A combined density functional theory and Green's function procedure is used to calculate the electrical characteristics of Watson-Crick and non-Watson-Crick nucleotides. Furthermore, We determine the molecular orbitals that participate in the electron-transfer process and the current-voltage characteristics through the base pairs. Sliding or passing the double strand through metallic electrodes or through a nanopore. We found that each base pair has its own electrical signature or current-voltage characteristic, which has a strong possibility for detecting the presence and sequence of DNA, including the detection of base-pair mismatches.