  The effect of heterogeneous sequence composition on the denaturation of
double stranded DNA is investigated. The resulting pair-binding energy
variation is found to have a negligible effect on the critical properties of
the smooth second order melting transition in the simplest (Peyrard-Bishop)
model. However, sequence heterogeneity is dramatically amplified upon adopting
a more realistic treatment of the backbone stiffness. The model yields features
of ``multi-step melting'' similar to those observed in experiments.
