8. DNA and RNA | Coconote

The President of the United States, Donald Trump, and his wife, have been arrested for the use of force in the United States. The president has been arrested for the use of force in Okay, let's get started. So, what I'm going to talk about in this lecture is DNA and RNA. The idea is for the next few lectures to give you a basic introduction to DNA and RNA. We'll go through some of the original experiments that determined DNA as a genetic material. and we'll talk about the structure and the process of replication of the DNA molecule. In this lecture, what we're going to talk about are those key experiments, and these are the learning objectives here. We'll talk about Griffith's experiment, which discovered bacterial transformation and the transforming principle. We'll then move on and talk about Avery McLeod and McCarty's experiment, which actually specifically identified DNA as the transforming principle. And then we'll talk about the Hershey-Chase experiment that showed the genetic material of phage T2 was DNA. So these are the sort of key learning objectives here for that, the sort of history. And of course that then brings us on to the discovery of the helical structure. by Watson and Crick and Franklin and Wilkins. And of course that comes from knowledge of purine and primidine ratios, nucleotide structures, and so on and so forth. And we'll cover all of those things as we go through this lecture. So these are the three classic experiments that led to Watson and Crick suggesting the double helical model. for DNA structure in 1953. So the first was Griffiths in 1928, and his experiments discovered the transforming principle. Then we have Avery McLeod and McCarty, which goes a step further and works out that transforming principle with DNA. And then we have Hershey and Chase that came up with this experiment, again finding DNA with the material using bacteriophage T2. So let's have a look at those experiments. So this is Griffith's experiments from 1928. And what he was interested in was streptococcus pneumoniae. So this is pneumococcus, and it infects the lungs. And what Griffith was interested in was how to go about coming up with a treatment or a vaccine for this disease. And there are these two different strains of pneumococcus. Serotype 2, and that looks like this down here. So it's this R strain, as you see here. And so it has this rough colony. So there's no capsule, but there's this rough colony. And this infects a mouse, but it has no effect. The immune system fights it off, and the mouse will survive infection. And on the left here we have serotype 3. This you can see has a capsule. So this is this smooth colony that you can see here. And this is pathogenic in mice. So when injected into the mouse, well, the bacteria colonize the lungs, the immune system doesn't get rid of the disease, and eventually the mouse succumbs to the infection and dies. So we can see that repeated here in these top two boxes. So smooth type 3, the mouse dies from pneumonia. Rough type 2, the mouse will survive. So what Griffiths did is an extra experiment here, where he took these pathogenic smooth type 3 bacteria and heat-killed them. and those heat kill bacteria had no effect on the mouse. The mouse survived. And, you know, it's showing quite clearly that that experiment has worked, that that process of heat killing is complete, and all of those smooth type 3 bacteria that are pathogenic when they're alive have been killed, and you can inject them effectively quite safely with no effect on the mouse. He then did an experiment where... He took the heat-killed smooth type 3 bacteria from above and put them into a mouse alongside rough type 2. So with this combination, well, you'd expect rough type 2. Well, that has no effect. It's non-pathogenic. The mouse should survive. And the heat-killed smooth type 3, we've just established that the mouse survives if injected with those heat-killed smooth type 3 bacteria. But actually what happens here is the mouse dies from pneumonia. And if you look at the lungs, then what you find is bacteria with these type 3 capsules. in the lungs of the mouse. And so what's going on here is that some property of these dead, heat-killed smooth type 3 bacteria is being transferred to those living rough type 2 bacteria. So what's happening is that the smooth type 3 bacteria are transforming the live rough type 2 bacteria. And of course we now know that transformation occurs because you get DNA released from these heat-killed smooth type 3 bacteria. That's then taken up by these living rough type 2 bacteria. and incorporating them to their genome and changing their properties. Effectively, those non-pathogenic rough type 2 bacteria take up DNA from these dead, heat-killed, smooth type 3 bacteria and effectively become pathogenic. So Dawson continued with this work, but actually changed it from a mouse-based system to an in vitro system. So effectively you're doing this in a test tube rather than having to do this using live mice. And what they did was they took those heat-killed 3S cells and showed that they transform living 2R cells in vitro. So effectively you can recover living 3S cells without the need to inject into mice. And it just simplifies the process. It makes the whole process of investigating and identifying the transforming principle, makes it all a little bit simpler in that you don't need those mouse hosts to actually do that experiment. And then in 1944, Avery, McLeod and McCarty They discovered DNA as the transforming principle. But of course DNA is a simple molecule and it was quite hard to convince people that DNA was the transforming principle. That idea was not universally accepted. So there are so many other better candidates, I guess, for what that transforming principle might be. More complicated molecules that would lead to that change in phenotype. Why would such a simple molecule like DNA be the thing that leads to that change in phenotype? So what Avery, McLeod and McCarty did, they took 75 litres of heat-killed 3S cells, and that gave them a soluble extract containing the transforming principle. And what they then did is they simply just went through a series of experiments where they removed different components of that mix. So they started off by removing the lipids. And once you do that, you can test, well, is the extract still functional? And yes, the extract is still active. So that tells you that the transforming principle isn't