NARRATOR: All might seem outwardly peaceful, but right now tiny battles are raging inside of you. The battles are being fought for control of the molecular machinery that runs your cells and keeps them, and you, alive. Who are the attackers? They are miniscule, yet deadly, viruses. If you’ve ever had chicken pox, the flu, or the common cold, your cells have fought and defeated viruses.
Your cells have factories with the power to create molecular machines. Viruses need to hijack that factory to copy themselves because they do not have factories of their own. Each virus is like a little tank that can blast a hole in the outer cell wall and send its forces in. These intruders change the blueprints in the factory’s headquarters so the factory starts producing more viral tanks. It doesn’t stop until it builds so many tanks that the cell bursts right open.
This is what a virus attack might look like if you were the size of a cell. Most viruses are little more than genetic information––DNA or RNA––inside a protein shell. They bind to a cell and inject their genetic code inside. The invaded cell treats this code as its own and starts making virus protein components to build new viruses. When enough viruses have been assembled, they burst through the cell membrane and the whole process starts again.
Fortunately, your cells have evolved their own defenses. When you get sick with the flu, at first the viruses win out, but then your immune system learns to recognize and destroy the invaders.
One immune response, which was only recently discovered in plants and some animals, is called RNA interference, or RNAi. Cells make a protein called DICER. It's always on the lookout for double stranded RNA, which is used by many viruses but rarely by cells. When DICER finds this RNA, it dices it up. DICER doesn’t stop all the viral RNA from getting through, but it doesn’t need to because RNAi has a great trick up its sleeve: it uses those chopped up pieces of RNA as weapons against the virus.
Those chopped up RNAs are shreds of the virus’s blueprint for copying itself. Now, when that virus tries to hijack the cell’s factory, RNAi molecules are ready: they check all the protein assembly instructions against the chopped up virus RNA snippets. Anything that matches is something that the virus is trying to make, so the cell slices it up before it makes a protein. And just like that, the cell defeats the virus.
RNAi is not only a great natural defensive weapon, but also a powerful tool for biological research. Scientists can use the same basic mechanism of RNA interference to turn off one gene at a time and study the effect on the cell and the organism. For example, turning off one gene may drain all of the pigment out of a purple flower. Another might prevent a plant from producing a toxic chemical, making it safe to eat. Scientists have not yet determined whether human cells naturally fight viruses with RNAi, but it is possible that one day we will be able to use RNAi to deactivate cancer-causing genes and genetic disorders. Meanwhile, viruses and cells will continue evolving ingenious weapons to try to gain the upper hand as the Great Virus Wars rage on.