We calculate the transverse current through double-stranded DNA nucleotides using ab initio techniques in order to establish a protocol to recognize the type and sequence of double-stranded DNA nucleotides. The distinct current-voltage features between nucleotides are used as signatures for their characterization and sequencing. Extended bulk gold electrodes as well as extensions of the DNA backbones are tested as contacts for the electron transport, yielding currents 2 orders of magnitude larger for the former. The addition of Na or H positive counterions improves the signal levels, thus leading to a better discrimination, especially when sodium cations are added.