  We develop a new approach to derive single state tight binding (SSTB) model
for electron transport in the vicinity of valence-conduction bands of
poly(G)-poly(C) and poly(A)-poly(T) DNA. The SSTB parameters are derived from
{\it first principles} and are used to model charge transport through finite
length DNA. We investigate the rigor of reducing the full DNA Hamiltonian to
SSTB model to represent charge transport in the vicinity of valence-conduction
band. While the transmission coefficient spectrum is preserved, its position
shifts in energy. Thymine is poorly represented and its peak height is
substantially reduced. This is attributed to the abstraction of the HOMO-LUMO
coupling to other eigen-states in the nearest neighbor DNA bases, and can be
corrected within $2^{nd}$ order time independent perturbation theory.
Inter-strand charge transport has also been analyzed and it is found that
hopping to the nearest neighbor in the complementary strand is the most
important process except in the valence band of poly(G)-poly(C), where hopping
to the second nearest neighbor between $3'-$ends is the most dominant process.
As a result, transport between $3'-$ends and $5'-$ends in the vicinity of
valence band of poly{G}-poly{C} is asymmetric.
