 Let's label the 5 prime and 3 prime ends of this nucleotide. Remember how I told you that the carbons, the fifth carbon in my pentose sugar attached to the phosphate and the third carbon is where we could potentially attach another sugar molecule or attach another nucleotide. This image shows you the actual chemical structure of a nucleic acid or a nucleotide. And you can see the actual carbons have been labeled here. So notice that this would be called the 5 prime end and this would be the 3 prime end. And I want you to think about the 5 prime end is the phosphate end and the 3 prime end has the sugar on it. And I want you to think about that because now we're going to put several nucleotides together into a DNA molecule, which is a double helix. And look, we've even got our 5 prime end and our 3 prime ends labeled on this drawing. But it makes complete sense. It's beautiful. Thank you very much, fellas. Here is one nucleotide. Oops. Okay, I'm going to undo that. This is one nucleotide and it is chemically bound to another nucleotide, which is bound to another nucleotide, which is bound to another nucleotide. Okay, you get the idea. In my double helix, it's like a ladder and you have a sugar phosphate backbone. Here's a phosphate, here's a sugar, here's a phosphate, here's a sugar, here's a phosphate, here's a sugar, here's a phosphate, same thing on the other side. And then we've got these ladder rungs, which are the nitrogen bases. And the nitrogen bases connect to each other with hydrogen bonds. That's those are the dotted lines that you can see there. So these guys, the nitrogen bases form the ladder on my, they form the rungs of the ladder. And the sugar phosphate backbone forms the, I don't know, the, I know I had a word for it, the things, the side things of a ladder, those things. What do you notice about the nitrogen bases? I love this. Do you notice that one is big? Like look at these guys, these are big bases. Look, this blue guy is a big base. Here's another big one, who's the blue guy? The guanine and the adenine are big. And I'll tell you right now that guanine and adenine, I'm going to draw it the other way, adenine and guanine are purines. Do you want to hear my crazy way of remembering this? Big bases have a short word, and cows are big, and ag makes me think of cows. And I'm not joking you, I have to think that every time I remember who are my purines and who are my pyrimidines? My pyrimidines, pyrimidines, pyrimidines are the little guys, the little bases, look at the little shorty short. Here's another little shorty short. A shorty short, a shorty short. Pyrimidines are C and T. They're short, long word. I don't know why that works for me, but it totally does. A purine must bind with a pyrimidine, otherwise the rungs of your ladder are not going to match up. And that's one of the rules. In fact, that was Old Boy Shargaff's research. Let's go back and find it and see. He noticed Shargaff, the playboy, the player, he noticed, look at this, he noticed that the number of A's and T's, adenines and thymines in a critter were roughly equal, while the number of guanines and cytosines were also roughly equal. And these are percentages, so they basically blenderized human DNA, and counted how many adenines and how many thymines and how many guanines and cytosines out of all of it, what percentage were each individual base. And he found this to be true all the way through, which they were thinking, dude, why is that the case? Why is it that the percentage of adenines equals the percentage of thymines and the percentage of guanines equals the percentage of cytosines? We look at it across the board and it's true in all these different critters that they figured it out with. The number of adenines and guanines, like, it's not equal, and we're going to look at, when we get to DNA function, we'll look at why, what is all this, why do we care? And you will know why we care about the, what, the distribution of A's, T's, G's, and C's. But Shargaff was on it back in the 40s when he figured out that these are equal. This was a key part of Watson and Crick determining the structure of the DNA molecule because they have to bind. If you have an adenine, you have to have a thymine in your DNA molecule in order to bind to that. And if you don't have a thymine, you're going to have a hole in your DNA molecule, which means it's not going to be very stable. That ain't going to fly, dog pounds. That ain't going to fly. DNA is not the only, it's not the only nucleic acid that we care about when we look at DNA function overall. So we're going to take a second to compare DNA to RNA and see if we can make sure we understand the structural differences between those two types of nucleic acids.