in the liquids. They then did the same with the protein, so they removed the protein, and again, the extract was still active. So that's telling you the transforming principle, it's not in the lipids, it's not in the protein, so it must be elsewhere. They then did the same with polysaccharides, and the extract was still active. So at this point, you know the transforming principle is in the lipids, it's not in the protein, it's not in the polysaccharides, and that doesn't really leave very much left. And so what's remaining at this point is the DNA, and the extract is still active. So that's suggesting here that that transforming principle is in the DNA. It's the only thing that's left in these tubes that can possibly transform those bacteria. And so to confirm that... What they did is they took an enzyme called DNAs and this breaks up DNA and that's all it does, it just digests DNA, breaks it up, breaks it down and finally at that point when the DNA was digested was the activity lost, and only at that point was the activity lost. So this is giving you really strong evidence here that, well, the transforming principle has to be in the DNA, that's the only thing left, and if we get rid of the DNA, we lose that ability to transform bacteria. So it's telling you the transforming principle is in that DNA fragment. So that's quite convincing. And it confirms DNA as the transforming principle. And it tells us that DNA has the potential to change the properties of a living organism. But of course what it doesn't tell us is that organisms use DNA as a genetic material. So it doesn't go that far, it just identifies what the transforming principle is. And so that's a follow-on step that has to be looked at and that sort of leap has to be made. And so that comes with this Hershey Chase experiment. And so Hershey Chase, they worked on bacteriophage T2, and you can see we have that bacteriophage here in the centre of this EM image, and you can see that it's got this head structure, this tail structure here, and then extruding from the head, you've got all of this material. This is actually the genetic material that's extruding from the head. So huge amounts of genetic material that are packaged into the head of that bacteriophage. And so they looked at the life cycle of bacteriophage. And it has this interesting life cycle. Effectively, what happens is that the bacteriophage will adhere to the surface of a bacterium. And it injects DNA into the cell. So this sort of purpley blue square here, this is the bacterial cell. And this is the DNA being injected into that cell. And so what that means is, once that's happened, what happens is you have effectively an empty head, and the sort of remnants of the bacteriophage and the genetic material is all inside of the cell. And that replicates, and you then get new production of new heads and tails. You can see those being made here. And effectively what then happens is the DNA is then packaged into these new heads and tails, the fibres are attached, and the bacterial cell is lysed, and those progeny phage are released into the host. And so this is quite interesting because what you can see from this is that there's only one small part that gets into the bacterial cell. It's only the DNA here, only the genetic material. that enters the infected cell. Everything else remains on the surface. And that allows you to do an experiment where you work out, well, what is happening? What is where? And so they did that by using radioactive labelling. And what they did is they labelled the coat proteins, so the heads and the tails, they labelled those with 35S that are shown here in red. And then they label the DNA with 32p, and that's shown here in blue. And so we've got this sort of clear distinction here. And so we can see what is where, and so effectively we've got the heads and the tails, they remain on the surface. And so what you should find is all of the 35S here is on the surface, none of it is in the cell. And for the DNA, well that's injected into the bacterial cell, so you'd expect all of that 32P to be inside the cell. And so what you do is you take these bacterial cells and you put them in a blender, you mix everything up, And it gives you this mix of phage ghosts and cells. And you spin that down. And you can see that here. And effectively, the cells are heavy. They spin to the bottom of the centrifuge. And all of these ghosts, these phage ghosts, they remain in the supernatant. And so this is what we've got here. We've got the sort of pellet at the bottom of the tube that contains the cells and the supernatant that contains the remnants of those phage ghosts above. And you can see from this, well, all of 35S is in the supernatant. All of the 32P, well, that is in the cells. And so you've got very clear evidence here that only the DNA is being passed into the cells on infection. And so that tells you, OK, well, that is the genetic material. And you see that we've got this really clear pellet here of 32p. That's how we label the DNA, and that's just in that cell phase. Now those experiments then culminated in the discovery of the double helical structure of DNA by Watson and Crick in 1953. And so this shows work from Morris Wilkins, Rosalind Franklin and Ray Gosling. And they were using X-ray diffraction. And you can see here we have this helical structure. So you've got these heavy bands that you can see here. And they're the recurring bases, and you've got this helical structure formed. So this is quite clear helical structure that you can see from that X-ray diffraction work. And it's this clear helical structure, the presence of these bands here, that led to Watson and Crick proposing the structure of DNA. And so that x-ray diffraction evidence shows a couple of things. It shows that the DNA is helical. And if you measure, you can see that the repeating units, there are these repeating units of 3.4 R. Armstrong and that's the distance between base pairs and then you have 34 as the distance taken by one turn of the helix okay so that's giving you that measure will show you later on we'll show you some pictures of DNA and you can see how those those measurements fit to the base pairs the nucleotides and turns of the hex. So X-ray diffraction is a key part of that evidence, but actually understanding how the base parent occurs comes from some chemistry, and it's these Chargaff's laws which were established in 1949 to 1953. And there's a couple of things they found just