The blueprint of life: DNA
You and I are largely physical products of the genetic information passed down to us from our ancestors through our biological parents. The fact that all the members of your family share a certain resemblance is no accident… you all share pretty similar genetic material. As a matter of fact, you and your biological siblings are actually hybrids of your biological parents. DNA (Deoxyribonucleic Acid) is the vehicle through which the information that governs and maintains each human’s physical form is passed down. The building blocks of DNA are called nucleotides and there are 4 of them – Adenine (A), Cytosine (C), Guanine (G), and Thymine (T). A legitimate argument could be made that these are the very building blocks of life itself because these same nucleotides make up the DNA of pretty much all living things on this planet. So the next time you see a tree on your way to work, find mold growing on a loaf of bread you forgot about, or pet your dog, know that their DNA is made up of the same constituent parts as yours and mine. Humbling right? Yeah… I know.
DNA takes the form of an elegantly structured double helix with each strand of the helix being a kind of mirror image of the other. Evolutionarily, it makes sense that DNA is made up of two intertwined helices because that makes it more stable. DNA is so stable that you can actually isolate it from human blood and put it in a refrigerator for years without worrying about disrupting its molecular structure. Even though DNA is highly stable, nature has additionally “hedged its bets” by giving all of us two copies of each gene in almost all nucleated cells. This is why human bodily (or somatic) cells are referred to as “diploid”, indicating that they contain two copies of every gene. As a result of this redundancy, an individual can still function perfectly in most cases even if one copy of a particular gene is busted for whatever reason. The molecular stability of DNA and its inherent redundancy are very important because degradation and/or random mutations in DNA lead to disruption of the genetic sequence which in turn can yield a whole host of nasty disorders.
DNA serves as an elaborately coded instruction manual that “tells” your body what proteins/molecules to manufacture in order to maintain your general health, growth, and survival. The instructions contained within our genome or DNA are not directly functional units of growth and sustenance. Rather, DNA must somehow be translated into proteins which then serve as the actual “worker bees” that facilitate the growth and maintenance of the body. In truth, a lot of your body is made up of proteins. Your muscles are made up of actin and myosin which are both proteins. Keratin is another protein found in your skin and hair that you may have heard about from watching hair product commercials. The process of translating DNA to protein is simple in principle. It goes something like this:
- An enzyme (a type of protein) called RNA polymerase makes a temporary single stranded copy of the section of DNA that your body wants to turn into protein. This process is called transcription and such copies of the genome are called mRNA (messenger Ribonucleic Acid). mRNA is a single helix compared to the double helix structure of DNA
- This mRNA copy is translated into protein by a cellular powerhouse known as a ribosome.
- The protein is then dispatched to fulfil its function in the body, and the mRNA copy degrades after translation because it is a single helix and not nearly as stable as DNA
- Rinse and repeat a ton of times for the rest of your life
The portion of DNA that is translated into protein as well as the physical location where said translation takes place may vary with external stimulus. For instance, if you eat a sugary meal, DNA translation will occur in the “beta cells” of your pancreas to produce insulin. Insulin is a hormone/protein that functions to help the cells of your body utilize the sugar in your blood as a source of fuel. Such timely and efficient translation of DNA into protein is a major part of the central biological dogma that supports and maintains all life. Remarkably, this translation of DNA into biologically functional proteins occurs automatically in our bodies without the need for any conscious thought or effort on our part. This has an evolutionary advantage because if we all had to remember to make insulin every time we had ice cream, a lot of us would forget to do it and eventually die as a consequence. Moreover, you and I would be hard pressed to find time for anything else if our conscious minds were constantly engaged in the translation of DNA to protein… we’ve got lives to live, books to read, and beautiful things to create. In addition to controlling a wide variety of metabolic processes in your body, your DNA is also responsible for maintaining your physical form. You look the way you do because of the genes that were passed on to you by your parents at the moment of your conception.
During conception, a male sex cell (sperm) and a female sex cell (ovum or egg) fuse to form a fertilized egg which is technically known as a zygote. Every living human being started life as a zygote… an impossibly small living entity. The fusion of the sperm and the ovum endows the zygote or fertilized egg with a full arsenal of DNA sequences or genes that are required for growth, development, and maturation… all other things being equal. At this point, you may be wondering if the fusion of the sperm and the egg cells yields a cell with 4 copies of every gene and the answer would be no. Nature in its infinite wisdom has already solved that problem by making sex cells the only “haploid” cells in the human body. In plain english, “haploid” just means that sex cells (sperm and ovum) have only one copy of each gene as opposed to diploid somatic cells which have two copies of every gene. Thus, the fusion of two sex cells yields a diploid organism which possesses 2 copies of every gene in each bodily cell. Roughly half of a zygote’s genes come from mom, and the other half come from dad. This is loosely why you might have an uncanny combination of your dad’s smile and your mom’s eyes for example. I for one have a weird combination of my father’s smile and my grandfather’s facial features.
As a practical example of how genes are passed on, let us look closely at the process of sex determination in a human being. For the purposes of the explanation below you can assume that chromosome means “really long genomic sequence” i.e. a long sequence of A’s, C’s, G’s, and T’s: the nucleotides that we discussed above. Your mother (assuming she is still amongst us) has two X chromosomes on her sex locus because she is female. Your dad (assuming he is still amongst us) has one X chromosome and one Y chromosome on his sex locus because he is male. A mother can pass on either of her X chromosomes to her offspring while the father can pass on either his X or Y chromosome. If the sperm that fertilized the egg had an X chromosome, the child will be female (XX) because the mother cannot help but pass on another X chromosome. If the sperm that fertilized the egg had a Y chromosome, the child will be male (XY). All genes are passed on in similar fashion from parents to offspring. The classical “punnett square” (or in my case, punnett rectangle) from genetics 101 below pictorially represents this transmission of genetic information.
The entire genetic code of a human being is about 3 billion nucleotides long and as discussed above, this code is present in almost every single cell of the body (red blood cells are a notable exception because they lack a nucleus). The fact that your genetic code is remarkably long and complex is very apropos because you my friend are an amazingly complex being. Think about it for a second… as you are reading the words on this screen, your eyes are converting photons (packets of light) into electrical impulses that your brain can grasp all while you are probably digesting the last meal you ate and circulating blood all around your body. To make it even more amazing, most of these aforementioned functions are all performed on complete autopilot with no conscious effort required on your part. It stands to reason therefore that an extremely comprehensive set of instructions would be required to simultaneously direct such an involved symphony of diverse operations in a fashion that is so seamlessly efficient, that we humans aren’t even consciously aware of what is going on within our bodies. So next time you look in the mirror, please refrain from nitpicking and finding fault with everything you don’t like about yourself like most of us do from time to time. Rather, let us all take time to realize and appreciate how awesome this majestic vessel called the human body truly is.