I N recent years substantial information has been gained concerning the size and topology of DNA (deoxyribonucleic acid) molecules of viruses (for reviews see VINOGRAD and LEBOWITZ 1966; COHEN 1967; THOMAS and MACHATTIE 1967) , bacterial plasmic and sex factor (ROTH and HELINSKI 1967; HICKSON, ROTH and HELINSKI 1967), Mycciplasma hominis (BODE and MOROWITZ 1967), bacteria (CAIRNS 1963) and mito...