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In this paper, changes in a large-scale 3D structure of chromosomes during stem cell differentiation is studied. The polymer coarse-grained model of a human interphase chromosome is introduced which reproduces the experimental Hi-C contact maps in chromosomes 12, 17 for both embryonic stem and differentiated cells with high accuracy.Model based analysis of Hi-C data suggests a mechanism of establishment of preferential longrange chromosomal contacts and compartmentalization replacement during cell stem differentiation. The model provides the conceptual basis for integration of data on the dynamics of chromatin interactions, the 3D structure of chromosomes and gene expression during stem cell differentiation or reprogramming.

To study the acquired radioresistance of tumor cells, a model system of two cell lines, Djungarian hamster fibroblasts (DH-TK-) and their radioresistant progeny, was established. The progeny of irradiated cells were isolated by treating the parental cell monolayer with a single dose of 20 Gy (PIC-20). The genetic and morphological features, clonogenic ability, radiosensitivity, cell growth kinetics, ability to grow in methylcellulose, and tumorigenicity of these cell lines were compared. The plating efficiency of PIC-20 cells exceeded that of DH-TK- cells. The progeny of irradiated cells were more radioresistant than parental cells. The average D0 for PIC-20 cells was 7.4 +/- 0.2 Gy, which is three times higher than that for parental cells (2.5 +/- 0.1 Gy). Progeny cell survival in methylcellulose after irradiation with a dose of 10 Gy was 15 times higher than that of DH-TK- cells. In contrast to parental cells, the progeny of irradiated cells showed fast and effective repopulation after irradiation with doses of 12.5 and 15 Gy. The tumor formation ability of irradiated progeny cells was higher than that of parental cells; after 15 Gy irradiation, PIC-20 cells produced tumors as large as unirradiated progeny of irradiated cells, whereas the tumor development of DH-TK- cells diminished by 70%. High radioresistance of progeny of irradiated cells was reproduced during the long period of cultivation (more than 80 passages). The stability of the radioresistant phenotype of PIC-20 cells allows us to investigate the possible mechanisms of acquired tumor radioresistance.

The origin of dose-response curves for radiation-induced chromosomal instability (CI) is studied using the mechanistic CI model. The model takes into account DNA damage generation and repair in the progeny of irradiated cells, cell passage through mitotic cycle, and intercellular signaling. It is shown that the dose-response curves are closely related to the dynamic curves. The principles underlying this relationship are analyzed.