  The zero-temperature transmission rate spectrum of a double-chain
tight-binding model for real DNA is calculated. It is shown that a band of
extended-like states exists only for finite chain length with strong
inter-chain coupling. While the whole spectrum tends to zero in thermodynamic
limit, regardless of the strength of inter-chain coupling. It is also shown
that a more faithful model for real DNA with periodic sugar-phosphate chains in
backbone structures can be mapped into the above simple double-chain
tight-binding model. Combined with above results, the transmission rate of real
DNA with long random sequence of nucleotides is expected to be poor.
