  The quite recent technological rise in molecular biology allowed single
molecule manipulation experiments, where molecule stretching plays a primary
role. In order to understand the experimental data, it is felt the urge of some
physical and mathematical models to quantitatively express the mechanical
properties of the observed molecules. In this paper we reconsider a simple
phenomenological model which reproduces the behaviour of single and double
stranded DNA under tension. The problem is easily solved via the cavity method
both in the small forces range and in presence of overstretching transition, so
that some properties such as bending stiffness and elasticity of DNA emerge in
a very clear manner. Our theoretical findings are successfully fitted to real
measurements and compared to Monte Carlo simulations, confirming the quality of
the approach.
