Have you ever wondered what DNA does when it’s not helping you with your family history? Understanding its day job can give us insights into how DNA behaves in our genetic genealogy studies. In honor of the 15th anniversary of DNA Day on April 25, 2018, I’ll be posting a DNA Basics series during the month of April. DNA Day celebrates scientific discoveries about DNA’s structure and function.
The Chemistry of DNA
At the most basic level, DNA is a chemical molecule, a very, very (very) long chemical molecule. It is made up of repeated units strung together like beads on a necklace. These types of molecules are known as polymers in the scientific lingo. (Tip for the lazy: Most biology terms are based on Latin or Greek roots that are mixed and matched to build new words. If you learn the roots, it’s easy to pick up scientific terms. In this case, poly means “many” and mer means “units” or “parts”.)
The units in DNA are called nucleotides, and each includes a component called a base. There are four bases, thus making four nucleotides in DNA, and they are almost always referred to by one-letter abbreviations: A (adenine), C (cytosine), G (guanine), and T (thymine). To create a polymer of DNA, thousands or millions of A’s and C’s and G’s and T’s are strung together, with each “bead” on the necklace attached to the ones before and after it by strong chemical bonds, like glue. These strong bonds are called “covalent” bonds.
Technically, DNA is really two molecules that are twisted around one another like strands of yarn. This is the famous double-helix structure discovered by Rosalind Franklin and published in 1953 by James Watson and Francis Crick.
In the animation below, the two outer twisty bands are the “backbones” of the two DNA molecules, and the crossbars are the individual A, C, G, and T bases reaching across to a partner on the other strand. A’s always pair with T’s, and C’s always pair with G’s. (The crossbars aren’t really linear; they’re two-dimensional, like flat rungs on a ladder. We’re only looking their narrow sides in this image.)
If you look closely, you’ll see that the two partners aren’t touching one another; that’s because they’re not connected by covalent bonds. Instead, the two partners on each strand are held together by weak chemical bonds, more like magnets than glue. Thousands of these so-called “hydrogen” bonds running up and down the two DNA molecules are what holds the double-helix together. These weak bonds can detach and reattach without damaging the underlying DNA. In fact, this happens all the time when the DNA is being “read” or when it’s being copied to make new cells. On the other hand, if the strong bonds are broken, the DNA itself can be destroyed.
The Language of DNA
More than just chemistry, DNA is a language, and A, C, G, and T are the alphabet. The language conveys a set of instructions for how our cells should operate. Some parts of our DNA are blueprints for proteins, which are like chemical machines that have specific functions in the cell. Those blueprints are called “genes”. Other parts of our DNA are on/off switches, telling our cells when to make certain proteins and when to stop making others. Yet others are structural regions, like centromeres (more on that in a later post) and telomeres. And finally, the vast majority of our DNA doesn’t appear to do anything at all.
Yes, your genome is a hoarder! It’s packed with tons of useless stuff. Why? I guess for the same reason some people are hoarders: because it’s too much trouble to get rid of the junk and because you might find a use for it someday. Thing is, all that junk DNA is a great thing for genetic genealogy.
DNA and Genetic Genealogy
Because most of the genes in our DNA are essential to life and to being human, we’re all pretty similar to one another genetically. In fact, each of us is about 99.9% identical, on average, to any other human. In genetic genealogy, we’re exploring the information in that remaining 0.1% where we differ from one another.
The genetic differences among us are caused by random errors that occurred usually when the DNA was being copied. These mutations are basically typos. Some happened tens of thousands or hundreds of thousands of years ago and were widespread in our respective ancestral populations; others happened more recently. Most of our differences are in those regions of DNA that don’t seem to have a function (the junk), because mutations there can’t hurt us. Mutations occur in genes, too, but they tend to be harmful and get weeded out quickly.
The various genealogical DNA tests that we do examine sites in our DNA that are known to have mutated at some point in the history of the species. The tests themselves—meaning what’s done in the laboratory—the results that we get, and the ways we interpret the results, differ depending on the the type of DNA and the type of mutations involved. We’ll talk about that in later posts.