Constancy of ribosomal RNA genes during aging of mouse heart cells and during serial passage of WI-38 cells.

The DNA content and ribosomal RNA gene copy number in heart of the inbred mouse strain C57BL/6 were determined at different ages. The DNA content of mouse heart remained constant, at about 150 micrograms DNA per heart, from 1 to 30 mth of age. The number of rRNA genes, as estimated by 28S rRNA . DNA hybridization, was not found to change significantly as a function of age. Likewise, the extent of rRNA hybridization to DNA from cultured human WI-38 cells at early and late passage levels was the same. These data support the notion that genomic rDNA sequences are not lost during in vivo and in vitro aging. However, the rDNA sequences are quite large and numerous small deletions or base pair substitutions would not have been detected in these studies.