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[Effect of single-stranded breaks on the ultrastructural organization and cytochemistry of the chromatin in tumor cells].

Single-stranded DNA breaks in Zajdela's ascitic hepatoma and transformed hamster fibroblasts were caused by treating alive cells with 1% dimethylsulfoxide for 2 h or 100 micrograms/ml bleomycin for 5 min and tested by alkaline and neutral DNA elutions. Electron microscopy of thin sections revealed decompaction of the loosened approximately 25 nm globules within diffuse chromatin into thin fibrillar mesh while supranucleosomal structure of the compact chromatin remained untouched. The chromatin enhanced its affinity for cationic dyes and contrast agents. It is concluded that the diffuse chromatin possesses torsional stress of DNA superhelicity and its loosened subunits represent a form for its organization. They probably correspond to the functionally active (dynamic) nucleosomes which display destruction under DNA domain relaxation caused by one-strand breaks.