Ever seen a photo like this one? I bet you have. Scientists love stock photos like this: beautiful molecules. (And I don’t really blame them. Us?) Maybe you don’t know what this is. Maybe you can recognize it as a DNA ladder or helix, but couldn’t say much more than that. Hopefully by the end of this primer, you’ll be able to show off to your friends next time you look at a magazine cover.
DNA. Deoxyribonucleic acid. That’s a mouthful. This is the molecule that codes every protein that makes you up.
DNA is composed of a sugar and phosphate-based “backbone,” the sides of the ladder. Attached to these sugars are nucleotides. There are four different kinds: Adenine, Thymine, Guanine, and Cytosine. The system is actually quite simple, as Adenine (A) can only bind to Thymine (T), and Guanine (G) can only bind to Cytosine (C). If you look at the blue photo, you’ll notice that each of the ladder “rungs” seems to be split in half. The split rungs are actually representative of these complementary base pairs: each half-rung is bonded to its complement. For example, if there was a nucleotide sequence of ATTGC going down one side of the DNA ladder, directly opposite would be TAACG. This is a handy tool because if a single-strand is floating around, such as after a cell divides, the body knows exactly how to construct the opposide side of the ladder using its complementary strand.
Each series of 3 nucleotides is called a “codon” and usually codes for an amino acid. There are only 20 amino acids total, and in many different arrangements, these building blocks create every protein in your body. A gene describes a region of DNA that codes for one specific protein. (If you must know, there are also “start” and “stop” codons, also designated by nucleotides, that indicate where the gene begins and ends.) Genes can be any variety of lengths and code for just a few or many amino acids, which line up and then fold up in a particular way to make the designated protein.
So imagine your cell has a single-strand of DNA and proteins are working to build the complementary strand. The human genome has over 3 billion nucleotides; what are the chances that there will be a mistake? Very likely. It happens all the time, in fact. We have evolved different “spell-check” mechanisms within the cell to help correct mistakes, such as accidentally pairing a C with a T, for example. When a change in the sequence slips through the cracks, it’s called a mutation.
There are a few different kinds of mutations. The one described above, with one nucleotide exchanged for another, is called a point mutation. A deletion is when a nucleotide is left out, and a duplication is when an extra is put in. When a point mutation occurs, the wrong amino acid is made from the incorrect codon, ending up with a faulty protein. Often, there is no significant change and the protein functions as if nothing happened. However, if that particular amino acid is critical, the protein will be non-functional.
Deletions and duplications are more serious mutations. If the codons are being read in triplets and you shove an extra nucleotide in or remove one, the entire template will be skewed, with wrong amino acids for the rest of the gene and a loss of function for the protein. Additionally, remember that the DNA ladder has a very stable structure, with the nucleotides pairing up with precision. If the DNA molecule becomes unstable because of a duplication or deletion, the protein-making system can become jammed-up, so that no protein is made at all.
Now that you’re all freaked out about all the mistakes your cells are making, all the duplications ruining your genes, listen to me: do not worry. Think of all the cells in your body. A mutation in the DNA molecule of one cell isn’t going to create dramatic change in how your body functions. (Unless it’s CANCER! Just kidding. Am I?) You can still breathe.
The real danger is from mutations that occur in the sperm and eggs before you were even created, or early on after fertilization. These are mutations that can be spread through the entire genome, as they are the origins of YOU. And that’s how we have genetic disorders.
Well, look at you now! You know more about DNA than Watson and Crick did when they discovered it. (With the help of a certain Rosalind Franklin..) Congratulations!
Send any questions or comments to me at culturingscience [at] gmail [dot] com