by looking at the chemistry here. They found that the total pyrimidines equals... the total purees. Okay, so we've got the number of T's and C's equals the numbers of A's and G's, our bases. And so that's the first of those laws. But the second goes a little bit further and it says, well, T is equal to A and C equals G. And so all of that is established based on chemical analysis of the DNA. But it gives you that indication, well, we've got pyrimidines equaling purines, T's equals A's, and C's equals G's. And that suggests the base pairing that occurs. And of course we know... That base pairing, we know how those bases work today, but this is where you get that information from, from the basic chemistry. You're expecting equal numbers, so T is paired with A, C with G. And the nucleotide structure is then the third piece of that evidence that comes from that work of Watson and Crick. And so this is what the structure of DNA looks like. So you can see that we have this this Ready pink sort of color. This is the backbone. This is a sugar backbone And so you've got two single strands here that wind around each other So if you have a look at this, you've got this first strand that goes on top here when it goes underneath Then it comes over the top again and eventually would go underneath again. The other strand, you can see, starts off going underneath, it then comes over the top, and then underneath here at the bottom. So these are the sugar, these are the two strands, and that's the sugar backbone. And then in the middle here, you can see in blue, you can see the bases. And so these are the different bases, and they're effectively holding the thing together. And so we have this Watson and Crick base pairing that hold the two DNA strands together. And you can see that more clearly here. So you can see that we've got a T base here, and that pairs with an A base. And you can see that there are these two bonds here. And then if we look at C and G, you can see, well, this is a C base, this is a G base, and in this case we have these three bonds that are formed between GC pairs. And so that's quite interesting in itself, that we've got this sort of stronger bond for Gs and Cs, and a weaker bond for As and Ts. And, you know, as we go through further lectures, you'll sort of realise, well, actually, where the two strands tend to fall apart is where we have lots of As and Ts. Okay, so... A and T's are weak bonds, and they're in good positions, if you like, for the start of replication. They easily melt, they fall apart, and allow other enzymes to get in there and interact with the DNA. So you can see here in this diagram, we've got the sugar. So these are these pentose sugars. And so you've got those pentose sugars that are connected to form the chain. The same on this side. And one very important point here is that the two strands are anti-parallel. So you can see this strand on the left is running from 5' at the bottom to 3' at the top. And on the right, it's the opposite way around. It runs from 5' at the top to 3' at the bottom. So the other thing to point out here is that if you look at the structure here, that you get this larger gap here. So a groove, this is what's known as the major groove. And then here we have what's known as the minor groove. And these are quite important because this is the point at which interaction with the DNA can occur. And so there are lots and lots of different things that will interact with the major groove. They get access to the different bases in the middle here. There are also some that will interact with the minor groove. But by and large, most things will interact with the major groove because there's more access to the bases at that point. So let's have a look at the basic unit of a nucleic acid. So that's the nucleotide. And so what we have here is we have a pentose sugar, and we have these carbons that are shown as 1' to 5'. So you've got the 1' carbon here, the 2' carbon here, 3' here, 4' here, and 5' here. And so the 5' carbon has a phosphate group attached. And that gives DNA... and RNA its negative charge. In fact, it's this phosphate that is where the two bases will connect. So the 3' carbon has a hydroxyl group attached. The 1' carbon, this has the purine or pyrimidine base attached to it at this point here. And the 2' carbon, well that depends on whether it's DNA or RNA. So if we have OH, the sugar is ribose. And if we have H here... then it's deoxyribose. So ribose for RNA, deoxyribose for DNA. And that's the difference here in that unit, that basic unit. So at this point here we have the purine or primidine bases. So let's have a look at the structure of those and how they work. So we've got two types of bases. We have purines, so adenine and guanine. And you can see they have this double nitrogenous ring structure. And then the primidines, well, they have a single ring structure, and they include cytosine and thymine and uracil. So the purines are adenine and guanine. The primidines in DNA are cytosine and thymine, but you don't get thymine in RNA, and instead you get uracil. So effectively, you can think of T and U as interchangeable. If you're looking at RNA, well, a T will be a U. And of course, if you're looking the opposite way, well, a U will be a T. So those nitrogenous bases are attached at this point here to the pentose sugar. And if we go back, you can see that attachment here at this point. So that's the point of attachment. And then, of course, we have this base pairing between these nitrogenous bases. So adenine here will pair with thymine. And then you have guanine here, which will base pair with cytosine. So you always get that same pairing. A with T and G with C. And so you can see the bonds. So adenine binds with bonds with thymine here. And these are the two points at which you get those hydrogen bonds. So you get those two hydrogen bonds. And for guanine and cytosine, you get three hydrogen bonds. So that's this point here. OK, we can just flick back. And you can see those bonds here. So here is C, the nitrogenous base C, that's bound here with G, with those three hydrogen bonds, and A with T, with just those two bonds. So the key difference between DNA and RNA is effectively this two prime carbon position here. What's attached to that? So if it's ribose, you have this OH group. If it's deoxyribose, so DNA, you get this H. Okay, so this is the key difference between the two, the presence of a hydrogen at the 2' carbon in deoxyribose versus