Denaturation of different types of double-stranded DNA molecules. (A) Denaturation of linear double-stranded DNA yields two linear single strands. (B) Denaturation of circular double-stranded DNA with at least one broken phosphodiester bond (nick) in one of the strands (blue) yields a linear single strand (blue) and a circular single strand (green). (C) The strands cannot come apart when double-stranded, covalently closed, circular DNA is denatured.