Almost two decades ago, measurements of force versus extension on isolated double-stranded DNA molecules revealed a force plateau. This unusual stretching phenomenon in DNA suggests that the long molecules may be extended from the usual B form into a new conformation. Different models have been proposed to describe the nature of DNA in its stretched form, S-DNA. Using atomic force microscopy combined with a molecular combing method, we identified the structure of λ-phage DNA for different stretching values. We provide strong evidence for the existence of a first-order transition between B form and S form. Beyond a certain extension of the natural length, DNA molecules adopt a new double-helix conformation characterized by a diameter of 1.2 nm and a helical pitch of 18 nm.

• Received 6 October 2010

DOI:https://doi.org/10.1103/PhysRevE.83.031903