Genes provide the instructions that cells use to produce, or synthesize, proteins, which are necessary for the proper functioning of an organism. Protein synthesis from genes occurs in two basic steps: transcription and translation. During transcription, instructions contained in DNA are copied to a single-stranded molecule called messenger RNA (mRNA). Messenger RNA is subsequently translated into an amino acid chain (a protein) whose sequence corresponds to that particular mRNA molecule.

Cells typically perform these steps flawlessly—producing vital proteins in just the right quantities, at just the right times. When things go wrong, it is commonly the result of the cells being "hijacked" by outside invaders. Viruses infect cells by inserting their own RNA "recipes" into cells. Viral attacks typically lead to abnormal development and disease.

RNA interference (RNAi) is a recently discovered mechanism that cells use to prevent such outcomes by interfering with protein synthesis. The RNAi mechanism is activated by the presence of double-stranded molecules of RNA in the cytoplasm, the fluid surrounding the cell nucleus. Double-stranded RNA indicates abnormality in a cell, as RNA normally produced by the cell is single stranded. Double-stranded RNA molecules may come from viruses or may be part of the cell's own mechanism to inhibit the production of certain proteins at certain times.

When the RNAi molecules detect an offending molecule, a protein complex, which scientists call "Dicer," cuts the double-stranded RNA into fragments. Next, a molecule called "RISC" binds to one fragment of the offending RNA and uses this fragment to detect single-stranded mRNA with the corresponding sequence. Whenever RISC encounters corresponding mRNA molecules, it cuts and degrades those molecules such that they can no longer be used to synthesize proteins. By degrading all mRNA with this sequence and inhibiting the production of the protein it codes for, RNAi can effectively limit the effects of viral RNA.

After scientists discovered RNAi, they soon realized that this naturally occurring mechanism could potentially be manipulated to block, or "silence," just about any gene's normal activity. Today, RNAi is being used experimentally to treat a small number of genetic disorders. However, because manipulation of gene function is not without its risks, scientists and doctors are proceeding as cautiously with RNAi as they have with similar potential medical therapies.

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