Sedimentation of DNA supercoils and bacterial nucleoids

The sedimentation of DNA supercoils and bacterial nucleoids is discussed in terms of an asymptotic expression for the size of branched supercoils exhibiting an excluded-volume effect between superhelical segments. A Kirkwood–Riseman approximation is adopted for the sedimentation coefficient. The theory predicts the sedimentation of DNA supercoils fairly well despite their relatively small size in current simulations and experiments. We introduce a crosslinked supercoil model for bacterial nucleoids that are known to contain a variety of adsorbed proteins. Sedimentation experiments of the 1970s are discussed.