Disclosed are: an artificial base pair Ds-(Pa derivative) [wherein Ds represents a 5-amino-7-(2-thienyl)-3H-imidazo[4,5-b]pyridin-3-yl group and the Pa derivative is a 2-nitro-1H-pyrrol-1-yl group having a substituent bearing a π-electron system bound to position-4 thereof], which can be replicated with high selectivity and high efficiency a method for amplifying a nucleic acid containing the artificial base pair a method for introducing an artificial base having a functional substituent bound thereto into DNA through a nucleic acid amplification reaction a method for replicating a nucleic acid containing an artificial base pair from a nucleic acid pool and collecting the replicated nucleic acid selectively and a method for determining a naturally occurring nucleotide sequence located adjacent to an artificial base in DNA for the purpose of achieving the highly efficient and highly selective replication of a nucleic acid containing an artificial base.
