Gene reorganization during serial divisions of normal human cells

We have followed during serial divisions of human fibroblasts the presence in chromosomal and extrachromosomal DNA, of two genes that are expressed in fibroblasts, actin and interferon, and of one that is not expressed, globin. The intensity of the blot hybridization of the actin and globin probes with chromosomal DNA diminished during serial divisions of diploid fibroblasts. The interferon gene remained constant throughout the human fibroblast life span. Chromosomal DNA sequences were present in extrachromosomal circular DNA which appeared at the end of the fibroblast life span. The results could explain some functional changes that occur in these cell populations when their division potential declines.