 Okay, let's talk about what they figured out. The first thing that I want to make sure we're all cozy on is the structure of a nucleotide. DNA is a nucleic acid, acid, that's one way to write acid, and it's made up of nucleotides. And this is review because we've already done this. Nucleotides consist of a phosphate, a pentose sugar, and a nitrogen base. We already learned in the last section that we have four possible nitrogen bases. We have this one right here, which is guanine. And do you remember, I don't know how to list them for you. Do you remember who guanine binds to? Cytosine. This is Shargath, who remember how he said, like, God, when you look at the number of guanines, it's the same as the number of cytosines. And if you look at the number of thymines, it's the same as the number of adenines. Why? How is that possible? It's because these nitrogen bases have bonding patterns. And guanine and cytosine form a bond with each other. And in fact, they form a bond that creates the rungs of our double helix ladder. Thymine and adenine also form that bond and make the rungs of the ladder. There's a couple of things that I want you to know about these, these bases. I'm going to highlight two of them. I'm going to highlight guanine. And I'm going to tell you right now. Guanine is similar to adenine. It doesn't bind with adenine, but it has a similar structure. Guanine and adenine are called so many color options. They're called purines. And I'm sorry about this. How's that? So all of them, guanine, cytosine, adenine, and thymine are all nitrogen bases. And so they hold that position in the molecule or in the DNA nucleotide. Guanine and adenine are purines. They're bigger than cytosine and thymine. So let's, you can imagine, right, that cytosine and thymine, oh my gracious, have another name. They are called pyrimidines. A purine always binds to a pyrimidine. How's that? So my purines are A, oops, I'm not sure what that was, A and G, and my pyrimidines are C and T. I have a way of remembering this, and I'm sure that you guys have better ways of remembering this, but purine is a smaller word, but ag is, makes me think of cows and makes me think of bigger ag, big cows, and those are the big nucleotides, nitrogen bases. Pyrimidines are smaller and they're like a cat, a cow in ag or a cat, a cat is small, but has the bigger word. The pyrimidine is a bigger word. I'm sure you have a better way of remembering that. Okay, before we go, that's going to be huge when we talk about how we're going to put the DNA together. The other thing that we need to do is we need to look at how DNA, I've got numbers on my pentose sugar and hopefully you're looking at that going, dude, what is that even? What does that mean? If you remember from how we draw chemical structures, the nodes in these big structures often are missing the molecule or the element that the atom that makes up that position, and it's always, if it's missing, it's always a carbon. Carbon. Okay, so you have five carbons, which makes sense because it's a pentose sugar. I better tell you the name of the pentose sugar in deoxyribose nucleic acid. It is deoxyribose. That's the name of our pentose sugar that makes up a DNA nucleotide. Those five carbons give us directionality in our nucleotide and this is helpful. This is something that helps us have context. The five prime end of the sugar attaches to the phosphate and establishes a five prime end of the nucleotide. The sugar's three prime or third carbon is the one where another nucleotide can attach, so this is called the three prime end. If you look at this, carbon number one is where you attach your nucleic acid, I mean your nitrogen base. We don't care so much about the nitrogen bases. It's interesting to know that it attaches to the first carbon, but the three prime and the five prime give us a directionality that's super important when we start putting lots of them together. So let's take a look at what the whole thing appears like. You'll notice, well what do you notice? This would be a place where you push pause and see what you see here. Do you see the individual nucleotides? Here's one, right? And then here's another one. Whoa, whoa, right? Here's another one. That's actually really hard to do and to not like accidentally grab anyone else's parts. Here's another one. Yikes. What do you notice about this double strand of DNA? It forms a sugar phosphate backbone here. To hear on both sides, we have a sugar phosphate backbone. The sugars and phosphates get added in a sequence that you end up with the whole molecule. If you just keep adding and adding and adding forever, you're going to always have a five prime end where this number five carbon is sort of exposed with this phosphate. I remember five prime phosphate. So if your nucleotide has a phosphate part, that's the five prime, I can say it, five prime carbon. And you have the three prime end. That's going to be the other end. There's no phosphate, just a sugar on the three prime end. If you look at the other side and this was the moment of where Watson and Crick, it was, and I think it was Crick who had the epiphany. He was imagining they were trying to build nucleotides and they were trying to put it together and they were imagining like trying to use the rules that Chargath had and trying to build a helix and how do you do this? And he had this epiphany that, oh my God, what if you flipped the nucleotides upside down so that the strands, they call it anti-parallel. They're running in opposite directions. I'm going to write that down. It's anti-parallel. From the five prime to three prime end one way is opposite from the five prime to three prime end the other way. I know another thing I need to tell you. I need to tell you that the nitrogen bases are connected to each other with hydrogen bonds. Interestingly, DNA is an incredibly stable molecule. We know hydrogen bonds aren't strong. They're actually easy to break. This gives us some idea of how DNA might replicate itself. Before we talk about DNA replication, which is one of the main points of this conversation, let's look at RNA and talk about how RNA structure is different from DNA.