t times like these, with the summer movie season just around the corner and super-powered mutants vying to rule or rescue the silver screen, it's natural to ask ourselves where we might be headed as a species, and whether there are strange new powers in our future—at least for a privileged few.
To answer that question, let's first talk a bit about what mutants are, and how they arise. The long chains of DNA in your cells are made of much smaller molecules called nucleotides, which form triplets called codons, which are read by teeny-weeny parts of the cell called ribosomes. Each codon represents an amino acid building block, and as the ribosome reads its way down a "tape" of RNA (copied from the DNA), it grabs the appropriate amino acids and sticks them together to form another large molecule called—you guessed it—a protein. A "gene" is simply a segment of DNA that commands the production of a particular protein, and a mutation is a change in the DNA which leads to a change in the form, function, quantity or quality of a given protein. Most of you probably know these things already, but a quick refresher never hurts.
The most common type of mutation is a single-nucleotide substitution, which replaces one of the 3 billion base pairs in your DNA with an alternate (of which there are three to choose from). These "point mutations" usually leads to harmless differences, and occasionally no differences at all, since many different codons can actually stand for the same amino acid. Generally speaking, most point mutations have a slight cosmetic or functional effect, which is very important in the scheme of things, because all human beings have the same number of genes serving all the same purposes, and the differences between individuals—the things that make each of us unique—are mainly these point differences or "single-nucleotide polymorphisms" (SNPs).
Monitoring the
invisible mutants
Now, like man-made tools and machines, proteins have a function that is determined mainly by their shape. This is also critical, because proteins are really just long, stiff, sticky chains that have folded up into three-dimensional shapes. This folding depends on the exact electrical charges of the DNA bases at critical points, and if a mutation falls directly on a corner or joint, it can change the shape of the protein and therefore its function.
Again, though, this is often harmless, since the "active sites" on the protein—the parts that do the work—are a small fraction of the overall shape. Think of a snow shovel: you can bend and dent it all sorts of ways and still get pretty good use out of it. Every now and then you may even improve it slightly! But the leading edge of the blade does need to be fairly flat and sharp to do its work, and if your mutation damages this property then the shovel's usefulness will be significantly degraded.
This is exactly what happens in sickle-cell anemia, where a single substitution alters the shape of the hemoglobin molecule, resulting in crescent-shaped red blood cells that are less efficient carriers of oxygen. Fortunately, higher animals always have two copies of every gene (one from the father and one from the mother), and this particular malformed gene is recessive, meaning it takes two copies of the same bad gene to produce the disease.
Another type of mutation occurs when a single nucleotide is inserted into the sequence, or deleted from it. These are generally more serious than substitution errors, since they screw up the three-letter coding system, making nonsense of the rest of the protein (this is known as a "frame shift"). If these errors occur close to the end of the chain, the result may be only a minor structural change, but if they occur close to the beginning, the shape and function of the protein will be completely random. Whatever effects this has on the development and operation of the body, they are really unlikely to be helpful. Sometimes, two frame shifts can cancel out, leaving a protein with normal ends but a malformed segment in the middle.
Other sorts of mutation affect the genome at a higher level, by rearranging the chromosomes. These include the fusion of two chromosomes into a single larger one, the fission (splitting) of one chromosome into two, and the duplication, deletion or inversion (end-to-end flipping) of a segment containing one or more genes. DNA segments can also suffer translocation, which moves them intact to a different part of the chromosome, or even to a different chromosome altogether.
These errors don't result in malformed proteins, but they do change the relationships between genes, possibly affecting the ways in which they're expressed. (In the language of geneticists, "expressing" a gene is the same thing as "running" a computer program.) Gene duplication in particular offers the opportunity for interesting changes, since it induces the body to create more of that particular protein.
This is often harmful, as with the neurological ailment "Chacot-Marie-Tooth" which results from an excess of a protein called peripheral myelin. However, if the protein in question is involved in, for example, DNA repair, then extra copies may have beneficial side effects such as an increase in lifespan or a reduction in the prevalence of cancer. Doubling of the gene that codes for neuroglobin (a special type of hemoglobin produced only in the brain) appears to confer a higher tolerance for the oxygen deprivation that occurs during a heart attack or stroke.
Duplication of entire chromosomes can also occur, but so many genes are involved—and so many body systems affected—that the results are invariably detrimental. For example, Down's Syndrome (whose symptoms include mental retardation and shortened lifespan) is caused by an extra copy of chromosome 21.
The promise of unnatural selection
Notably, visible mutations (other than cancers) do not generally occur in adult organisms. Rest easy; you're in no danger of growing new limbs or unsightly blue scales, although your descendants someday might. But can evolution be speeded up, so that the number of mutants in the human population increases? Most definitely yes. Normally, mutations accumulate slowly, but as comic books have taught us since time immemorial, the process can be accelerated with low to moderate doses of ionizing radiation.
To introduce new traits randomly into plant seeds, breeders sometimes borrow a trick from Homer Simpson (episode 231,"E-I-E-I-D'oh!") and employ controlled X-ray doses. For what it's worth, bombardment with neutrons would be about 100 times more mutagenic for the same energy, but that's harder to arrange. Mutagenic chemicals such as mustard gas may also be used. Most of the plants that grow from these seeds are sickened in some way, but a few develop useful traits such as early ripening or resistance to particular diseases, which can then be studied and even transferred to other, unmutated plants.
We've all heard of natural selection—survival of the fittest and all that—and it's the main reason why most harmful mutations die out sooner or later. This is especially true in successful species, whose stability and environmental harmony are difficult to improve on. Beneficial mutations are more likely to occur when an organism is in a new environment, or when its established genome is suboptimal for coping with new pressures such as pollution, resource competition, and overpopulation. Pesticide resistance in insects and antibiotic resistance in bacteria are examples of beneficial traits which have arisen in this way, right before our eyes, and at the molecular level we humans are not so different.
Still, there are limits. It's not hard to conceive of mutations that might, among other things, make someone stronger or slipperier, with a longer tongue or an improved ability to heal and regenerate. Almost any trait we have can be increased or decreased if the right genes are tweaked. Even the ability to control magnetic fields is not completely impossible; although this doesn't occur anywhere in the animal kingdom, an electric eel with coils of wire attached could make a good try of it. The power to emit laser beams is considerably more involved, and would require dozens or perhaps hundreds of very specific mutations. It would never happen by accident—at least not within a realistic human time frame. Controlling the weather and walking through walls are even more problematic, since we don't even have technology on the drawing boards that can do these.
But if you want a bit of cinematic thrill, think of this: while you've sat there reading this article, your cells have accumulated (statistically speaking) another 0.004 millirads of DNA-scrambling radiation, mainly from natural sources. Bon apetit!
Wil McCarthy is a rocket guidance engineer, robot designer, science-fiction author and occasional aquanaut. He has contributed to three interplanetary spacecraft, five communication and weather satellites, a line of landmine-clearing robots and some other "really cool stuff" he can't tell us about. His short writings have graced the pages of Analog, Asimov's, Wired, Nature and other major publications, and his book-length works include the New York Times notable Bloom, The Collapsium and most recently The Wellstone and a related nonfiction book, Hacking Matter.
