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DNA molecules extracted form the papovavirus SV40 (bar = 500 nm). Molecules of SV40 DNA exist in two major forms. (A) When it is isolated from the virions, most of the DNA occurs as a double-stranded, supercoiled, circular molecule (superhelix). (B) If one of the DNA strands is nicked, the superhelix becomes a circle.
(Courtesy Dr. P. Sharp.)
