Rosette-like structures from nuclei with condensed (chromomeric) chromatin but not from nuclei with diffuse (nucleomeric or nucleosomic) chromatin.

The structure of chromatin of rat hepatocyte nuclei has been studied. At low ionic strength (20-50) chromatin in isolated nuclei, depending on the concentration of MgCl2 in the solution (0-2 and 4-5 mM), may be present in two states, respectively, diffuse and condensed. The major structural component of the nuclei with condensed chromatin is globular structures 100 nm in diameter, i.e. chromomers. By treating chromomer-containing nuclei with heparin and dextransulfate (polyanions/DNA equal 1), one can isolate rosette-like structures having an electron-dense core and numerous loops (the number loops in the rosette, 15-30, total length of all the loops, 15-20 micrometers, core diameter, 30-60 nm). The action of endogeneous nuclease on the nuclei and DNase I (but not RNase) on the rosette results in the break-down of the loops. Pronase or higher concentrations of polyanions (polyanions/DNA equal 4) induces partial or total decondensation of the rosette core and unfolding of the loops into a continuous linear structure. Rosette structures are not isolated from nuclei with diffuse chromatin. Rosette structures are discussed in terms of the known levels of the organization of chromatin.