FIGURE 1.

Sketch of the system considered. (a) In a typical experimental setup, the two ends of an RNA molecule are attached to beads, the position of which is controlled, e.g., with optical tweezers. (b) In a theoretical model, the potential of the optical trap can be approximated by a linear spring (with spring constant λ). This spring is connected in series with a highly nonlinear elastic element (the RNA molecule, usually with an additional linker). The total extension, R_t, is externally controlled, whereas the individual extensions, R_s and R, undergo thermal fluctuations.