[Ions and chromatin structure--morphologic and x-ray microanalytic findings]

It is quite common that the different procedures for isolation of nuclei make use of the purifying effect of sedimentation through concentrated sucrose. For this reason we investigated by light and electron microscopy the changes of chromatin structure in dependence on the amount and sort of ions present in the solution used for isolation and resuspension of nuclei. A decreasing concentration of monovalent cations (Na+, K+) leads to a swelling of the whole nuclei in consequence of the decondensation of the chromatin fibres. This change of nuclear structure can be reversed by small additions (0.5-1.5 mM) of divalent cations (Mg2+, Ca2+, Cu2+). Inasmuch as each change of the electrostatic balance within the nucleoplasm causes morphological alterations also the pH of the solution influences the chromatin structure. Data of X-ray-microanalysis have established the importance of divalent cations for the maintenance of a certain nuclear structure under conditions of low ionic strength.