[Electron-microscopic study of the morphology of scaffold-like structures in chromosomes, formed by formamide]

Isolated human metaphase chromosomes treated with formamide and prepared for electron microscopy by protein monolayer technique have an appearance of loop-shaped chromatin fibers coming off the central scaffold-like structures, such chromosomes having approximately the same histone content as those before treatment. The morphology of scaffold-like structures at different formamide concentrations is described. It is shown that during formamide treatment the protein-protein and/or protein-DNA interactions are weakened because of disruption of hydrogen bonds. However, the intermolecular interactions in the chromosomes allow them to preserve their shape and size. The changes in the structure of formamide-treated chromosomes become readily visible after spreading on the hypophase surface. It is shown that after removal of formamide from the incubation solution by dialysis, chromosomes condense drastically. The data obtained are in good agreement with the loop model of chromosome organization. However they evidence also that the scaffold in the form of a rigid characteristic chromosome structure arises as a result of redistribution and/or aggregation of chromosomal proteins during chromosome preparation.