On the Duplication of Ribosomal RNA Cistrons in Chinese Hamster Cells

Because of the redundancy of ribosomal RNA cistrons and in analogy with a few known special cases, one may postulate that a few “master genes” for rRNA direct the synthesis of multiple copies of themselves per cell cycle. We have shown that in Chinese hamster cells rRNA cistrons duplicate like the bulk of DNA, once in a cell cycle. The time during the S‐phase at which rRNA genes duplicate has also been determined by means of RNA‐DNA hybridization studies. The time of duplication of rRNA genes occurs between t = 1.5 and t = 3.0 hours from the beginning of the S‐phase.