I have had a number of science-related posts in mind since I first started this blog, but time and again I find myself putting them off as too complicated. One of my goals is to make science friendly to the average person, and so I try to provide some background in order to provide the casual reader with a basic understanding of the subject. The result is that some of my posts start to get a little lengthy, and so I have not written about a number of topics that I find interesting because I fear that I will spend way too much time on the background and lose interest before reaching the main point. So after giving this some thought, what I have decided to try is writing a few posts about the basic science behind some of the topics about which I intend to write so that I can reference them in future articles.
One of the common themes in both my writing and research is the nature of heredity. What distinguishes humans, cats, fish, butterflies, and bacteria? How is this information shared from parent to offspring, and how does it change? We often simplify such ideas to make them friendly to the public and easy to teach, but the reality is often far more complicated, and not as well-understood as people like to think. I considered starting with the idea of genes, which seems like a good place to start when talking about heredity, but I wonder if it might not help to have some understanding of DNA first? Of course, to really understand DNA, we need to talk about chemistry, and to talk about chemistry we need a little primer on physics…see how my mind works? I am making it too complicated already.
So I have chosen to start with nucleic acids, which represent one of the major categories of macromolecules that make up living organisms, along with carbohydrates, lipids, and amino acids (proteins). The most familiar is probably DNA (pictured in the image above), which is an abbreviation for deoxyribonucleic acid (Deoxyribo Nucleic Acid). Another very important nucleic acid is ribonucleic acid (RNA). Nucleic acids are required for life as we know it, as they are used to both encode and express the genetic blueprints that give rise to a living organism. In general, the role of DNA is primarily to serve as a repository for genetic information in the organism, while RNA plays a number of different roles related to the expression of that information.
Structurally, nucleic acids are polymers, which means that they are made up of a repeated subunit. DNA and RNA are made up of almost identical subunits that consist of three components. The first component is a sugar that is made up of five carbons that form a ring, known as either ribose or deoxyribose. The ring that makes DNA (deoxyribose) differs in only one regard from the ring that makes RNA (ribose), as you can see in the image to the left. Ribose possesses an extra hydroxl group (an oxygen and hydrogen), while the deoxyribose has only a hydrogen in that location. The polymer consists of a “backbone” or “chain” that is made up of many of these sugars that are connected by second component, which is a phosphate group. This forms an alternating chain of sugar – phosphate – sugar – phosphate and so on, as you can see in the image on the right. The third component is known as a nitrogenous base (represented by the B in the image on the right). Together, these three components make up a nucleotide (defined by the area in brackets labeled n in the image on the right). DNA and RNA share three different bases in common (adenine, cytosine, and guanine), while there is a fourth base that is unique to either DNA (thymine) or RNA (uracil). You can see the difference in the image to the left. These bases actually bind together in a very specific fashion, like puzzle pieces, and allow nucleic acid chains to join together, as we will see in a moment. As you might imagine, drawing out nucleic acids like this all of the time becomes cumbersome, so you will usually see them depicted as a strand or a ribbon. The image below represents a strand of RNA and a double strand of DNA in this fashion, and shows how the base pairs can join together.
As a result of their functions, DNA usually exists in a double-stranded state and is much longer than RNA, which is more often found in a single-stranded state and in much shorter lengths. This is an oversimplification, of course, as the size and structure of these nucleic acids can change, and certainly varies depending upon the role that it is playing at the time, but for convenience we often depict them in this fashion. This was a fairly brief overview of the molecular structure of nucleic acids. In upcoming articles, I will take a look at the roles of DNA and RNA, and start addressing genes and heredity.
Thanks for reading!