FIG 5.

Ratio of active to inactive rDNA in old and young cells. Psoralen cross-linked rDNA was digested with AflIII, and band retardation was assessed after electrophoresis. (A) Recognition sites of AflIII and position of the probe for the experiment shown in panel B. (B) Southern blot analysis to detect the psoralen cross-linked rDNA by mobility retardation in young and old mice. Transcription-active rDNA efficiently intercalates psoralen, which retards migration during gel electrophoresis. Mouse identification (ID) numbers are given (Fig. 3B). (C) Ratio of copy numbers of active to inactive rDNA. Band intensities from the experiment shown in panel B were measured, and the ratio of active to inactive rDNA was calculated. The error bars represent SDs.