  In this work we propose a model for DNA double helix within the tight-binding
framework that incorporates the helicity of the molecules. We have studied
localization properties of three DNAsequences,the periodic poly(dG)-poly(dC)
and poly(dA)-poly(dT) sequences and the random ATGCsequence, all of which are
coupled to backbone withrandom site energies representing the
environmentalfluctuations. We observe that due to helicity of DNA, electron
transport is greatly enhancedand there exists almost a disorder-strength
independent critical value of the hopping integral, thataccounts for helicity
of DNA, for which the electronic states become maximally extended. We havealso
investigated the effect of backbone energetics on the transmission and I-V
characteristics of DNA.
