Rat liver nuclei, after preliminary isolation in 2.2 molar sucrose solution, were separated into density classes by centrifugation at 95,000 g for 45 to 85 minutes in a sucrose density gradient (density range, 1.28 to 1.33). Nuclei from normal liver separated into three bands with average DNA phosphorus content per nucleus of 0.67, 0.84, and 0.93 picogram for top, middle, and bottom bands, respectively. Nuclei from regenerating liver (26 hours after one-third hepatectomy) yielded three bands and a pellet fraction with average DNA phosphorus content per nucleus of 0.76, 1.02, 1.38, and 1.51 picograms (top to bottom of tube). This method appears capable of yielding nuclei which have increased their DNA content prior to mitosis, and this procedure should be valuable in studies of biochemical changes which occur in nuclei preparing for mitosis.

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