Scanning Microscopy


Ion concentrations in isolated lymphocyte nuclei subjected to KCl or MgCl2 media of varying ionic strength were measured by X-ray microanalysis. Values were corrected for the contribution of free ions by estimating the volume fraction of the water space morphometrically. The amount of bound cations and Cl was constant and independent of the widely varying free ion concentration.

It is concluded that the mechanism of binding is counterion condensation but with limited cooperativity. In contrast to classical counterion condensation theory, the binding of ions occurs at oppositely charged clusters at the surface of the chromatin. Therefore, both cations and anions are bound and binding cannot be completely delocalized.

The bound ions stabilize the basic chromatin fibre but are not involved in the regulation of the transition between the condensed and decondensed state.

Using earlier data, we estimated the concentration of free cations in rat liver nuclei under in-vivo conditions to be in the order of about 80 mM.

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