  We present the results of our numerical analysis of a "composite" model of
DNA which generalizes a well-known elementary torsional model of Yakushevich by
allowing bases to move independently from the backbone. The model shares with
the Yakushevich model many features and results but it represents an
improvement from both the conceptual and the phenomenological point of view. It
provides a more realistic description of DNA and possibly a justification for
the use of models which consider the DNA chain as uniform. It shows that the
existence of solitons is a generic feature of the underlying nonlinear dynamics
and is to a large extent independent of the detailed modelling of DNA. As
opposite to the Yakushevich model, where it is needed to use an unphysical
value for the torsion in order to induce the correct velocity of sound, the
model we consider supports solitonic solutions, qualitatively and
quantitatively very similar to the Yakushevich solitons, in a fully realistic
range of all the physical parameters characterizing the DNA.
