RNA Interference (RNAi)

RNA interference (RNAi) is a cellular mechanism for regulating gene expression. RNA, in contrast to DNA, is usually single-stranded in cells. Exogenously-introduced double-stranded RNA (dsRNA), which can come from a variety of sources such as viruses and synthetic dsRNA (i.e. for experimental purposes), trigger the RNAi pathway in the cell.

The RNAi pathway:

1) The enzyme dicer chops the dsRNA into small double-stranded RNA fragments of 20-25 base pairs. These fragments unpair to become single-stranded RNA, which are called siRNA (short interfering RNA).
2) The single-stranded siRNA's are incorporated into a complex called the RNA-induced silencing complex (RISC).
3) When the siRNA's base-pair with complementary RNA sequences such as in messenger RNA (mRNA), the business end of the RISC - argonaute - degrades the target mRNA.

Recall the central dogma of biology:

DNA --> mRNA --> protein

DNA is transcribed into mRNA, which exits the nucleus and is translated into protein, which mediates biological functions. Since RNAi results in the degradation of mRNA, the effect is that protein production is reduced, or "knocked down."

Biological Functions of RNAi

Why does the RNAi pathway exist? There have been theories about RNAi playing a role in the immune system. It has been reported in plants that RNAi plays a role in cleaving an intruding viral dsRNA. The immunological role of RNAi in animals and humans is less understood.

RNAi in Research

RNAi has become an indispensable tool of molecular biological research. It has become one of the major ways to downregulate a specific protein's expression. In simplistic terms, scientists often want to study the function of a protein. So how do you know that Protein A mediates Function A'? You can increase the expression of Protein A and see if the effect (A') is increased. This is called overexpression. You can reduce the expression of Protein A and see if the effect is decreased. RNAi is one of the main techniques used to reduce protein expression. The other major technique is a genetic knockout, which is more cumbersome, and sometimes can result in lethality of the animal or cell line if the target protein is essential for survival.