The structure of partly decondensed metaphase chromosomes

The technique of freeze-drying was applied to examine the submicroscopic organisation of metaphase chromosomes from Chinese hamster after removal of bivalent cations with EDTA and removal of histone HI with 0,6 M NaCl. Treated chromosomes increased in size, and nucleosomal filaments appeared at the periphery of the chromosomes. Removal of bivalent cations is accompanied with the appearance of regularly organized structures of the "beads-on-a-string" type. The regular organization of the fibers is damaged as soon as histone H1 is removed. After decondensation in a 0.6 M NaCl solution the metaphase chromosomes were treated with staphylococcal nuclease in situ on EM grids nd the residual structures analysed using electron microscopy. Nucleohistone fibers wer visible at the periphery of the chromosomes at the beginning of digestion. After complete elimination of the nucleohistone fibers in the course of digestion the remaining proteinaceous material was represented by aggregates of irregular shape and of varying size. These were either concentrated along the central axis of the chromatids or, at the final step of digestion, scattered evenly over the entire area that had been occupied by the chromosome. Presumably, in the chromosome prior to digestion, the material did not form an integral protein structure similar to a scaffold in dehistonised and spread chromosomes. An alternative interpretation for the fragmentation of protein material in the chromosome considers possible degradation of the protein scaffold in the course of digestion.