I don't know how you feel, but I always feel very frustrated that Rosalind Franklin isn't better, more widely recognized for her incredibly massive contribution to our understanding of DNA structure. It's really hard to overstate how revolutionary our understanding of DNA structure has been to science in the last 70 years. It really has changed absolutely everything, including our sequencing of most species now, the human genome. It's been revolutionary in terms of medicine, and I predict it'll get even more revolutionary as we get better able to take what we know of human DNA sequence and turn it into better therapies and drugs and really a massive contribution that she made in science. The structure of DNA is a double helix, as we said earlier.
So it's got these two strands winding around each other. And they're really... two molecules, so we'll explain how this works in a minute. RNA is a single molecule, so here's the backbone. We'll talk a little bit more about the backbone in a minute.
Both DNA and RNA have nucleobases, so in the case of RNA, they're composed of cytosine, guanine, adenine, and then uracil. In the case of DNA, we have the same first three, cytosine, guanine, adenine, and then the fourth base in the case of DNA is called thiamine. And you'll notice that in DNA these bases are, they're actually hydrogen bonded together, so they're loosely bonded, whereas the backbone is made of covalent bonds.
And then in RNA the bases are hydrogen bonded, sometimes they hydrogen bond to each other in a single molecule, whereas this is actually two molecules hydrogen bonded together. All right, let's take talk a little bit more about base pairing. So in DNA, Adenine always base pairs with thymine, and cytosine, guanine always base pairs with cytosine.
And we talk about this strand being complementary to this strand. So what we're saying is they're not identical, but they're predictable, they're complementary. So if, for example, you know that this strand, its sequence is A, G, T, C, the complementary bases in DNA will always be T, C, A, G, because A always base pairs with T, and C always base pairs with G. I'll point out another feature here.
Do you see that this strand is 3'on this end and 5'on this end? And this strand is 5'on this end and 3'on this end. We talk about directionality. So DNA, when we think about how it gets synthesized, it's always built in the 5'to 3'direction. So here again we're going 5'to 3'and you'll notice that they're going the opposite direction.
So we say this, they're anti-parallel. And I'm going to talk more about that in a moment. Another thing I want you to notice is just that there, the hydrogen bonds between adenine and thymine, there's actually two hydrogen bonds.
And then between cytosine and guanine, there are three hydrogen bonds. Okay. So two hydrogen bonds between adenine and thymine, and always three hydrogen bonds between cytosine and guanine.
So this is a, this is a stronger. interaction between cytosine and guanine because it's got another hydrogen bond holding those two bases together. Now an RNA structure, and this is DNA here, right? We have our double strand and they're hydrogen bonded, but you see how it's starting to kind of unzip and that's important later on for function. We also have our nitrogenous bases in RNA.
However, in RNA, sometimes There is some base pairing that happens, but it's usually within the same molecule. So here is an RNA molecule. Do you see how it's a single molecule?
It's wrapped around itself. But do you see how in the middle here, some cytosine is base pairing with guanine and some adenine is base pairing with uracil? So this occurs in RNA.
It's called intramolecular hydrogen bonding. And it's very important for the structure of RNA, for example, transfer RNAs. So you'll see this later on when we talk about protein synthesis, this intramolecular hydrogen bonding. that we sometimes see with RNA. All right, I want to go back to the anti-parallel nature of DNA, the double-stranded DNA molecule.
This gets important in terms of understanding DNA replication, and DNA replication is fundamental to how genes are passed down from one cell to the next and one from one generation to the next. So we're going to spend a little bit of time on it. The first thing I want to do is point you to the sugar.
That's the sugar molecule. Let's come over here for a moment and look at it kind of blown up. Do you notice that this is, we've got this one prime here, this is carbon number one.
That's what the number stands for. Carbon number two. carbon number three, carbon number four, and carbon number five.
So five prime refers to this carbon, and three prime refers to this carbon. Let's go back to our molecule over here. Do you see that this is the five prime carbon here, and it's bound to a phosphate? So this is the end of this strand, so we say this is the five prime end, because this carbon is sticking out, and the five prime carbon is what's is at the end of this particular strand of DNA. And then remember that it's a sugar, in this case, phosphate sugar, phosphate sugar, phosphate sugar.
So we're alternating. So the phosphates hold on to the 5'carbon and then 3'carbon. See, this phosphate has a bond to the 3'and then the 5'and then the 3'and then the 5', etc. And when you look at the bottom of this strand, you'll notice it's the 3'.
carbon that's sticking out at the bottom. So this strand runs 5 prime to 3 prime this direction. On the other strand, notice how the sugars are upside down? And this is the 5 prime carbon.
So now the 5 prime carbon is at the bottom. And so we have 5 prime, 3 prime, 5 prime, 3 prime, 5 prime, 3 prime. All right, so now the 3 prime end is what's sticking out at the top.
and the five prime is at the bottom. So this strand runs the opposite direction. So this strand is running down and this strand is running up.
So we have two complementary strands of DNA running in opposite directions, and this is what we mean when we say anti-parallel. So this is an important slide and you may want to come back to it after we talk a little bit about DNA replication. This may help you understand why DNA is synthesized the way it is because this anti-parallel nature of DNA structure is important to consider when thinking about DNA replication. Alright, so the next video is going, we're going to talk about the structure of DNA and how this, the structure tells us about DNA replication and how DNA gets copied.
And that was why Francis Crick and James Watson were so excited when they realized they had the structure because they didn't just get the structure of DNA. When they looked at the structure, they understood that that explained how DNA works.