[Maintenance of torsional tension in DNA of transactivated genes during the cell cycle].

Changes in DNA topology during the cell cycle of murine fibroblasts in vitro were studied by microfluorometric method. At interphase stage, 21% of DNA were found to be torsionally stressed and hypersensitive to the relaxing activity of DNAase I. This DNA is presumably transcriptionally active. At metaphase stage, about 75% of DNA are under torsional stress with low sensitivity to DNAase I. After cytokinesis, one half of the stressed DNA relaxes, the other half of DNA remains under the stress. Later, one hour after cytokinesis, the stressed DNA attains hypersensitivity to DNAase I and remains hypersensitive during interphase. Taking into account that the cells in vitro steady maintain some domains of torsionally stressed DNA during the whole cell cycle, we suppose that these DNA domains contain genes, activated to transcription after mitosis. This may mean that the torsional stress preserved after mitosis is necessary for activation of the genes, which were active at the previous interphase stage.