  We study the propagation of very large amplitude localized excitations in a
model of DNA that takes explicitly into account the helicoidal structure. These
excitations represent the ``transcription bubble'', where the hydrogen bonds
between complementary bases are disrupted, allowing access to the genetic code.
We propose these kind of excitations in alternative to kinks and breathers. The
model has been introduced by Barbi et al. [Phys. Lett. A 253, 358 (1999)], and
up to now it has been used to study on the one hand low amplitude breather
solutions, and on the other hand the DNA melting transition. We extend the
model to include the case of heterogeneous chains, in order to get closer to a
description of real DNA; in fact, the Morse potential representing the
interaction between complementary bases has two possible depths, one for A-T
and one for G-C base pairs. We first compute the equilibrium configurations of
a chain with a degree of uncoiling, and we find that a static bubble is among
them; then we show, by molecular dynamics simulations, that these bubbles, once
generated, can move along the chain. We find that also in the most unfavourable
case, that of a heterogeneous DNA in the presence of thermal noise, the
excitation can travel for well more 1000 base pairs.
