  The melting behavior of long, heterogeneous DNA chains is examined within the
framework of the nonlinear lattice dynamics based Peyrard-Bishop-Dauxois (PBD)
model. Data for the pBR322 plasmid and the complete T7 phage have been used to
obtain model fits and determine parameter dependence on salt content. Melting
curves predicted for the complete fd phage and the Y1 and Y2 fragments of the
$\phi$X174 phage without any adjustable parameters are in good agreement with
experiment. The calculated probabilities for single base-pair opening are
consistent with values obtained from imino proton exchange experiments.
