Varmus made extensive use of gel electrophoresis in his study of the genetics of retroviruses, in this case to examine the integration of proviruses (the DNA form of viral RNA) of Avian Sarcoma Virus into the DNA of host cells. He found that proviruses have sequences at the end of their DNA chains that are repeated many times, and that proviruses are integrated into cellular chromosomes at many different sites. These findings helped explain how ASV transforms normal cells into cancer cells.. The ten lanes shown in this figure were formed when electrically charged (ionic) DNA fragments, which had been dissolved in a solvent and spread on an agarose gel as a support medium, migrate across an electric field and are separated according to the speed of their migration as well as according to changes they undergo during their migration.
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