Q.Gene expression can be controlled with the help of RNA. Explain the method with an example.

RNA interference (RNAi) is the method by which gene expression is controlled using RNA molecules. In this process, a double-stranded RNA (dsRNA) is introduced into a cell, which then gets cleaved into small interfering RNAs (siRNAs) by an enzyme called Dicer. These siRNAs bind to a protein complex called RISC, which then uses one strand of the siRNA to find and destroy any complementary mRNA in the cell. This effectively silences the target gene because the mRNA is degraded before it can be translated into a protein. …

The idea that RNA can control gene expression was a revolutionary shift in molecular biology. For decades, the central dogma held that DNA made RNA, and RNA made protein — RNA was merely a messenger. But we now know that certain RNA molecules can actively shut down genes. This mechanism is called RNA interference (RNAi).

RNAi works through a simple but elegant principle: a double-stranded RNA molecule (dsRNA) is introduced into a cell, and it triggers the destruction of any messenger RNA (mRNA) that has a complementary sequence. Since mRNA carries the genetic instructions from DNA to the protein-making machinery, destroying it effectively silences the gene. No mRNA means no protein.

The process unfolds in two main steps. First, the long dsRNA is chopped into small fragments, about 21–23 nucleotides long, by an enzyme called Dicer. These fragments are called small interfering RNAs (siRNAs). Second, these siRNAs are loaded onto a protein complex called RISC (RNA-induced silencing complex). The siRNA unwinds, and one strand guides RISC to find and bind to any mRNA that matches its sequence. Once bound, RISC cuts that mRNA, preventing it from being translated into protein.

A powerful example of RNAi in action comes from the field of agriculture, specifically in the fight against the nematode worm Meloidogyne incognita, a root parasite that devastates crops like tobacco, tomato, and cotton. These worms infect the roots of plants and cause massive yield losses. …

Method — walk the dsRNA→silencing pathway once, then plug in the nematode example

Generic pathway: dsRNA introduced → Dicer cuts it into siRNAs → siRNAs guide degradation of the matching mRNA → that gene's protein is never made. …