rRNA were found to resist melting at temperatures 10° or more above the T_m. This property permits enrichment of these genes by thermal fractionation.

Speculation: Ribosomal RNA genes are probably present on several human chromosomes. The ability to enrich these genes in relation to the whole human DNA indicates that they are clustered.

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