Various charge migration mechanisms in the DNA are studied within the
framework of the Peyrard-Bishop-Holstein model which has been widely used to
address charge dynamics in this macromolecule. To analyze these mechanisms we
consider characteristic size and time scales of the fluctuations of the
electronic and vibrational subsystems. It is shown, in particular, that due to
substantial differences in these timescales polaron formation is unlikely
within a broad range of temperatures. We demonstrate that at low temperatures
electronic transport can be quasi-ballistic. For high temperatures, we propose
an alternative to polaronic charge migration mechanism: the
fluctuation-assisted one, in which the electron dynamics is governed by
relatively slow fluctuations of the vibrational subsystem. We argue also that
the discussed methods and mechanisms can be relevant for other organic
macromolecular systems, such as conjugated polymers and molecular aggregates