a hydroxyl group at the 2' carbon in the ribose sugar. Now, there's some terminology here that I guess we should go through that sort of complicates sort of how we understand these bases. So we've got nucleoside with an S. This is just the sugar. and the base. Okay, so the sugar, the pentose sugar, and the base. And then we have the nucleotide, which is the sugar, the pentose sugar, the nitrogenous base, and the phosphocrypt. Okay, so nucleoside and nucleotide. And so this gives you the nomenclature of nucleosides and nucleotides. So if we look at the base, if the base is adenine, then the nucleoside is adenosine. And if we look at the nucleotides, well in RNA, that is AMP, and in DNA, it's DNA. AMP. So for guanine, the nucleoside is guanosine, and the nucleotide in RNA is GMP, and in DNA it's DGMP. For cytosine, the nucleoside is cytidine. In RNA, it's CMP, and in DNA, DCMP. Thymine, the nucleoside is thymidine. And so that doesn't exist in RNA. So we just get that in DNA, and that's DTMP. And then for uracil, the nucleoside is uridine, and the nucleotide in RNA is UMP. And again, that doesn't exist in DNA. And so these are the nucleotides. Now they also exist in di- and triphosphate forms. So you're probably more familiar with hearing about dNTPs, or DATP, CTP, GTP, and so on. And these are the things that we use if we're setting up a PCR reaction. We would get the dNTPs and use as the building blocks for those reactions. So you've got the monophosphate form, so DAMP. is the monophosphate form, DADP is the diphosphate form, and DATP is the triphosphate form. So that's the nomenclature of nucleosides and nucleotides. And as I say, if you're setting up a PCR reaction, then you need the triphosphate form of those nucleotides to utilize them in molecular biology, to set up a PCR, for example. So we've already talked about the complementary base pairing and you can see that in a little bit more detail on this slide. So you've got T with A with these two hydrogen bonds. And then you have C with G with these three hydrogen bonds. And you can see that if we have a T on one strand, then on the other strand we have an A. That's the complementary base pair. If we have a C on one strand, then on the other strand we have an A. we have a G. If we have an A on the left strand, then we're going to have a T on the right strand. And then at the bottom here, we have a G on one strand, then we have a C on the other. Okay, so that's the complementary base pairing with those two hydrogen bonds between A's and T's and those three hydrogen bonds between the C's and the G's. And as we said, already the strands are anti-parallel so you can see effectively on the left, you can see that we have the 5' end here at the top and the 3' end at the bottom. And so it's the phosphate group at that 5' end, shown in this sort of pink colour. And then the opposite strand, well, it's the opposite way around. It has 5' at the bottom and 3' at the top. So at the 5' end, we've got this free phosphate group that you can see here. And that's not involving forming a bond with another nucleotide. All of these other phosphate groups are involved in forming a bond with the next nucleotide down in the chain. And then, and it's exactly the same on the other strand. So we have this free phosphate group here at the 5' end, and then everything else, all of these other phosphate groups are involved in forming a bond with the next in the chain. And then at the three prime end, either here or here, we have this free hydroxyl group. So when new nucleotides come in, well, they're added to the 3' end. We'll talk about that more in just a moment, but effectively, a chain is always extended at the 3' end. You don't add anything new to the 5' end. What you do is you add new bases to the 3' end. So this is what that structure looks like. So this is a diagram showing that structure. So in the sort of grey, blue colourish, this is the phosphodiester backbone. So this is the sugar backbone. and you can see the two strands coiling over one another. So this one goes on top here and then underneath and then back out and the strands underneath and then on top. And you then get, as you saw previously, you get that major groove here in red and you get the minor groove here in yellow. And in the middle here, in this sort of orange and red colour, these are the bases. and you You can see they're stacked effectively on top of one another, a bit like a pack of cards. You get one base on top of the other going all the way up through that double helical structure. And you can see there's lots and lots of access here. For replication, effectively, you can get lots of access to those bases in the major groove. It's much more difficult to get access in the minor groove. And you do have enzymes that will interact with both, but predominantly with that major groove. So DNA is a right-handed helix. And you can sort of see that here. So it has this right-handed coil. and so there are actually three forms of DNA that we have listed here and the one really to sort of be concerned with is is B so B is the standard form of DNA and say it has this right handed helix. And if we look at some of the numbers here, well, between the turns, well, you get 10 base pairs per turn. And the distance between base pairs is 3.4. And the diameter of the helix is 90. And if we compare that to other forms, well, the A form, okay, this is quite similar to the B form. This is, again, it's a right-handed helix. But instead of having 10 bases per term, we have 11. The distance between the base pairs is a bit smaller, and the diameter of the helix... is a bit wider. So effectively the A form is very similar to the B form, the standard B form, but it's effectively squashed down. So you get less distance between the base pairs, you get a fatter helix, and there are 11 base pairs per turn. And then the Z form, well that's a little bit more complicated, that's a left-handed helix, but it zigzags. rather than coiling elegantly. And so the one to be predominantly concerned about is this B form, which is what you find in the natural states. You get a lot of A form when you're looking at interactions between DNA and RNA. In the lab, presumably also, that occurs naturally. And the Z form you find when you have lots of repetitive structures. So let's have a look at that B form of DNA. So you've got this very large major groove here and the narrow minor groove. And it's that major groove that's recognised by DNA binding