DNA topoisomerases: enzymes that control DNA conformation

It is common knowledge today that DNA molecules are not simple, rigid double helices, but molecules which have considerable conformational flexibility. This flexibility allows coiling of DNA into complex higher order structures. In most living cells or organelles, including many virus particles, these higher order structures occur on the basis of negative supercoiling or, as it is also called, negative superhelicity. Negative supercoiling is the conformational consequence of a winding deficit in DNA, i.e., the DNA under conditions of defined constraints forms in vivo or in vitro fewer helical rotations of one strand about the other than it assumes if left unconstrained