Patent ID: 8895712

Claim:
A double-stranded nucleic acid comprising a first nucleic acid strand and a second nucleic acid strand, wherein the first nucleic acid strand contains a nucleotide or nucleotides each having an unnatural base represented by Formula I: wherein, A 1 represents N or CH; A 2 represents CH; R 1 represents hydrogen or an amino group; and R 2 represents a substituent selected from the group consisting of a 2-thienyl group, a 2-thiazolyl group, a 2-imidazolyl group, a 2,2′-bithien-5-yl group, a 2-(2-thiazolyl)thien-5-yl group, a 5-(2-thienyl)thiazol-2-yl group, and a 2,2′,5′,2″-terthien-5-yl group or a derivative thereof as the base, and the second nucleic acid strand may or may not contain a nucleotide or nucleotides each having an unnatural base represented by Formula I or a derivative thereof as the base; and the unnatural base represented by Formula I or the derivative thereof in the first nucleic acid strand forms a base pair with any natural base or the unnatural base represented by Formula I or the derivative thereof in the second nucleic acid strand; wherein the unnatural base derivative thereof is represented by Formula I in which the R 2 is further modified by addition of C 1 -C 3 alkyl group, a C 1 -C 3 alkenyl group, or a C 1 -C 3 alkynyl group; and wherein the unnatural base derivative thereof is represented by Formula I further comprising a protecting group selected from a group consisting of: a phenoxyacetyl group, an isobutyryl group, and a dimethyl formamidyl group.