The ABCs of RNAi
The term RNA interference was coined in 1998, when scientists demonstrated they could insert small pieces of RNA into cells of a roundworm to reduce, or "knock down," the effects of specific genes.
The method "silences" genes by preventing their coded instructions from being used to make proteins, the building blocks of the body. Normally the cell makes a copy of the gene's DNA "recipe" for a particular protein, then sends that copy — called messenger RNA or mRNA — out of the nucleus. Tiny factories called ribosomes tap the mRNA copy's genetic information to build the protein.
Using RNAi, scientists can intercept the mRNA copy before it reaches the ribosomes. They do so by sending into the cell a small segment of RNA — often in a form called a "short hairpin" — that contains an identical sequence, or "address," to that of the mRNA template. The interfering RNA activates enzymes that chop up and destroy the mRNA, thereby blocking the gene's ability to make proteins.
RNAi is not the first technique designed to interrupt gene functions, and it can't thwart every piece of mRNA, but it appears to be more effective and faster than others. RNAi can be used to infect cells in the laboratory or in living animals, causing almost immediate gene silencing. "By contrast, it might take a year or more for one researcher to breed one knockout mouse that lacks the gene in question," explains Boehm. "In theory, it would take some 30,000 researcher-years to create knockouts of all the human genes." Knockout mice haven't been rendered obsolete, by any means, but with RNAi gene silencing, many questions about a gene's function can be answered in a matter of months instead of years. Putting a large set of RNAis to work on a cell simultaneously can yield a vast amount of information in a short time.
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