Figure 1.

Sketch of the setup for the single DNA molecule unzipping experiment. The DNA molecule is attached to the surface (left) and a bead (right) held in an optical trap at distance L from the surface through rigid linkers of total length ${\ell }_{\text{ds}}$. When the unzipping force is equal to its average value $f_{\text{av}}$, the single strands corresponding to the n unzipped basepairs of the molecule have extension $2n{\ell }_{\text{ss}}$, and the bead is displaced by ${\ell }_{\text{av}}$ from the center of the trap. When the force deviates from $f_{\text{av}}$ the extension of the ssDNA strands and the displacement vary according to their stiffnesses, $K_{\text{ss}}\left(n\right)$ and $K_{\text{trap}}$.