Spatial model of the organization of chromatin in the nucleus of a biological cell according to small-angle scattering data

Small-angle neutron and X-ray scattering data for HeLa nuclei with the normal and suppressed transcription activities are reported. Small-angle neutron scattering data demonstrate the presence of a bifractal structure inside a nucleus. The logarithmic fractal structure is observed in the range from the size of the nucleus to several hundreds of nanometer, whereas the volume fractal structure exists at smaller scales down to a nucleosome structure. Small-angle X-ray scattering data show that the presence of the volume fractal structure correlates with the transcription activity of a cell. In view of the successful description of chromatin by the fractal globule model (Hi-C method data), a scenario for the formation of the bifractal structure inside the nucleus has been proposed. A system of transport channels (logarithmic fractal) is located inside close-packed chromatin, whereas active chromatin is localized near transport channels or inside them and forms volume fractal structures due to the transcription activity. Thus, the logarithmic fractal structure ensures the uniform distribution of voids at various scales, which is potentially necessary for the transcription and transport of substances inside the nucleus, whereas the volume fractal structure is due to the transcription activity of the cell.