Varmus made extensive use of gel electrophoresis in his study of the genetics of retroviruses, namely to determine the quantity and structure of viral DNA prepared from extracts from the nucleus and cytoplasm (the substance surrounding the nucleus) of cells infected with Avian Sarcoma Virus (ASV). The technology enabled Varmus, for example, to show the existence of proviruses of ASV, the double-stranded DNA version of the double-stranded RNA of which the ASV genome is made. By forming a provirus, retroviruses are able to integrate themselves into the DNA of host cells.. The 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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