DNA on surfaces

Abstract The development of the ideas of linking number, twist and writhe (Chapter 2) has provided powerful concepts for understanding the geometry of supercoiled DNA. However, we have seen that the actual values of twist and writhe are difficult to determine, except in a few specialized cases (Chapter 2, Section 2.4.1). An important factor that can modulate DNA supercoiling is the wrapping of the DNA on a protein surface, where, in a closed-circular molecule, the writhing of the DNA around the complex and any changes in twist of the DNA helix contribute to the overall supercoiling of the molecule.