Circular DNA is a type of double-stranded DNA in which the 5' and 3' ends of each strand are joined by phosphodiester bonds (figure 25.12a). (...) The circular DNA shown in figure 25.12b has been unwound by four complete turns of the helix. The strain introduced by this unwinding is spread uniformly over the entire molecule by the introduction of four superhelical twists (figure 25.12c).

Plasmids