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RNAi Explained

  • Teacher Resource
  • Posted 08.09.07
  • NOVA scienceNOW

This interactive activity from NOVA scienceNOW illustrates the function of RNA interference (RNAi), a mechanism by which cells interrupt the process of protein synthesis. By effectively silencing genes, RNAi helps to fight infections. Scientists hope that one day they will be able to use this powerful mechanism in the treatment of a wide variety of genetic diseases.

NOVA scienceNOW RNAi Explained
  • Media Type: Interactive
  • Size: 35.7 KB
  • Level: Grades 9-12

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Source: NOVA scienceNOW: "RNAi Explained"

This media asset can be found as "RNAi Explained" on the NOVA scienceNOW web site.


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.

To learn more about the discovery of RNAi and its function in the cell, check out RNAi Discovered.

To learn more about how RNAi may be used to treat genetic disorders and infectious diseases, check out RNAi Therapy.

To learn more about protein synthesis, check out From DNA to Protein, DNA Workshop, and How Do Cells Make Proteins?.

To learn more about how genetic mutations can affect the synthesis of various proteins, check out One Wrong Letter, A Mutation Story, and How Cancer Grows.

Questions for Discussion

    • Why did cells evolve enzymes that recognize and destroy double-stranded RNA molecules?
    • What is the function of the RISC/RNA complex?
    • How can RNAi be used to treat a disease?

Resource Produced by:

					WGBH Educational Foundation

Collection Developed by:

						WGBH Educational Foundation

Collection Credits

Collection Funded by:

						National Science Foundation

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