Interference RNA, known as RNAi, is a class of molecules that naturally occurs to interfere with protein expression, resulting in gene silencing. This natural mechanism is usually triggered by the presence of a foreign DNA, a micro RNA (miRNA) or a double stranded RNA (dsRNA) of viral origin, suggesting an important role in pathogen resistance. The RNAi can cause a process called Post-Transcriptional Gene Silencing (PTGS), where the gene already started to be expressed, but is stopped in the way to encode a protein.

In the genome, most genes that encode proteins are transcribed by RNA polymerase II first into messenger RNA (mRNA) that is exported to the nucleus of the cell to the cytoplasm, where rybosomes catalyse translation of the mRNA into polypeptide chains that fold into proteins. This is where some forms of RNA can have silencing effects. There are several types of regulatory small RNA, such as small inteferring RNA, also known as siRNA, which are derived from long double stranded RNA, or a miRNA. Those siRNA binds to dicer, an endonuclease protein that cuts the RNA into short segments (most siRNA’s are aproximately 21 nucleotides long). After that, it binds to an argonaut (AGO) protein where only one strand of the RNA will stay to work as a guide, called the RNA Induced Silencing Complex (RISC). siRNA’s direct RISC to bind to an specific mRNA by matching the nucleotide sequence between them. siRNA are usually perfectly complementary to their target mRNA. AGO then catalyses the cleavage of mRNA that will then be degraded, and therefore translation being inhibited.