  We discuss the effect of heterogeneity on the chaotic properties of the
Peyrard-Bishop-Dauxois nonlinear model of DNA. Results are presented for the
maximum Lyapunov exponent and the deviation vector distribution. Different
compositions of adenine-thymine (AT) and guanine-cytosine (GC) base pairs are
examined for various energies up to the melting point of the corresponding
sequence. We also consider the effect of the alternation index, which measures
the heterogeneity of the DNA chain through the number of alternations between
different types (AT or GC) of base pairs, on the chaotic behavior of the
system. Biological gene promoter sequences have been also investigated, showing
no distinct behavior of the maximum Lyapunov exponent.
