  Recent single-molecule pulling experiments have shown how it is possible to
manipulate RNA molecules using optical tweezers force microscopy. We
investigate a minimal model for the experimental setup which includes a RNA
molecule connected to two polymers (handles) and a bead, trapped in the optical
potential, attached to one of the handles. Initially, we focus on small
single-domain RNA molecules which unfold in a cooperative way. The model
qualitatively reproduces the experimental results and allow us to investigate
the influence of the bead and handles on the unfolding reaction. A main
ingredient of our model is to consider the appropriate statistical ensemble and
the corresponding thermodynamic potential describing thermal fluctuations in
the system. We then investigate several questions relevant to extract
thermodynamic information from the experimental data. Next, we study the
kinetics using a dynamical model. Finally, we address the more general problem
of a multidomain RNA molecule with Mg2+-tertiary contacts that unfolds in a
sequential way and propose techniques to analyze the breakage force data in
order to obtain the reliable kinetics parameters that characterize each domain.
