  We consider the regime in which the bands of the torsional acoustic (TA) and
the hydrogen-bond-stretch (HBS) modes of the DNA interpenetrate each other.
Within the framework of a model that accommodates the structure of the double
helix, we find the three-wave interaction between the TA- and the HBS-modes,
and show that microwave radiation could bring about torsional vibrations that
could serve as a pump mode for maintaining the HBS-one. Rayleigh's threshold
condition for the parametric resonance provides an estimate for the power
density of the mw-field necessary for generating the HBS-mode.
