The structural basis of DNA flexibility

Although the average physico-chemical properties of a long DNA molecule may approximate to those of a thin isotropic homogeneous rod, DNA behaves more locally as an anisotropic heterogeneous rod. This bending anisotropy is sequence dependent and to a first approximation reflects both the geometry and stability of individual base steps. The biological manipulation and packaging of the molecule often depend crucially on local variations in both bending and torsional flexibility. However, whereas the probability of DNA untwisting can be strongly correlated with a high bending flexibility, DNA bending, especially when the molecule is tightly wrapped on a protein surface, may be energetically favoured by a less flexible sequence whose preferred configuration conforms more closely to that of the complementary protein surface. In the latter situation the lower bending flexibility may be more than compensated for on binding by a reduced required deformation energy relative to a fully isotropic DNA molecule.