protein. So that's your access point, your predominant. access point to the DNA molecule with some interaction in the minor groove and you can see that there are there are 10 base pairs per turn and you've got 3.4 Armstrong here is the is the nucleotide, and you've got 19 across. That's the width. Now, if we can compare that to the A form, you can see that it's just a bit squashed. In fact, that B form has been squashed down to form this A form. And you get these 11 base pairs per term. The nucleotide, it's 2.6, as opposed to 3.4 here. And then you've got this width here of 23. as opposed to 19. So it's just a bit fatter. It's squashed down. It's still coiling in the same direction, but it's squashed down. It's compressed. And this is a form that you get by double-stranded RNA. And if you get RNA-DNA hybrids, its significance in nature isn't all that clear, but it's something that we see commonly in the lab. And then there's this Z form of DNA, and this is left-handed instead of right-handed. It's sort of zigzag DNA. It has this very narrow diameter, 18, and it's effectively stretched. So you've got 12... base pairs per term, 3.7 is the distance for a nucleotide, and it's just this stretched form of DNA. And you find it when you get repeated DNA sequences like this. So you've got GC, GC, GC, GC. So these sort of dinucleotide repeat systems. And it's something that you see in vitro, and you'd expect it to also form transiently in vivo during gene transcription. So if you're producing RNA... you'd expect this to form. Now, one very important property of DNA is that you can denature it and renature it. And so, effectively, we go from this double-stranded DNA molecule here to a single-stranded molecule by denaturing. Okay, and back to denature, all we need to do is apply heat. So if we were doing this in something like a PCR reaction in the lab, then we'd heat to sort of 94, 96 degrees C, and effectively the two strands would fall apart. But we can also get them to re-anneal. by simply cooling the thing down. And this is a technique that we use if we want to put primers on, if we want to do a PCR reaction, anything like that, then effectively we would denature the DNA by heating to separate the strands out, and we would then add a primer. primer and that would allow us to replicate. So this is a really important property that we can heat to denature, we can separate the strands, I'll then form these random coils that you see here, we can also then go back the other way simply by cooling, they should re-anneal. So as it says at the bottom here, you know, this is important in genetic techniques. And of course, it's also the process by which DNA is replicated. It's a very important process, both in the lab and in vivo. So RNA is a single-stranded molecule. And it's very similar to a single strand of DNA, but as we mentioned earlier, it contains uracil instead of thymine, and it has that ribose sugar instead of deoxyribose. And there are three important types of RNA in cells. So there's messenger RNA, or mRNA. So this is RNA that is copied from DNA. It's a set of instructions, effectively, to produce a protein. So the messenger RNA is copied from DNA and used to make that protein. And it's effectively an intermediate state between DNA and protein. so you carry out what you'll have is you'll have the messenger RNA is produced in the nucleus it will then move across into the cytoplasm where it will go to a ribosome and that gives the genetic code the instructions the building blocks if you like for making that particular protein so ribosomal RNA or rRNA these are really important genes And effectively what they do, they don't encode protein, but they coil upon themselves to form a secondary and a tertiary structure. So they're quite long molecules, and they just, the single strand of... effectively calls on itself and that process of coiling will produce a structure a little bit like a protein will produce a structure and this is a functional RNA component so it effectively is the translational machinery in the cells okay so there's no protein involved in that it's just ribosomal RNA so you've probably heard of you know things like the small sub unit and large sub unit ribosomal RNA genes Sometimes they're called 18S and 28S genes. So the ribosomal RNA genes, they're present across all life, and they effectively form those ribosomes, the site of protein synthesis in the cell. And then we have tRNA here. And so these are the transfer RNA molecules, and they're involved in the process of translation. And what they do is they bring amino acids to the ribosomes during translation. And so rRNA and tRNA, they sort of form similar structures, but we'll have a look at tRNA. tRNA has this much simpler structure, so these are very short molecules. And so RNA is single-stranded, so unlike DNA is single-stranded, but you can see what happens is you get the single strand here that effectively coils on itself. to form this cloverleaf structure. And you can see that clearly here. So this is the two-dimensional representation of that cloverleaf. And you can see what happens is that you have complementary base pairing. like on either sides of these stems you see here so we've got some initial start here we've got this stem then we have a loop around here in this sort of ready pinky color and then we have the stem on this side and the stems are held together by standard Watson and Crick base pairing so C with G C with G G with C G with C a with you that's the same And here we've got G with C, A with U, G with C, G with C, G with C. And so that's what forms the structure. Effectively, you have a single-stranded RNA molecule that calls on itself to form that 2D structure. So this is the tRNA. If we were looking at ribosomal RNA genes, well, effectively, it's a similar array. but it's much, much longer and much more complicated. And so this is the tRNA. Its job is to bring in amino acids to the ribosome. And this point here is where those amino acids are attached onto those tRNA molecules. And so this is the 2D structure. And if we look at that in 3D, well, this is what it looks like. You have that 3D structure of that here. But effectively, it's a single strand that calls upon itself to form that structure. There's no encoding for proteins there, effectively. It's a functional molecule on its own, as is the ribosomal RNA. Okay, so we'll break now for 10 minutes. And then after the break, we'll talk about replication. Thank you.

