Question

What is the phenomenon of RNA interference (RNAi)? Explain and describe where these RNAi molecules originate from, how they are processed, and how they could potentially lead to the silencing of certain genes. Be specific.

Answer 1

RNA interference is a process by which the translation of mRNA is inhibited by specific RNA molecules. It is also called a post-transcriptional gene silencing.

There are mainly two types of RNA molecules involved in this mechanism- microRNA (miRNA) and small interfering RNA (siRNA). They are synthesized by the cleavage of a double-stranded RNA molecule. A specialized enzyme called Dicer cleaves this RNA to give miRNA or siRNA.

The siRNA works similarly to that of miRNA. They are usually double-stranded, 20-25 bp in length. These siRNA associate with a protein complex called RNA Induced Silencing Complex (RISC). After this association, double-stranded siRNA becomes single-stranded. This single-stranded RNA is complementary to the mRNA to be degraded. So they bind and cleave the mRNA. This cleaved mRNA is then degraded. This RNA is degraded by an RNase H enzyme Argonaute (Slicer).

miRNA is a non-coding RNA and it is usually about 22 nucleotides long. This mRNA is complementary to the mRNA. Once it binds to the target mRNA, the following processes can take place:

• mRNA will be cleaved to two pieces
• The polyA tail will be shortened. This destabilizes the mRNA
• The translation of mRNA will be less efficient

The miRNA Is similar to siRNA. But miRNA is formed from the foldback regions of the hairpin of the double-stranded RNA. The siRNA is formed from the longer double stranded region.