Quantitative estimation of nucleolar cistrons (rDNA) in proliferating normal and tumor cells

The saturating hybridization levels of DNA with rRNA (hereafter abbreviated as rDNA/DNA) in adult liver, bone marrow, and fetuses in Long-Evans rats were 0.042, 0.036, and 0.029 approximately 0.033%, respectively, showed a lower level in rapidly growing tissues. We consider that the lower rDNA/DNA in rapidly growing tissues might be due to the replication of rDNA in the late S phase, or in other words, to the depression of rDNA/DNA in each S phase. Based on these findings, the relative levels of rDNA per G1 geome were estimated in cells of these normal tissues and 4 diploid tumors by multiplying rDNA/DNA and DNA/genome. By this procedure, rDNA/genome was shown to be constant in normal G1 cells, while a 31 approximately 48% increase was found in the tumor cell genome. Direct measurement of rDNA in synchronized #2 trisomy leukemia cells confirmed the replication of rDNA in the late S phase, with a 21.4% increase of rDNA per G1 genome.