and we'll talk about the structure and the process of replication of the DNA molecule. In this lecture, what we're going to talk about are those key experiments, and these are the learning objectives here. We'll talk about Griffith's experiment, which discovered bacterial transformation and the transforming principle. We'll then move on and talk about Avery McLeod and McCarty's experiment, which actually specifically identified DNA as the transforming principle. And then we'll talk about the Hershey-Chase experiment that showed the genetic material of phage T2 was DNA.

So these are the sort of key learning objectives here for that, the sort of history. And of course that then brings us on to the discovery of the helical structure. by Watson and Crick and Franklin and Wilkins. And of course that comes from knowledge of purine and primidine ratios, nucleotide structures, and so on and so forth.

And we'll cover all of those things as we go through this lecture. So these are the three classic experiments that led to Watson and Crick suggesting the double helical model. for DNA structure in 1953. So the first was Griffiths in 1928, and his experiments discovered the transforming principle.

Then we have Avery McLeod and McCarty, which goes a step further and works out that transforming principle with DNA. And then we have Hershey and Chase that came up with this experiment, again finding DNA with the material using bacteriophage T2. So let's have a look at those experiments. So this is Griffith's experiments from 1928. And what he was interested in was streptococcus pneumoniae.

So this is pneumococcus, and it infects the lungs. And what Griffith was interested in was how to go about coming up with a treatment or a vaccine for this disease. And there are these two different strains of pneumococcus.

Serotype 2, and that looks like this down here. So it's this R strain, as you see here. And so it has this rough colony. So there's no capsule, but there's this rough colony.

And this infects a mouse, but it has no effect. The immune system fights it off, and the mouse will survive infection. And on the left here we have serotype 3. This you can see has a capsule.

So this is this smooth colony that you can see here. And this is pathogenic in mice. So when injected into the mouse, well, the bacteria colonize the lungs, the immune system doesn't get rid of the disease, and eventually the mouse succumbs to the infection and dies.

So we can see that repeated here in these top two boxes. So smooth type 3, the mouse dies from pneumonia. Rough type 2, the mouse will survive. So what Griffiths did is an extra experiment here, where he took these pathogenic smooth type 3 bacteria and heat-killed them. and those heat kill bacteria had no effect on the mouse.

