the RNA-interference pathway
/ AR-en-ay in-ter-FEER-ence PATH-way /
Imagine a cell with a built-in immune memory not for germs but for sequences. If it encounters double-stranded RNA — a tell-tale sign of a virus or a runaway gene — it can grab that sequence, shred it, and then hunt down and destroy every other RNA in the cell that matches. RNA interference, or RNAi, is this sequence-specific silencing system.
The pathway works in steps. A double-stranded RNA is first chopped by an enzyme into short fragments about 21 to 23 base pairs long, called small interfering RNAs (siRNAs). One strand of each small fragment is then loaded as a guide into a protein complex. That complex uses the guide to find any messenger RNA whose sequence matches, base-pairs to it, and cuts it in two so it can no longer be translated — silencing the corresponding gene. Because the guide directs the complex purely by base-pairing, the silencing is exquisitely sequence-specific: only RNAs matching the guide are destroyed.
RNAi was discovered in 1998 in roundworms (a Nobel-winning finding) and turned out to be ancient and widespread — a natural defense against viruses and against jumping genes (transposons), and a regulator that overlaps with the microRNA system, sharing the same core machinery. It also became one of the most powerful tools in biology: feed a cell a chosen double-stranded RNA and you can switch off almost any gene to see what it does. That tool-use, and the engineered guide RNAs of gene editing, are covered elsewhere; here the focus is RNAi as the cell's own pathway.
A plant infected by a virus chops the virus's double-stranded RNA into siRNAs, then uses them as guides to seek out and destroy any remaining viral RNA in the cell — a sequence-targeted antiviral defense, the cell's own version of search and destroy.
Double-stranded RNA is diced into guides that direct the cell to destroy matching RNAs.
RNAi silences after transcription — it destroys or blocks the RNA message — rather than editing or deleting the gene in the DNA. The gene is untouched and intact; only its RNA output is suppressed, which is why the effect is reversible and not inherited.