The differentiation of embryonic skeletal muscle cells is closely coupled with the cessation of normal DNA replication. Once these cells begin to differentiate, they normally never undergo semiconservative replication of DNA during the entire life time of the muscle cell. Cessation of DNA synthesis has been shown to be accompanied by a loss of 80–90% of the replicative DNA polymerase activity of these cells. Despite this loss the studies reported here demonstrate that muscle cells retain the ability to synthesize DNA of a repair type after UV irradiation. These results suggest that the control exercised over semiconservative DNA synthesis during differentiation of these cells does not extend to repair synthesis after UV irradiation.

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