Most people are, at this stage, familiar with the general concept of a ‘gene’ and DNA. Both are used, it seems, as part of everyday vernacular, and the basic understanding seems to be there: genes are things you inherit from your parents, and define characteristics about you, and DNA is the key molecule in you that makes you you, and contains all the instructions to make you who you are. Given these common pieces of knowledge, it should come as no surprise that genes are to be found in your DNA. So genetics, the study of genes and their interactions within living organisms, is, in one sense, a study of DNA. But what exactly is DNA?
It’s important to introduce some basic scientific language here, and I will bring it in slowly and show that it can be used as helpful shorthand to help us rather than confuse us, as it so often seems determined to do. Everything in this planet (particle physics notwithstanding) can be thought of as being made of atoms. Atoms come in a number of sizes, and when atoms join together in one of the number of ways they can, they form molecules. DNA is a large molecule, and because it is found exclusively in living systems (like you or I), it is called a biomolecule. More importantly, DNA is a biopolymer.
What’s a polymer?
A polymer is just a molecule made of repeating units (these units are called monomers). Think of a model train set. You have a number of carriages, and not all of them are exactly the same, but some of them might be. What’s important is that all of the carriages can fit together, and they do so in the same way every single time. You join a red carriage to a white carriage to a coal carriage to a passenger carriage, but you never connect a carriage to, say, a toothbrush. Similarly, not all monomers in a polymer are necessarily identical, but they are all of the same type and they usually connect in very similar, if not identical, ways. So that’s what a polymer is, and biopolymers (such as DNA) are just polymers which exist only in living organisms.
If DNA is a biopolymer, what monomers is it made of?
DNA is made of a type of a monomers called nucleotides. All nucleotides have three components, two of which are identical amongst all nucleotides in your DNA. Those two components are: a sugar molecule called deoxyribose (the D in DNA), and a small phosphate group. The final, variable component is called a base. In DNA, there are four categories of base: Adenine, Thymine, Cytosine, and Guanine. Each nucleotide is referred to by the identity of its base. These four bases are all reasonably similar, but their order is extremely important. The DNA in your body contains roughly 12 billion nucleotides in specific orders, so for shorthand geneticists refer to the nucleotides as As, Ts, Gs, and Cs. So, DNA is a biopolymer consisting of a series of nucleotides which each have one of four distinct identities. The order of these nucleotides encode the instructions to create and sustain a living organism – just like how binary code encodes instructions to run computer programs. And your exact code is uniquely your own – nobody else in the world has the same DNA as you (identical twins excepted).
Where do I find my DNA?
Your DNA is found in your cells. You’re probably familiar with the idea of cells, but if you’re not, I’m going to shock you by telling you that your body, despite appearing to be one single functioning machinery, is actually made up of thirty trillion microscopic blobs called cells. There are lots of different kinds of cells in your body, and they perform different functions depending on where they’re located, what type of cell they are, and how old you are, but almost all of them have the exact same set of DNA in them. That’s why CSI cops can get a tiny sample of saliva or skin from a crime scene and find a killer within an hour of TV (containing ads, no less). There are a couple of notable exceptions – red blood cells, which contain no DNA, and your sex cells (sperm or eggs), which contain more or less exactly half your DNA.
Cells are home to all kinds of different cellular machinery and different compartments and chemicals, but luckily DNA is all found in one place – the nucleus (plural nuclei). Just as all your cells have DNA, all your cells have membrane-bound nuclei containing the DNA, but do all living cells have nuclei containing their DNA? The answer is no! This is somewhat academic if you’re only interested in human genetics, but it’s worth noting that most single-celled organisms, like bacteria, don’t have a nuclei, instead having a general unbound region of the cell called the nucleoid which contains the bacterial DNA.
The shape of DNA
This is perhaps the hardest aspect of DNA to properly explain, but thankfully it’s also its most well-known characteristic. The shape of DNA, as discovered in the 1950s by Rosalind Franklin, James Watson, and Francis Crick, is the famous double helix.* A double helix is two strands running in parallel, with both strands moving in a helical shape. This is shown below.
On the inside of the double helix, forming links between the opposing strands, are the bases. Bases from one strand form weak bonds with the corresponding base on the opposing
strand. Importantly A only binds with T, and G only binds with C (and vice versa)**. The strands themselves are composed of the sugar and phosphate components of the
nucleotides (referred to as the sugar phosphate backbone). Of course, that image is a simplification of the actual molecular structure of DNA. Below is an image showing a more accurate computer model including all of the atoms involved in the DNA, colour-coded. This isn’t important to understand in any real detail, but it is good as a reminder that very simple diagrams are just that – simple diagrams designed to help us understand more complicated systems.
