  We report on our theoretical investigations of the electronic states in a DNA
molecule. We have used a two-leg charge ladder model where electron-electron
interactions and the electron spin have been taken into account. The energy
spectra for G-C and A-T base pairs obtained by numerically diagonalizing the
Hamiltonian reveal a gap structure and the interaction is found to enhance the
energy gaps. We also present the charge distribution in the ground state and
low-lying excited states for the A-T and G-C base pairs.
