RNA Interference

Degradation of the dsRNA is not the end of the process, however. The presence of these fragments also prevents the expression of mRNA containing the same sequences. That is, if the host cell has used its own gene to create a single-stranded mRNA, and that mRNA is present in the cytoplasm along with dsRNA fragments with matching nucleotide sequences, the mRNA will be degraded, and the protein it codes for will not be made. This is the "interference" that gives the phenomenon its name. Indeed, it was this process that led to the discovery of RNA interference: Scientists found that introducing double-stranded RNA reduced, rather than increased, production of the encoded protein.

Note that not all mRNA activity in the cell is suppressed: Only those mRNAs of similar sequence are targeted. This provides a clue to the mechanism of suppression. Experiments have shown that dicer targets mRNA by using the dsRNA fragments themselves as guides. While the details are not yet clear, it is believed that one side of the dsRNA is matched with the complementary mRNA sequence, making a new dsRNA, which is itself then degraded. This mechanism also allows the process to be self-sustaining, as each new round creates new fragments that can target any new mRNA.

The recognition process also lends further credence to the belief that RNA interference is a protective mechanism. By targeting only mRNA sequences previously identified as double-stranded (and therefore dangerous), a cell can avoid creating proteins that may be derived from viruses, albeit at the risk of turning off one or more of its own genes at the same time.