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The DNA comet assay (neutral version) showed that irradiation of CHO cells in a dose of 1 Gy (gamma-radiation, (60)Co) is followed by an increase in the degree of DNA fragmentation. These changes were observed immediately after irradiation and on days 7-21. On days 2-4 and 23-28 after irradiation, the degree of DNA fragmentation in the descendants of irradiated cell did not differ from that in control samples. The increase in the degree of DNA fragmentation on days 7-21 probably results from induction of apoptosis. This assumption is confirmed by the study of cell death. The sensitivity of cells to repeated irradiation in a dose of 10 Gy significantly increased on days 9, 11, 16, and 18 after irradiation. However, these cells were resistant to repeated irradiation on days 21-28. Our results confirm the hypothesis that genomic instability is a selective mechanism, which mediates the formation of radioresistant cell clones.

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