  The mechanical unfolding of a simple RNA hairpin and of a 236--bases portion
of the Tetrahymena thermophila ribozyme is studied by means of an Ising--like
model. Phase diagrams and free energy landscapes are computed exactly and
suggest a simple two--state behaviour for the hairpin and the presence of
intermediate states for the ribozyme. Nonequilibrium simulations give the
possible unfolding pathways for the ribozyme, and the dominant pathway
corresponds to the experimentally observed one.
