Abstract
<jats:p>A DNA macromolecule deformation model that includes external and internal deformation components and their coupling is introduced. The external components describe twisting and stretching of the double helix. The internal component is associated with relative displacements of structural elements within base pairs. Coupling of both twist and stretch to the internal component is taken into account. A counterintuitive response is obtained: under tension, the DNA double helix can increase its twist. This anomalous behavior persists up to a critical force, above which the conventional untwisting regime is recovered. The results show that DNA mechanics in the pN force range is strongly affected by coupling between external deformation and internal conformational mobility. The unusual mechanical response is attributed to the influence of the conformational state on deformation through coupling, with the conformational component remaining within a local minimum over the relevant force range.</jats:p>