Bridged artificial nucleoside and nucleotide

A compound represented by the following formula I or formula II or a salt thereof:

• wherein,
• Base represents a purin-9-yl group or a 2-oxo-1,2-dihydropyrimidin-1-yl group that may have any one or more substituents selected from group α, wherein the group α consists of a hydroxyl group, a hydroxyl group protected with a protecting group for nucleic acid synthesis, linear alkyl groups having 1 to 6 carbon atoms, linear alkoxy groups having 1 to 6 carbon atoms, a mercapto group, a mercapto group protected with a protecting group for nucleic acid synthesis, linear alkylthio groups having 1 to 6 carbon atoms, an amino group, linear alkylamino groups having 1 to 6 carbon atoms, an amino group protected with a protecting group for nucleic acid synthesis and halogen atoms;
• R1 represents a hydrogen atom, an alkyl group having 1 to 7 carbon atoms that may be branched or form a cyclic group, an alkenyl group having 2 to 7 carbon atoms that may be branched or form a cyclic group, an aryl group having 3 to 12 carbon atoms that may have any one or more substituents selected from the group α and may contain heteroatoms, an aralkyl group with an aryl moiety having 3 to 12 carbon atoms that may have any one or more substituents selected from the group α and may contain heteroatoms, or a protecting group for an amino group on nucleic acid synthesis;
• R2 and R3 represents, each independently, a hydrogen atom, a protecting group for a hydroxyl group on nucleic acid synthesis, an alkyl group having 1 to 7 carbon atoms that may be branched or form a cyclic group, an alkenyl group having 2 to 7 carbon atoms that may be branched or form a cyclic group, an aryl group having 3 to 12 carbon atoms that may have any one or more substituents selected from the group α and may contain heteroatoms, an aralkyl group with an aryl moiety having 3 to 12 carbon atoms that may have any one or more substituents selected from the group α and may contain heteroatoms, an acyl group that may have any one or more substituents selected from the group α, a silyl group that may have any one or more substituents selected from the group α, a phosphate group that may have any one or more substituents selected from the group α, a phosphate group protected with a protecting group for nucleic acid synthesis, —P(R4)R5 (wherein R4 and R5 represent, each independently, a hydroxyl group, a hydroxyl group protected with a protecting group for nucleic acid synthesis, a mercapto group, a mercapto group protected with a protecting group for nucleic acid synthesis, an amino group, an alkoxy group having 1 to 5 carbon atoms, an alkylthio group having 1 to 5 carbon atoms, an cyanoalkoxy group having 1 to 6 carbon atoms or an amino group substituted with an alkyl group having 1 to 6 carbon atoms.);
• X represents an oxygen atom, a sulfur atom, an amino group or a methylene group;
• m is an integer from 0 to 2;
• and
• n is an integer from 0 to 1.