RNAi: How siRNA Molecules Inhibit mRNAs

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  • 0:01 What is RNAi?
  • 1:27 Discovering siRNA
  • 3:21 How does RNAi Work?
  • 4:14 RNAi in Biology
  • 5:25 RNAi in Research
  • 6:25 Lesson Summary
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Lesson Transcript
Instructor: Betsy Chesnutt

Betsy teaches college physics, biology, and engineering and has a Ph.D. in Biomedical Engineering

In this lesson, you will learn how siRNA helps cells fight viruses and controls gene expression and how scientists are using siRNA to understand genetic diseases and develop new therapies.

What is RNAi?

For years, scientists have known that DNA in the nucleus of a cell is used to produce messenger RNA, also known as mRNA. mRNA is then translated by ribosomes in the cell and used to make proteins. Each mRNA molecule contains the genetic code needed to produce one protein, but that's not the end of the story. It's clear that some proteins are produced much more abundantly than others despite similar amounts of mRNA, but for a long time, we didn't really know why.

The first clue about what was really going on came in the early 1990s when scientists, in the U.S. and the Netherlands, genetically engineered petunia plants to have extra copies of a gene that coded for an important pigment-producing enzyme. They expected that petunias that had the extra genes would make more enzyme and therefore have darker flowers.

When they got flowers that were partially or totally white instead, they were shocked! What could have caused such a strange thing to happen? At the time, no one really knew, but now we understand that this is happening because of something called RNA interference, or RNAi.

In 1998, two scientists, Andrew Fire and Craig C. Mello, discovered exactly why the flowers turned white. In the process, they discovered an important gene regulatory pathway that has revolutionized the way we study gene expression and develop treatments for diseases. They found that there were short segments of RNA in the cell cytoplasm that can bind to mRNA and prevent a protein from being made. These pieces of RNA, typically only 20-25 base pairs long, are called small interfering RNA, or siRNA.

Now we know that there are also even smaller RNA pieces, called micro RNA, that have a similar function. The way that small RNAs disable mRNA and prevent protein production is called RNA interference, or RNAi. In cells, RNAi is important in regulating gene expression and defending against infection by viruses.

In the petunia plants, the extra copies of the gene caused the genetically modified plants to make more mRNA that coded for the enzyme, and this caused the mRNA to be targeted by the RNAi pathway and destroyed before the pigment-producing enzyme could be made, resulting in a flower with little or no pigment. The discovery of RNAi has not only allowed us to make pretty flowers and have a better understanding of how gene expression is regulated, but also to research the genetic mechanisms underlying many terrible human diseases, such as cancer and AIDS. RNAi is also accelerating the development of treatments for these and many other diseases.

How Does RNAi Work?

RNAi is found in many eukaryotic cells, including the cells in your own body. The RNAi pathway begins when the enzyme DICER breaks a long strand of double stranded RNA into to siRNA. The two strands then unwind, and one of them joins with a protein complex known as RNA-induced silencing complex, or RISC.

Once RISC has siRNA bound to it, it becomes active and looks for mRNA with a complementary base pair sequence to that are found on the siRNA. When it finds a complementary sequence, it binds to it and then breaks up the mRNA into many pieces, making it nonfunctional and preventing protein production.

RNAi in Biology

RNAi is a very important part of the innate immune response to viruses across many organisms, particularly in plants and invertebrates. A virus is typically just a single strand of RNA encased in proteins.

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