A double coil chromatin sub-unit model.

A model is proposed for the structure of DNA in chromatin sub-units. Each sub-unit is proposed to contain two turns of an inner coil, with a pitch of about 40 A and an external diameter of 70 A. Around the inner coil is wound, in opposite handedness, a slightly larger amount of DNA at a diameter of about 150 A. The total contour length consistent with the electron micrographs and X-ray scattering is 600-700 A, or about 200 base pairs. It is suggested that the inner coil is protein rich and contains all of the histones except H1, which is associated with the outer coil. The double-coil model is consistent with previous biochemical and biophysical studies of chromatin. The existence of 200 and 100 base pair digestion fragments and a 6 to 1 DNA compaction are readily explained. This model is based upon the electron microscopic observation of replicas of frozen chromatin and X-ray and neutron scattering. Structural details of 25 A are preserved and visualized by the freeze electron microscopy techniques employed.