If you already know something DNA and molecular biology techniques, I’m sure that you too will be laughing your knockers off about the pun in this post’s title. You see? An understanding of science really does make the world a brighter place!
However, if you’re one of the normal people, you almost certainly won’t get the joke. And this makes me sad. I want this blog to appeal to people across the spectrum. So occassionally I’m going to write about a broad topic that is crucial to science, and try and de-mystify it. Today I want to tackle some of the basics of DNA. It’s a big topic, so I will do this over a couple of posts I think.
We are living in a world where it is becoming increasingly important to have some understanding of DNA. There seems to be a more or less endless stream of stories in the papers talking about scientists finding the gene for this and that. In terms of the challenges that the increasing global population poses to sustainable food production, we’re going to have to start facing the reality of more and more GM foods. You don’t have to be a molecular biologist to be in a position where some understanding of genetics is necessary and/or assumed.
I’m sure everyone’s heard or read that “DNA is what makes you you” or words to that effect. Perhaps you’ve heard it described as the “blueprint to you”. These descriptions, though true in broad strokes, are not really terribly informative. How does DNA do this? What is so special about DNA that bestows it with these properties?
The DNA in your cells is essentially a vast molecular machine that is central to a cascade of events that shape each individual cell. This is a point I really want you to take with you. All too often people talk about DNA in almost the same way they talk about magic and ghosts. But when you strip away the hyperbole, DNA is merely a massive molecule. Of course, I say “merely”, but it is the molecular nature of DNA that makes it what it is. We’ll come back to this, but for now, I just want to make the point that it is not magic.
At this point, we’re going to introduce a new phrase: genome. The genome of an organism is the total DNA contained within the nucleus of a cell. In humans, that genome is spread over 22 paired chromosomes plus 2 sex chromosomes (the X and Y chromosomes). On the other hand, bacterial genomes, for example, tend to be found as a single circular loop of DNA, also known as a chromosome. The human genome has 3 billion base pairs in it, which is a huge number. Sequencing initiatives, such as the Human Genome Project, have allowed us to read the sequence of all those bases, and we’ve been able to do it on smaller scale for some time now.
This brings us onto what can really be considered the most important concept about DNA. What does all this talk of base pairs, bases and sequences actually mean? Here I want to show you that it is the molecular structure that is all-important with regards to the function of DNA. In the sub-microscopic molecular world, DNA consists of repeating motifs with 4 basic variations. What you have is 3 molecules stuck together. This group of molecules is called a nucleotide. One is a phospate backbone. One is a sugar molecule called pentose. The third one is a “base” molecule.
Crucially, the phospate and the sugar remain the same from nucleotide to nucleotide and it is the base molecule that can vary. This is the key to the whole reason why DNA behaves the way it does.
The 4 bases are called adenine, thymine, guanine and cytosine. It is from these names that we get the code with which we denote the sequence of nucleotides: ATGC. As it is only the bases that vary, we need only note these to tell us the nucleotide sequences.
Right. Deep breath. We’re about to uncover another crucial aspect of DNA. The phosphate molecule can join to the bottom of the sugar from another nucleotide, forming a phosphate backbone. This is how you get chains of nucleotides (eg GCCGTAATGC…) This process is, in theory, open ended: there will always be sugar at the bottom of the chain for another phosphate to attach to.
You’ll notice in the above diagram that the different bases have different shapes. You can also see in that picture that A looks like it would connect to T nicely. This simple schematic actually represents the real molecular system fairly well. The bases are different shapes and A does bind to T nicely. As it happens, C also binds to G in an equally specific way. Critically, other combinations won’t fit and so nucleotides always bind to a consistent partners: A to T and C to G.
This property gives rise to base pairs forming and another strand of DNA being assembled to complement the sequence of the first strand. The partner nucleotides to the sequence in the first strand pair up and are able to join together giving rise to the second, complementary strand.
This big composite molecule, a macromolecule, exists in 3D, not just the flat 2D world of a diagram. In the 3D world, the two strands coil around each other to form the famous double helix.
I think that covers enough for one post. I don’t want to overdo it as I know there’s quite a lot to absorb here, especially if you’re not familiar with it. In the follow up to this post we will go on to find out what genes are, what chromosomes really are and what the winning lottery numbers will be.
One of those promises may not be true.