Nature Reviews Genetics features an excellent high quality computer visualization (including an associated slideshow) of the elegant mechanism of RNA interference. RNA interference is a defence mechanism that enables double-stranded RNA molecules (such as those derived from transcribing the genomes of viruses that invade the cell) to be degraded and destroyed.
RNA interference essentially involves a four-step pathway. These steps are:

1. The recognition of double-stranded RNA (dsRNA) and its subsequent cleavage into small RNA segments by a protein complex, containing a nuclease, called “Dicer.”
2. The dsRNAs are then loaded onto another protein complex called an RNA Induced Silencing Complex (RISC). RISC separates the two RNA strands and releases one of them. The other RNA strand (the so-called “guide strand”) has a sequence that is complementary to the target RNA.
3. The RISC complex, bound to the small interfering RNA (siRNA), is subsequently recruited to the RNA target where it facilitates the binding of the siRNA with the complementary target.
4. This results in cleavage of the target RNA. The cleavage is catalyzed by a component of the RISC complex — an endonuclease enzyme — called “Argonaute.”