Chromosomes
I have a confession. I studied biology, and with it genetics, for four months before I finally googled what chromosomes were. My teachers simply assumed we all knew what chromosomes were, and whilst I had heard the word before, I wasn’t entirely sure what exactly they were. So, I won’t assume you do either.
Chromosomes can be thought of as a kind of secondary structure of DNA. Not all of ou
r DNA is bundled into one very long double helix accounting for all of our genes. It’s actually bundled into 23 pairs of very long double helixes, and each of these bundles is a chromosome. Most human cells have 46 chromosomes, each containing between 48 and 249 million base pairs of DNA (a base pair is just a bonding pair of nucleotides, one from each strand, and is the basic unit of measurement of physical length for DNA). We get 23 of these chromosomes from each parent, and they all have names. Thankfully, the names are mostly just numbers – most chromosomes have a number between 1 and 22 (you have two of each), and then finally you have your sex chromosomes – these are both X chromosomes if you’re biologically female, and an X and a Y chromosome if you’re biologically male.***
A small technical note that chromosomes don’t consist solely of DNA. DNA does not exist free floating in our cells, it exists bundled around large molecules called histone proteins. These proteins, and several other molecules constitute what is known as the epigenome, and the structure and shape of these molecules affect many health conditions and physical characteristics.
So where do genes come into all of this?
So far we’ve discussed the basic structure of DNA and how it can be read as a form of code consisting of As, Ts, Gs, and Cs. Now comes time to narrow in on our definition of a gene. Genes, for most people’s general purposes, are simply sections of DNA which provide specific instructions to code specific proteins. Not all sequences of DNA constitute genes (particularly in complex multicellular organisms like humans), but all genes are made of DNA. It’s important here to quickly clarify what I mean by protein. Proteins are just a different type of biopolymer to DNA. Where DNA is constructed of nucleotides, proteins are formed from a category of monomers called amino acids. Proteins can be thought of as the machines that do all the work in a cell. They can be motor proteins, which assist in moving things around the cell, enzymes, which work to help certain chemical reactions go ahead, structural proteins, which form structures within the cell, or something else entirely. Whereas DNA only has diversity insofar as the order of its nucleotides and its length, proteins have diversity with regards to size, shape, and constituent parts. In fact, they are so diverse and at times complex that for years it was believed almost without question that genes existed in proteins, and any thought that genes might exist in something as simple and limited as nucleic acids was scoffed at.
The concept of genes and DNA so far seems quite simple (hopefully). The final question I want to ask is, if it’s all this simple, why is this such a complex area of research?
Genes and phenes – where it all starts to break down
Compton rapper Kendrick Lamar is on the record saying that he has not only royalty, but also loyalty, inside his DNA. But what does this mean? Has he got a sequence of DNA on his second chromosome which makes him royal? Has he got the loyalty gene? Could he possibly be making it all up? The reality is that for most attributes we have, the relationship between gene and attribute is rarely 1:1.
In mid-19th century Austria, botanist and Augustinian monk Gregor Mendel wrote a paper based on an experiment he’d conducted with pea plants in his monastery, and in doing so discovered the foundations of modern genetics (Mendel is often referred to as the father of genetics). Unfortunately, Mendel was more of a monk than a self-publicist, so this work went on almost completely unknown until the beginning of the 20th century. He did experiments on peas which correlated ‘discrete units of heritability’ (what we now call genes) with physical traits. Physical traits affected by our genetic code are known as ‘phenotypes’.
Mendel was a great scientist and a huge influence on modern biology. He was also insanely lucky. His experiments with peas studied seven phenotypes which all, in the species he was studying, were almost entirely controlled by one gene each. The usual reality of genes and phenotypes is a little less direct. Most observable phenotypes depend on interactions between more than one gene, and most genes influence more than one phenotype. The complex reality is, phenotypes, including diseases, are outcomes of extremely complex genetic interactions, the exact extent of which is not even close to being properly understood. There is a long way to go, still.
So, whilst Kendrick probably doesn’t have a short sequence of nucleotides endowing him with royalty, or a gene making him loyal, he probably wasn’t making it up. The royalty and loyalty is in there, somewhere. We just need to know how to find it.
I hope you learnt something new!
Thanks for reading,
Jack
Read more:
Any tertiary level biology textbook you can get your hands on, or any online learning resource on biology (I like Khan Academy for introductory stuff like this, but edX and Coursera have some great stuff with more detail).
*The exact nature of its discovery is a pretty spicy piece of science history and a story worth looking into if you like gossip and drama, especially the kind with microscopes and thinly veiled sexism.
** There are exceptions to this (there are exceptions to almost everything in biology), but these exceptions will be covered elsewhere and this rule generally holds very well.
***A caveat that these are simply biological definitions of sex and not gender. Importantly, there are further genetic elements to contemporary biological definitions of human sex, but these are a little complicated to be introduced in a beginner’s guide and will be discussed elsewhere.
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