Abstract
Ehrlich ascites tumour cells were treated with Trioxsalen and long wave ultraviolet light to crosslink DNA in vivo. Despite the crosslinking the cells retained to some extent their ability to incorporate radioactive thymidine. Part of this incorporation was due to repair DNA synthesis and part to semiconservative DNA synthesis. DNA synthesized as a result of repair represented short stretches covalently linked to the high molecular weight DNA. The semiconservative DNA synthesis that had initiated between and had terminated in the vicinity of the crosslinks resulted in the formation of DNA fragments that contain the initiation site and were not ligated to the high molecular weight DNA. They were released by denaturation or by S 1 nuclease digestion in single strand or double strand form, respectively, and were separated from bulk DNA by electrophoresis or sucrose density gradient centrifugation. The reassociation analysis showed that these DNA fragments contained sequences repeated about 10 4 times per genome, which probably represented the region of the Ehrlich ascites tumour origin of replication.
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