  The formation of polaron and its migration in a DNA chain are studied within
a semiclassical Peyrard-Biship-Holstein polaron model. Comparing the energetics
of the polaron system found from the quantum chemical and semiclassical
calculations, we extract the charge-phonon coupling constant for poly DNA
sequences. The coupling constant is found to be larger for the G-C than for the
A-T pairs. With this coupling constant we study tunneling in the DNA molecule.
The rates and the nature of tunneling have strong dependence on the DNA
sequence. By changing the trap positions in the molecular bridge the tunneling
rate can by varied up to seven orders of magnitude.
