[Chromatin packing in the nuclei of mammalian cells studied by flow cytometry].

A phenomenon of limited accessibility of mammalian cell nuclear DNA to external agents is described, which is revealed by altered binding of DNA-specific fluorescent dyes after fixation of nuclei with glutaraldehyde. This approach is demonstrated to permit visualizing different degrees of DNA shielding by proteins in cells of diverse differentiation and functional state, as well as measuring an alteration of nature chromatin conformation after different treatment. The treatment of nuclei with single-stranded DNA was found to result in a considerable increase in nuclear DNA accessibility to fluorescent dyes, probably by activation of endogenous nuclear DNA-dependent serine proteases. The data obtained allow to suppose the existence of a specific nuclear DNA accessibility regulating system based on protease action and regulated by the appearance of single-stranded DNA stretches which accompanied such genetic processes as DNA repair, transcription, recombination and replication.