The mouse survived. And, you know, it's showing quite clearly that that experiment has worked, that that process of heat killing is complete, and all of those smooth type 3 bacteria that are pathogenic when they're alive have been killed, and you can inject them effectively quite safely with no effect on the mouse. He then did an experiment where...

He took the heat-killed smooth type 3 bacteria from above and put them into a mouse alongside rough type 2. So with this combination, well, you'd expect rough type 2. Well, that has no effect. It's non-pathogenic. The mouse should survive.

And the heat-killed smooth type 3, we've just established that the mouse survives if injected with those heat-killed smooth type 3 bacteria. But actually what happens here is the mouse dies from pneumonia. And if you look at the lungs, then what you find is bacteria with these type 3 capsules.

in the lungs of the mouse. And so what's going on here is that some property of these dead, heat-killed smooth type 3 bacteria is being transferred to those living rough type 2 bacteria. So what's happening is that the smooth type 3 bacteria are transforming the live rough type 2 bacteria. And of course we now know that transformation occurs because you get DNA released from these heat-killed smooth type 3 bacteria. That's then taken up by these living rough type 2 bacteria.

and incorporating them to their genome and changing their properties. Effectively, those non-pathogenic rough type 2 bacteria take up DNA from these dead, heat-killed, smooth type 3 bacteria and effectively become pathogenic. So Dawson continued with this work, but actually changed it from a mouse-based system to an in vitro system. So effectively you're doing this in a test tube rather than having to do this using live mice.

And what they did was they took those heat-killed 3S cells and showed that they transform living 2R cells in vitro. So effectively you can recover living 3S cells without the need to inject into mice. And it just simplifies the process. It makes the whole process of investigating and identifying the transforming principle, makes it all a little bit simpler in that you don't need those mouse hosts to actually do that experiment.

And then in 1944, Avery, McLeod and McCarty They discovered DNA as the transforming principle. But of course DNA is a simple molecule and it was quite hard to convince people that DNA was the transforming principle. That idea was not universally accepted.

So there are so many other better candidates, I guess, for what that transforming principle might be. More complicated molecules that would lead to that change in phenotype. Why would such a simple molecule like DNA be the thing that leads to that change in phenotype?

So what Avery, McLeod and McCarty did, they took 75 litres of heat-killed 3S cells, and that gave them a soluble extract containing the transforming principle. And what they then did is they simply just went through a series of experiments where they removed different components of that mix. So they started off by removing the lipids.

And once you do that, you can test, well, is the extract still functional? And yes, the extract is still active. So that tells you that the transforming principle isn't in the liquids.

They then did the same with the protein, so they removed the protein, and again, the extract was still active. So that's telling you the transforming principle, it's not in the lipids, it's not in the protein, so it must be elsewhere. They then did the same with polysaccharides, and the extract was still active. So at this point, you know the transforming principle is in the lipids, it's not in the protein, it's not in the polysaccharides, and that doesn't really leave very much left.

And so what's remaining at this point is the DNA, and the extract is still active. So that's suggesting here that that transforming principle is in the DNA. It's the only thing that's left in these tubes that can possibly transform those bacteria.

And so to confirm that... What they did is they took an enzyme called DNAs and this breaks up DNA and that's all it does, it just digests DNA, breaks it up, breaks it down and finally at that point when the DNA was digested was the activity lost, and only at that point was the activity lost. So this is giving you really strong evidence here that, well, the transforming principle has to be in the DNA, that's the only thing left, and if we get rid of the DNA, we lose that ability to transform bacteria.

So it's telling you the transforming principle is in that DNA fragment. So that's quite convincing. And it confirms DNA as the transforming principle. And it tells us that DNA has the potential to change the properties of a living organism. But of course what it doesn't tell us is that organisms use DNA as a genetic material.

So it doesn't go that far, it just identifies what the transforming principle is. And so that's a follow-on step that has to be looked at and that sort of leap has to be made. And so that comes with this Hershey Chase experiment.

And so Hershey Chase, they worked on bacteriophage T2, and you can see we have that bacteriophage here in the centre of this EM image, and you can see that it's got this head structure, this tail structure here, and then extruding from the head, you've got all of this material. This is actually the genetic material that's extruding from the head. So huge amounts of genetic material that are packaged into the head of that bacteriophage.

And so they looked at the life cycle of bacteriophage. And it has this interesting life cycle. Effectively, what happens is that the bacteriophage will adhere to the surface of a bacterium.

And it injects DNA into the cell. So this sort of purpley blue square here, this is the bacterial cell. And this is the DNA being injected into that cell.

And so what that means is, once that's happened, what happens is you have effectively an empty head, and the sort of remnants of the bacteriophage and the genetic material is all inside of the cell. And that replicates, and you then get new production of new heads and tails. You can see those being made here.

