Analysis of the penetrable space within the nucleus.

Radioactive glycogen molecules have been used as passive probes to compare cavity systems within nuclei and isolated chromatin. Isolated chromatin was found to possess a narrow range of microspaces with mean effective diameters between 4.0 and 4.5 nm (40 and 45 A) depending on shape assumptions. Intact nuclei contained a far larger class of free spaces with average diameters in the order of 11.0-15.0 nm. This clearly shows that DNase-I (diam. 4.1 nm) can penetrate and occupy a large proportion of nuclear space even though this enzyme does not readily attack the undisturbed nuclear structure. A structure which simulated the pattern of penetrability and incorporated other known properties of chromatin was used to explain this DNase-I resistance of intact nuclei in terms of an ordered, compact, local structure interspersed by much larger spaces. A system for this local packing is suggested and the functional implications of this type of organization considered.