Orientation of the nucleosome within the higher order structure of chromatin

We have used electric dichroism to investigate chromatin fragments isolated from chicken erythrocytes. Both within the extended "10 nm filament" present at low salt and the condensed "30 nm solenoid" induced by the addition of divalent cations, the data give a quantitative description of the DNA conformation in the higher order structure of chromatin. In the 10 nm filament both the spacer DNA and the flat faces of the core particle discs must be oriented within 20 degrees of the fiber axis. Within the Mg++-induced 30 nm solenoid the flat faces of the core particles must also be oriented close to parallel to the solenoid axis, the estimated angle depending upon the disposition of the spacer DNA. We suggest a model for the 30 nm solenoid.