Well, the story of water is over, for now. Water will come up again, over and over, as we get into other topics, because—well, like you should know by now, water is really important in a lot of different ways. But for now, we’ll start looking into some other stuff. Let’s talk about DNA. DNA is fun.
Pretty much everybody has at least heard the term, DNA, and a lot of people know that it has something to do with genetics and heredity, but that’s about as far as most people can go. However, if we are trying to build scientific literacy, we need to go a little further, because it’s kind of a big deal. Genetic engineering, which is really just a term for the manipulation of genes (DNA) by people, is one of the most important and impactful (not to mention potentially world-changing) scientific developments in human history. Genetic engineering has been important, scientifically, economically, medically and otherwise for decades, even centuries, in some cases, since selective breeding of plants and animals is a form of genetic engineering. Gene therapies are already being used to address some really horrible diseases. Genetic engineering (among a lot of other biomedical science) gave us Dolly the sheep, the first cloned mammal, back in 1996. I’ll be doing a whole future article about the truly transformative and potentially world-changing scientific and technological advances in human history. Gene science is one of those relatively few advances.
DNA stands for deoxyribonucleic acid. It is one of those important molecules found in all living cells that we’ve touched on before. First, let’s figure out what it is. It might be helpful if you were to pull up a picture of a DNA molecule on the internet. A DNA molecule is an incredibly beautiful, elegant structure, and you should look at some really good images and videos of it online to better appreciate it. The easiest way to start learning about DNA is to look at the name. Obviously, it’s an acid. If it wasn’t, it would be called just DN, and that would be silly. Most people, when they think of “acid”, think of battery acid or something that eats through metal and suchlike. While there are certainly acids like that, most of the time, acids are not so dramatic. What the term “acid” in chemistry means is that it is just a chemical which can donate hydrogen ions to chemical reactions. Sulfuric acid (battery acid) is a strong acid that tends to be very reactive. It’s one of those that can eat into metal and cause burns to your skin. Water is also an acid, from the chemical perspective, albeit a very weak one. There are lots of acids that are important in biochemistry. Nucleic acids, like DNA, and amino acids (the building blocks of protein molecules) are just two very common examples of common acids in living things. We can do an article on acids and bases sometime if anyone is interested. Some pretty cool stuff, there. So, we’ve done the “A”. Let’s do the “D”. The “D” is for deoxyribose, which is a type of sugar that contains 5 carbon atoms. Deoxyribose, though it is, chemically, a sugar, isn’t a dietary sugar like sucrose (table sugar) or fructose (fruit sugar). It is a carbohydrate that is part of the basic structure of a DNA molecule. The “N” is for “nucleic”, which refers to a category of nitrogen-containing molecules that are found mostly in DNA and the related molecule, RNA (ribonucleic acid). A DNA molecule is made up of subunits called “nucleotides”. All DNA nucletides have the same “D” part (the sugar) and the same “A” part (which is a phosphate group, made up of phosphorus and oxygen atoms). There are four different “N” parts, so there are four different nucleotides, adenine, thymine, cytosine and guanine (those are the A,T,C, and G you might have seen in relation to DNA molecules).
Image. A Nucleotide: The part with the P’s and O’s on the left is the phosphate group (the acidic part). The ring structure in the middle is the sugar. The nitrogen-containing BASE on the right is the part that differs among the 4 different nucleotides.
By Binhtruong – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=25453268
DNA molecules fit, structurally, into a class of macromolecule (“macro” just means “big”—nucleic acids are one of the four main macromolecules found in all cells, along with proteins, carbohydrates, and lipids) called polymers. You’ve probably heard the word “polymer” in relation to plastics. Polymers are large molecules that are made up of a lot of subunits attached to each other in a chain. The subunits of DNA are nucleotides, so a DNA molecule is basically a long chain of nucleotides hooked to each other. Similarly, a protein is a polymer of subunits called amino acids. In plastics, a polymer, like polystyrene (Styrofoam), is made up of very long chains of carbon-and-hydrogen-containing subunits (hydrocarbons) called styrenes. That might be a little more clear if you look up “polymer” or “macromolecule” on the internet. The take-home message is just that DNA (and RNA) are large molecules that are composed of long chains of just four different nucleotides. Think of the nucleotides as red, green, yellow and blue Legos, one stacked on the next.
DNA molecules are usually very large—among the largest molecules found in living cells. If you think back to when we were talking about basic chemistry, we talked about elements and atomic masses and things. Just to give you an idea of what I mean when I say DNA molecules can be very large, let’s compare it is a small molecule, like water. Water is made up of one oxygen (atomic mass, 16) and two hydrogens (atomic mass, 1) so the molecular weight of a water molecule is 18. DNA molecules often have molecular weights in the millions. If you took the DNA in just one single human cell and straightened it all out, it would be about 2 m (more than 6 feet) long. So when I say “big”, I mean “big”.
DNA is basically information storage molecule. All of your genetic information, basically the biological recipe that makes you, you, is encoded in your DNA. The same is true for a bacterium or a tulip. Every living thing uses DNA as its genetic material, and all use the same four nucleotides. Although that is common knowledge today, it has only been a little over 70 years since the structure of DNA was discovered. The scientific paper that described “the double helix” structure of DNA was published in 1953. Before that, no one really knew how genetic information was stored, copied or passed from generation to generation. The fact that characteristics were passed from one generation to another was clear—all sorts of traits (blue eyes, black fur, pink flowers, certain diseases, etc.) are all passed on, for instance. What wasn’t at all clear, however, was how that information passed. Obviously, some real, physical thing (as opposed to energy or something like that) was passed on, either sexually or asexually (depending on the organism), but how that worked was a mystery. Early theories were that the information had to be stored in protein molecules, because only protein molecules are large enough and complex enough (there are 22 different amino acids that make up proteins) to hold that much information. People knew that DNA was a large molecule and that there seemed to be a LOT of it in all cells, but since it only contained four “ingredients”—A,T,C, and G; the thinking was that it couldn’t hold genetic information because it wasn’t complex enough.
There was a HUGE competition in the biology world to answer the question of how genetic information was encoded and passed on. At the time, one of the most famous scientists in the world, certainly the most famous biochemist, was Linus Pauling, a scientist at CalTech. Most people nowadays have never heard of him, but he remains the only person in history to win TWO Nobel Prizes all by himself (4 other people have also won twice, but they shared awards with other people). One of the other big groups working on the project was at King’s College in England, led by another respected scientist, named Maurice Wilkens. You thought I was going to say something about Watson and Crick, didn’t you? They are certainly part of the story, but there’s much more to the story than most people think. As usual.
Source: Magladem96, Creative Commons
We’ll finish up with the tale of DNA next week. Between now and then, why don’t you contact me with your suggestions on interesting topics you might want to hear about? Why is the sky blue? Why do plants wilt if you don’t water them? What are kidneys for? There is an almost infinite list of questions. If you don’t, my article two weeks from now might be about quantum mechanics, and it’ll be really dry and hard to understand. There might even be equations. Don’t make me pull out the equations. If you thought the chemical bonding articles were dense…