Long molecules of free DNA in the sheared chromatin preparation

Hydrodynamic shearing of chromatin in the presence of Mg²⁺ ions produces two discrete types of particles: (1) molecules of completely free DNA which comprise 20–23% of the total DNA and (2) histone-covered DNA molecules which contain all five histone fractions. The average length of free DNA molecules depends on the intensity of shearing and can be as high as 1000 base pairs or more. Shearing of chromatin in the absence of Mg²⁺ produces a heterogeneous population of DNP particles; no free DNA is liberated. However, the addition of Mg²⁺ to this preparation results in appearance of free DNA molecules and in a complete restoration of the above bimodal distribution.These findings support a previously proposed asymmetric hairpin model of DNA packing in the chromatin [1–3].