  We describe quantitatively a RNA molecule under the influence of an external
force exerted at its two ends as in a typical single-molecule experiment. Our
calculation incorporates the interactions between nucleotides by using the
experimentally-determined free energy rules for RNA secondary structure and
models the polymeric properties of the exterior single-stranded regions
explicitly as elastic freely-jointed chains. We find that in spite of
complicated secondary structures, force-extension curves are typically smooth
in quasi-equilibrium. We identify and characterize two
sequence/structure-dependent mechanisms that, in addition to the
sequence-independent entropic elasticity of the exterior single-stranded
regions, are responsible for the smoothness. These involve compensation between
different structural elements on which the external force acts simultaneously,
and contribution of suboptimal structures, respectively. We estimate how many
features a force-extension curve recorded in non-equilibrium, where the pulling
proceeds faster than rearrangements in the secondary structure of the molecule,
could show in principle. Our software is available to the public through a
`RNA-pulling server'.
