Ask a Scientist: Alien DNA?
By: Sienna Schaeffer
Edited by: Katherine Hill, Namrata Damle
The X-Files is my favorite show of all time. I feel like it has a lot of good science but I know I can’t take everything on it as gospel truth. Throughout the series, Scully and Mulder frequently encounter people with alien DNA. Not that I would ever question The Great Ones, but it made me wonder; what would alien DNA look like and how could you tell it apart from normal human DNA?
Submitted by: Someone who wants to believe
I can’t really attest to the quality of scientific information presented on the X-Files, but since it’s a police procedural about aliens from the 90’s, I have to say that it probably does not qualify as a good source for scientific information. But don’t worry, that’s what I’m here for.
First things first, I think it would be helpful to look at terrestrial DNA and the features that make it oh-so-special. Now, as exquisite as the double-helix structure of DNA is, it’s not the hereditary material of all living things on Earth because it could win a molecular beauty contest. Of course, DNA’s elegant structure is key to its function, but we’re going to look at the more general features of DNA that enable it to serve as a hereditary material. You see how this is related to your question, yes? No? You’ll get there.
There are four major features that any hereditary material needs to have: the ability to encode information, the ability to replicate, the ability to change, and stability. That last one kind of ruined the pattern I had going. Let’s look at each of these requirements more closely.
The ability to encode information. Hereditary material by definition must carry all of the information necessary to build an organism from scratch. DNA encodes information in its sequence. You may have heard this word a lot in recent years. Websites like 23 & Me offer DNA sequencing and analysis to look for mutations and find where your ancestors came from*. You’ve probably inferred that they are somehow ‘reading’ information contained in your cells. So what does that actually mean?
A DNA molecule is made up of two backbones. Each of these backbones is bound to a sequence of four bases: A,T,G, and C. These bases can bind to each other, A with T and G with C, connecting the two backbones that twist together to make the characteristic double helix shape. Perhaps you learned this in high school, or perhaps you noticed this very helpful picture I’ve displayed just to the left here (normal structure is the top part). The order of these bases is the basis of a code that cells use to make proteins. Cells also use this code to decide when to make proteins, how much of each, and where to send them.
The ability to replicate. If you’re going to pass on information, you need to be able to make a copy of it. All biological entities on Earth, living and nonliving, must have some strategy to replicate their genome so that it can be passed on to progeny. I won’t go into how DNA replicates itself, but the picture above does have a few hints.
The ability to change. Change is a constant in life (too cheesy?). Changes in DNA are the molecular basis for evolution and the diversity of life on Earth. Natural selection, one of the main processes that lead to evolution, happens because of naturally occurring variations between organisms that make certain individuals better suited to their environment than others. These variations arise from random changes in DNA. These changes can come from mutations, recombination, retroviruses, damage and repair, etc. Hereditary material needs to have this capacity to change. Without the capacity to change, organisms using that material would not be able to evolve. All of the organisms living together in a group would have exactly the same genes. One big environmental change or even a temporary disaster could end up wiping out the whole population.
Stability. This requirement may seem a little counterintuitive since we just discussed how a whole population can die if the hereditary material is too stable. Still, stability is incredibly important. There are two kinds of stability that are key for DNA. There’s stability of sequence, which is pretty important when you want to pass down accurate information. As discussed above, sequences should be able to change, but too much change and you end up changing the message, which could be good but is usually bad. Stability of the actual material is also very important. You don’t want the material to react with a lot of stuff in its environment. You don’t want it to spontaneously fall apart or change forms. You want it to be sturdy and reliable and able to stay in the same form for years and years. DNA is an extremely stable molecule and cells take plenty of precautions to augment that stability.
So far we’ve discussed what makes DNA capable of being the hereditary molecule of Earth. In case you haven’t guessed yet, these are also the features that we would expect to see in alien ‘DNA.’ Perhaps this has all been a little too vague for your taste. Luckily for you we talked to Dr. Jake Bailey, a professor in the Department of Earth Sciences at the University of Minnesota. As a geobiologist, Dr. Bailey studies the metabolism and diversity of some of the most unusual microbes on Earth. He also instructs students on the topic of astrobiology. He offered some more detailed information on what we might expect to see in alien hereditary material.
As discussed above, DNA’s ability to change is the basis of genetic variation between individuals. This genetic variation provides the individual differences that enable the process of evolution. According to Dr. Bailey, “…evolution is a hallmark of living systems.” So we can safely assume that if Material X is what an alien species uses as its hereditary material, something about the structure or organization of Material X should show that it is being acted upon by evolutionary processes. “I would expect that evolutionary processes would still manifest themselves as a series of variable and conserved regions in that information molecule,” Dr. Bailey explained.
We see these patterns in DNA. We can look at DNA sequences and see regions that are shared across species/between species (conserved) and we can see regions that are uncommon or relatively dissimilar even between members of the same species (variable). Sequences that are useful for all kinds of organisms in a variety of environments tend to be relatively well conserved. For example, the gene sequences that carry the information to create protein synthesis ‘machines’ are the most well conserved sequences on Earth (1). Everybody needs to be able to make proteins, after all. Variable regions are often more specialized, as these regions create the differences that allow different life forms to excel in different environments. Detrimental sequences, or sequences that make an organism less likely to survive and reproduce in its environment, also tend to be poorly conserved.
What this means is that if we found some Material X, we could hypothesize that it was a hereditary material if it had variable and conserved regions in its structure that were produced through the processes of natural selection. These regions would occur in discernible patterns and would not be random. As Dr. Bailey stated, “While mutation is random, natural selection is not.”
“So?” you might ask. “This has all been so vague! What I really want to know is if aliens would use a molecule like DNA, where the info is encoded in a sequence like you were talking about earlier!” Unfortunately for you, this is where science and Dr. Bailey draw the line. “That would be a fair question,” he said, “but beyond the realm of what I would speculate on.”
Dr. Bailey is right — to consider anything more than the hazy outlines of what we’ve discussed here would be to go beyond what science can actually tell us. Everything we can even conceptualize as life is shaped entirely by our understanding of the life we’ve encountered on Earth. All of our understanding is therefore confined to a world where DNA is the hereditary material. To speculate on life outside of our little blue marble is a bit like speculating on a color you’ve never seen. I can tell you what a color is and I can tell you about the ones I know, but there’s no way I can truly describe or even imagine one I’ve never seen. As Dr. Bailey said, “For now, we know of only one example of life, and that is the life on our own planet.”
*I have MANY complaints about this but this is not the article for that. Suffice to say nationalities mean NOTHING to your DNA. It does not care. Or more accurately, nations have existed for such a short time even in the course of human history (plus there’s a fair degree of migration and mating between them) that they really haven’t left a significant mark on your enormous genome.
References
- Isenbarger, T. A., Carr, C. E., Johnson, S. S., Finney, M., Church, G. M., Gilbert, W., . . . Ruvkun, G. (2008). The Most Conserved Genome Segments for Life Detection on Earth and Other Planets. Origins of Life and Evolution of Biospheres,38(6), 517–533. doi:10.1007/s11084–008–9148-z
