Abstract
Superhelical closed circular DNA is not only of widespread occurrence but has been extensively utilized in studying the conformation of DNA. Superhelix densities of these DNAs have routinely been calculated using the assumption that ethidium unwinds the primary helix by 12 °. This assumption has been questioned by Paoletti & LePecq (1971 a) who proposed that ethidium winds the duplex by 13 °. In this paper we show by a combination of chemical and enzymatic means that ethidium unequivocally unwinds the duplex, but that the unwinding angle and consequently superhelix densities are probably between two and three times the previously assumed values.
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