  The electrodynamic response of DNA in the millimeter wave range is
investigated. By performing measurements under a wide range of humidity
conditions and comparing the response of single strand DNA and double strand
DNA, we show that the appreciable AC conductivity of DNA is not due to photon
activated hopping between localized states, but instead due to dissipation from
dipole motion in the surrounding water helix. Such a result, where the
conductivity is due to the constrained motion of overdamped dipoles, reconciles
the vanishing DC conductivity of DNA with the considerable AC response.
