A DNA polynucleotide is described herein having a modified sequence of a target gene, wherein any one type of nucleotide in the target gene sequence has been chemically modified to another type of nucleotide; and a complementary sequence of the unmodified target gene; wherein either one of the modified sequence or the complementary sequence is in a reverse orientation to the other sequence; and wherein the RNA sequence transcribed from the DNA polynucleotide forms a duplex between the modified sequence and the complementary sequence so that a long double stranded RNA (IdsRNA) duplex forms between the modified and complementary sequences with base pair mismatches where the nucleotides have been modified, the IdsRNA duplex being capable of inhibiting expression of the target gene. RNA polynucleotides and IdsRNA duplexes transcribed by the DNA polynucleotide are also described, as is a method for producing the IdsRNA duplexes. These IdsRNA duplexes can be used in gene silencing.