Quantitative Changes in Unscheduled DNA Synthesis in Rat Muscle Cells after Differentiation |
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Authors: | GEORGE M. HAHN DONALD KING SHUN-JEN YANG |
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Affiliation: | 1.Department of Radiology,Stanford University School of Medicine,Stanford |
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Abstract: | THE incorporation of tritiated thymidine (3H-thymidine) into cells not engaged in normal DNA replication has been called unscheduled DNA synthesis1. The phenomenon has been observed after X-irradiation1, ultraviolet irradiation2 and after exposure to the monofunctional alkylating agent methyl methane sulphonate3 (MMS) and other carcinogens4. In all published reports the cells showing unscheduled DNA synthesis had retained their proliferative capacity (and hence at least their potential ability to synthesize DNA). We have investigated whether differentiated cells—that is, cells which presumably will never have to initiate normal DNA synthesis—are still capable of unscheduled DNA synthesis. We used multinucleated rat muscle cells in vitro. Myotubes have been found to form by fusion of separate, mononucleated cells5,6, the nuclei of which no longer synthesize DNA. YalTe and Gershon7 have shown that such cells can reinitiate DNA synthesis after viral infection. They found it necessary, however, for fusion to continue during viral infection; in the absence of further fusion no new DNA synthesis was observed. The trigger for DNA synthesis after viral infection must therefore have come from cells which had been transformed before differentiation and fusion. This left open the question of whether differentiated cells could initiate DNA synthesis in the absence of trigger from transformed cells. |
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