ISOPYCNIC CENTRIFUGATION OF CHROMATIN IN RENOGRAFIN SOLUTIONS

Solutions of Renografin (30–60%) can be centrifuged to form density gradients in the range from 1.0 g/cm(3) to 1.4 g/cm(3) or, alternatively, preformed gradients can be made which under appropriate conditions of centrifugation have an indefinite stability. Such solutions have a low viscosity and a relatively low ionic strength. The density of DNA in such solutions is surprisingly low (∼1.14 g/cm(3)). Crude chromatin can be sedimented to an equilibrium position in such gradients, corresponding to a density of 1.2(4) g/cm(3), or slightly lower, depending on the method of preparation. The complex is shown to contain DNA, RNA, protein, and possibly some lipoprotein. Most of the RNA can be removed with RNase without any significant effect on the density of the chromatin.