How does rnai result in suppression of expression of a particular gene?

Certain small RNA molecules inhibit the expression of genes or their RNA with the complementary sequences by base pairing with them to prevent protein synthesis and/or targeting the mRNA with complementary sequences for cleavage.

Explanation:

RNA has the ability to regulate gene expression by a process known as RNA interference (RNAi). In RNAi, small RNA molecules such as small interfering RNAs, microRNAs, etc. inhibit the expression of genes or their RNA transcripts.

Small interfering RNAs (siRNAs) are double-stranded molecules about 23 nucleotides in length. They inhibit the expression of genes by selectively cleaving mRNA molecules with base sequences complementary to the siRNA, thereby inactivating them. MicroRNAs (miRNAs) are single-stranded RNA molecules about 21 to 22 nucleotides long that inhibit the translation of mRNAs by base pairing with them.