Hi, in this video we're going to be talking about the structure and function of DNA. So we've already reviewed this a little bit and I'm sure you've gone over most of this in your intro classes. But we're just going to hit it home one more time so that we can all be on the same page both in this current topic and in the next few topics that we're going to be talking about. So nucleotides are the building blocks of DNA. There are four bases: these are A, C, T, and G. Or, how I have them ordered, A, C, G, and T.
They're divided into two classes. So the pyrimidines have one carbon ring and the purines have two carbon rings. So, if we just take a second to look at this, you can see here there's one carbon ring, so this is a pyrimidine. And this is two carbon rings, so this one here is a purine, and I have them here. The purines are A and G, and the pyrimidines are C and T. Let me disappear for a second. So yes, the pyrimidine here or the one ring; the purines are the two rings.
Let's talk about Chargaff's rules. These are rules that say which nucleotides pair with each other. So, A pairs with T, and C pairs with G. Each time these pair together it's called a base pair, and they pair through either two or three hydrogen bonds. So you can see here that when A pairs with T, you have two hydrogen bonds. But when G pairs with C, you have three hydrogen bonds. Each one of these is called a base pair.
Due to the size and the chemical nature of each base, a purine can only pair with a pyrimidine. So here we have a purine of G and a pyrimidine of C; we have a purine of A and a pyrimidine of T. These individual bonds are eventually linked together to form the backbone of DNA. This is created between bonds between sugar and phosphate groups on adjacent nucleotides. So the 5' phosphate group binds to the 3' hydroxyl group in the neighboring molecule, giving it directionality. You always have the 5' here and the 3' here, and their binding in this direction.
Because there are these charged phosphate groups, the bonds between sugars and phosphate groups make them polar, which means they can interact with water really easily. So, if we're looking at the DNA strand here, you can see that there's a 5' end and a 3' end on each strand. So each strand has directionality. You can see here that you have these base pairs here between the bases and you have the sugar-phosphate backbone. See the phosphate here and the sugars here, and these bonds form the sugar-phosphate backbones that allow the nucleotides to face into each other.
When this structure is formed, it actually folds into a double helix, which is the most energetically favorable form that it can have. The DNA double helix is created through bonds between two linear strands of DNA. The sugar-phosphate backbone forms the outside edges with the bases facing the center. So you can see this is exactly what is shown here, and that's exactly what it will be shown down here in this example, with the phosphate backbone circling around and then you have the bases in the center. For every helical turn, there are 10 base pairs, and one helical turn adds 3.4 nanometers to the length of DNA, which means there's 0.34 nanometers per nucleotide.
The strands in the DNA double helix are complementary because they bind together. Complementary means that A and T bind together, and C and G bind together. So each strand is complementary. If there's a T on one strand, there's going to be an A on another; if there's a C on one strand, there's a G on the other. It also means that it runs antiparallel, which refers to the directionality that I've mentioned repeatedly. The structure of a DNA double helix has a major groove, which is larger, and a minor groove, which is smaller. This is just how the DNA double helix forms.
You're probably familiar with at least most of this just from your intro classes, but there's a new layer that we are going to add on to this in cell biology. There are three types of DNA double helixes, labeled B, A, and Z. B is the most common and the one you're most familiar with. A is rarer and shorter and is considered a right-handed helix, which means if you're looking down on the helix, it turns to the right. Z is a left-handed helix and its significance is completely unknown. You can see that there are significant differences in their structure, both when presented in this format and when looking at a cross section.
With all of that review and a little bit of new information, let's move on.
