The present invention relates to unnatural base pairs of Ds (a 7-(2 thienyl)-3H-imidazo[4,5-b]pyridine-3-yl group) and a Pa derivative (a 2-nitro-1H-pyrrole-1-yl group bearing a substituent having a π-electron system attached at position 4) that can be replicated with high selectivity/high efficiency, and methods for replicating nucleic acids containing the unnatural base pairs. The present invention also relates to methods for incorporating an unnatural base bearing a functional substituent attached thereto into DNA by a nucleic acid replication reaction. The present invention also relates to methods for replicating and selectively collecting a nucleic acid containing an unnatural base pair from a nucleic acid pool. The present invention also relates to methods for determining a sequence of natural bases in the proximity of an unnatural base in DNA for achieving highly efficient and highly selective replication of a nucleic acid containing the unnatural base.