Th e deformability of double-stranded DNA is critical for its packaging in the cell, for its recognition by other molecules, and for the opening up of base pairs for replication, repair, and transcription. One measure of deformability is how easily DNA can be bent. Th e stiff ness of DNA is characterized by its persistence length. DNA segments that are longer than the persistence length can bend without a signifi cant energy penalty, whereas DNA segments shorter than the persistence length are relatively rigid. Th e persistence length of B-form DNA is approximately 500 Å. Th is corresponds to 14 or 15 helical turns, or roughly 140–150 base pairs