This video segment adapted from NOVA scienceNOW explores a mechanism called RNA interference (RNAi), which has evolved in cells to prevent viral infection. Cells must carefully regulate the synthesis of proteins from genes to ensure normal development and to prevent disease. Having accidentally discovered this mechanism while experimenting with color in petunias, scientists now know that RNAi serves a critical role in this process.
In 1998, scientists discovered that petunias, nematodes, fruit flies, mice, and even humans use a built-in cellular mechanism to protect their genome (set of genes) from attacks by viruses. Normally, when a gene is turned on, it makes an RNA copy of itself that leaves the nucleus and serves as a message to direct the production of a specific protein. The RNA that is part of this process exists as a single strand. When a virus attacks the cell's genome, it carries a double-stranded form of RNA that enters the cell. A newly discovered cellular mechanism called RNA interference, or RNAi, recognizes this double-stranded RNA as a dangerous invader. It sets out to destroy not only all double-stranded RNA but also any single-stranded RNA messages in the cell that have a sequence like that found in the double-stranded RNA.
A few years later, scientists discovered that many, if not most, organisms use RNAi not just to prevent viral infections, but to regulate the expression of their own genes. Scientists have recently discovered a new class of RNA molecules called microRNAs. These microRNAs are produced during the early stages of an organism's development. They bind with matching single-stranded RNA molecules that are involved with protein synthesis. When a microRNA binds to its matching messenger RNA, it forms a double-stranded RNA that is recognized by the RNAi system, which then destroys it. This RNAi mechanism silences the expression of that particular gene.
Today, scientists are using the RNAi mechanism to learn more about what particular genes do and how to alter their function. Determining gene function is a relatively simple matter of inserting double-stranded RNA molecules that have a particular sequence into cells and observing the effects after RNAi silences the corresponding gene.
Conceivably, this method may one day be used to silence gene mutations that cause human diseases such as Huntington's disease, rheumatoid arthritis, cancer, and many others. By using either the body's own mutations or viral invaders, scientists may develop a new type of drug—for example, one that switches off the genes of a cancer cell and leaves healthy cells unaffected. However, because RNAi's potential effects are so powerful, scientists must first determine that they can control the mechanism so that only the target gene is silenced, and not others.
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