[Maintenance of torsional tension in DNA of transactivated genes during the cell cycle]. / Sokhranenie v kletochnom tsikle torsionnogo napriazheniia v DNK transkriptsionno-aktivnykh genov.

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.

[Torsional tension in metaphase chromosome DNA]. / Torsionnoe napriazhenie v DNK metafaznykh khromosom.

The torsional tension in DNA of isolated metaphase chromosomes from murine fibroblasts was measured by the microfluorescent method. The method is based on the ability of a fluorescent dye ethidium bromide to compensate for the negative torsional tension in topologically closed DNA by intercalation between DNA base pairs. The value of the relative twist difference delta Tw/Tw = -0.1 was found in a bulk (about 3/4) of unconstrained chromosomal DNA. In interphase nuclei, the torsionally stressed DNA comprises about 15%, with value of delta Tw/Tw = -0.075. We suppose that the tension in chromosomal DNA was created in the prophase stage of mitosis by condensines, the drivers of chromosomal condensation.