Molecular DNA-RNA Hybridization as a Means of Chromosome Mapping

Abstract

We have used molecular DNA-RNA hybridization as a direct means of assayign a human gene. This assay provided a logical method of chromosome mapping. Comparison of rRNA hybridization with normal and monogoloid DNA was used to measure ribosomal RNA genes on the 21st chromosome. The DNA-rRNA reaction was chosen because of the relative abundance of rRNA and its characteristic sedimentation in sucrose. Mongoloid tissue was used since the 21st chromosome is known to contain a nucleolar organizer. Th enucleolus is the site of ribosome synthesis in other eucaryotes.The assay of the rRNA genes required purification of the DNA and rRNA. We found that approximately 14 × 10−5 of the human DNA hybridized with rRNA under conditions where the specific complementary sites were saturated. Assuming a DNA content per cell of 9.2 pg enough DNA complementary to rRNA was present in each cell to code for approximately 300 ribosomes.In two different mongoloid patients in whom we assayed for rRNA genes, we found a 14% and a 20% increase in hubridization. These initial studies provide a foundation for increasing the sensitivity of the reaction. We should be able to utilize tissue with specific deletions of the 21st chromosome for fine mapping of the chromosome and exact quantitation of the genes. This type of study also provides a method of mapping other human genes for which we can obtain purified gene products. These include hidden viral genomes, transfer RNA's and purified messenger RNA's such as hemoglobin messenger.