The word “mutant” suggests aberrations. Whether we think of Jeff Goldblum’s stomach-churning physical deterioration in The Fly or even the unwarranted demonization of the X-Men by non-mutants, that term is hardly one of endearment. Just try saying it out loud: mutant. Your face has to crease in apparent disgust as you articulate that “mew” sound. Maybe that’s why geneticists are now moving away from the word “mutation” when describing disease-causing variants in our DNA. But mutations are normal: they are in fact a central pillar in creating the diversity that is shaped by evolution, making all of us and every living organism a mutant. The coronavirus is also a mutant, but we need to explain what it means for a mutation to arise so that we can dial back our anguish appropriately.
Mutations arise in large part because no one writes perfectly, not even the molecules in our body. Think of what your text messages would look like without auto-correct (fewer instances of “ducking”, sure, but fat thumbs on a tiny screen naturally create a lot of incomprehensible statements). And if you were asked to type up a manuscript, you would make the occasional mistake (that’s why there’s a backspace key). There’s a molecule in every living thing called a polymerase (literally “enzyme that will make long chains of repeated units”), and this molecule also makes mistakes when copying long strings of letters, like those in our DNA. Many versions of the polymerase have access to a basic spellcheck and backspace key so they can correct the mistakes they detect, but even with this proofreading activity, some mistakes go uncorrected. We often call these mistakes mutations. It’s easy to assume that all of these mutations are bad, but actually many have no effect on the living thing. Some have negative effects and some are beneficial to its survival (cue Darwin and natural selection and survival of the fittest). It’s also important to note that mutations can also arise when our DNA gets exposed to certain chemicals, to high-energy light waves (like ultraviolet light, X-rays, gamma rays), to many metals and even to certain viruses and bacteria. And there are major recombination events that happen like the key parties of old that I will put aside for now.
But why is it that some mutations are good for us, others are bad, and many are neutral? It depends on where they are. DNA is a blueprint (others will call it a recipe), and it contains the instructions to make proteins… but only discreet parts of our DNA called genes code for proteins. The rest? Some regions are important to regulate how many proteins get made; other regions have no known function at the moment. If a mutation arises in one of these “dead zones”, then nothing (as far as we know) happens. Imagine adding a line to an empty corner of an architectural drawing. The house that is eventually built will look just the same. Now, even if a mutation arises inside a gene, it may be of no consequence. The reason has to do with how the information is encoded. An architectural plan is a fairly intuitive representation of what will be built, but DNA uses a code. Three sequential letters are read as one (like a mini-UPC code) and these three letters are equivalent to one specific protein building block. For example, GGC tells the cell to reach for a glycine to add to the growing protein chain… but GGA also means glycine. In fact, in this example, that third letter is inconsequential because this code has redundancies. So if the DNA mutates from GGC to GGA, the protein will look the same. But if the mutation turns GGC into AGC, the cell will add a different building block altogether, which may affect how the protein performs. I’ve given the example of a mutation affecting one letter, but there are all kinds of mutations, like insertions, deletions, and massive changes that can cause a lot of havoc.
So how does this apply to the new coronavirus? The virus has a genetic blueprint, like we do, except that it’s a molecule called RNA (very similar to our DNA). It’s also much, much smaller than ours and thus codes for many fewer proteins. And the virus has a polymerase to make copies of its RNA so that each new viral particle can carry this blueprint… and this polymerase, though it has a backspace key, makes mistakes. They all do. Mutations arise that are not corrected. In fact, viruses are known to exist in what has been termed “quasispecies.” If you take a sample of the virus, you will see that the RNA of all of these viral particles in the sample is slightly different. A mutation here, a mutation there. That’s normal. And when mutations arise, they may not have any appreciable impact on the virus.
But some mutations do make a difference. In fact, the new coronavirus needed to mutate in order to become good at infecting humans. It is possible that, in the future, a mutation could arise in a gene that codes for the external part of the virus that gets recognized by our immune system as foreign. This is called “antigenic drift” and it happens a lot with RNA viruses. It would thus create a new strain of the virus to which we would not be immune. But this has not happened yet and the virus appears to be genetically stable, all things considered.
One last note on mutations: they are not proof of intelligence. When we hear, often from questionable sources, that a virus is “smart” because it can mutate and adapt, we have to remember that viruses do not have brains. They do not display cunning in mutating. Mutations are random. They are accidents. Just like there are happy accidents, some of these random mutations will end up helping the virus. Cleverness need not apply.
Take-home message:
- The genetic code of all living things often contains changes called mutations that are typically (though not exclusively) caused by the equivalent of typing errors
- Most mutations are neither good nor bad because they do not affect the recipe that carries the instructions on how to make a protein
- When a virus mutates it is not a sign of intelligence, as viruses don’t have brains and mutations are random
Leave a comment!