And effectively what then happens is the DNA is then packaged into these new heads and tails, the fibres are attached, and the bacterial cell is lysed, and those progeny phage are released into the host. And so this is quite interesting because what you can see from this is that there's only one small part that gets into the bacterial cell. It's only the DNA here, only the genetic material.

that enters the infected cell. Everything else remains on the surface. And that allows you to do an experiment where you work out, well, what is happening?

What is where? And so they did that by using radioactive labelling. And what they did is they labelled the coat proteins, so the heads and the tails, they labelled those with 35S that are shown here in red. And then they label the DNA with 32p, and that's shown here in blue. And so we've got this sort of clear distinction here.

And so we can see what is where, and so effectively we've got the heads and the tails, they remain on the surface. And so what you should find is all of the 35S here is on the surface, none of it is in the cell. And for the DNA, well that's injected into the bacterial cell, so you'd expect all of that 32P to be inside the cell. And so what you do is you take these bacterial cells and you put them in a blender, you mix everything up, And it gives you this mix of phage ghosts and cells.

And you spin that down. And you can see that here. And effectively, the cells are heavy. They spin to the bottom of the centrifuge.

And all of these ghosts, these phage ghosts, they remain in the supernatant. And so this is what we've got here. We've got the sort of pellet at the bottom of the tube that contains the cells and the supernatant that contains the remnants of those phage ghosts above.

And you can see from this, well, all of 35S is in the supernatant. All of the 32P, well, that is in the cells. And so you've got very clear evidence here that only the DNA is being passed into the cells on infection.

And so that tells you, OK, well, that is the genetic material. And you see that we've got this really clear pellet here of 32p. That's how we label the DNA, and that's just in that cell phase. Now those experiments then culminated in the discovery of the double helical structure of DNA by Watson and Crick in 1953. And so this shows work from Morris Wilkins, Rosalind Franklin and Ray Gosling. And they were using X-ray diffraction.

And you can see here we have this helical structure. So you've got these heavy bands that you can see here. And they're the recurring bases, and you've got this helical structure formed. So this is quite clear helical structure that you can see from that X-ray diffraction work. And it's this clear helical structure, the presence of these bands here, that led to Watson and Crick proposing the structure of DNA.

And so that x-ray diffraction evidence shows a couple of things. It shows that the DNA is helical. And if you measure, you can see that the repeating units, there are these repeating units of 3.4 R.

Armstrong and that's the distance between base pairs and then you have 34 as the distance taken by one turn of the helix okay so that's giving you that measure will show you later on we'll show you some pictures of DNA and you can see how those those measurements fit to the base pairs the nucleotides and turns of the hex. So X-ray diffraction is a key part of that evidence, but actually understanding how the base parent occurs comes from some chemistry, and it's these Chargaff's laws which were established in 1949 to 1953. And there's a couple of things they found just by looking at the chemistry here. They found that the total pyrimidines equals... the total purees. Okay, so we've got the number of T's and C's equals the numbers of A's and G's, our bases.

And so that's the first of those laws. But the second goes a little bit further and it says, well, T is equal to A and C equals G. And so all of that is established based on chemical analysis of the DNA. But it gives you that indication, well, we've got pyrimidines equaling purines, T's equals A's, and C's equals G's. And that suggests the base pairing that occurs.

And of course we know... That base pairing, we know how those bases work today, but this is where you get that information from, from the basic chemistry. You're expecting equal numbers, so T is paired with A, C with G. And the nucleotide structure is then the third piece of that evidence that comes from that work of Watson and Crick. And so this is what the structure of DNA looks like.

So you can see that we have this this Ready pink sort of color. This is the backbone. This is a sugar backbone And so you've got two single strands here that wind around each other So if you have a look at this, you've got this first strand that goes on top here when it goes underneath Then it comes over the top again and eventually would go underneath again. The other strand, you can see, starts off going underneath, it then comes over the top, and then underneath here at the bottom. So these are the sugar, these are the two strands, and that's the sugar backbone.

And then in the middle here, you can see in blue, you can see the bases. And so these are the different bases, and they're effectively holding the thing together. And so we have this Watson and Crick base pairing that hold the two DNA strands together.

And you can see that more clearly here. So you can see that we've got a T base here, and that pairs with an A base. And you can see that there are these two bonds here. And then if we look at C and G, you can see, well, this is a C base, this is a G base, and in this case we have these three bonds that are formed between GC pairs. And so that's quite interesting in itself, that we've got this sort of stronger bond for Gs and Cs, and a weaker bond for As and Ts.

And, you know, as we go through further lectures, you'll sort of realise, well, actually, where the two strands tend to fall apart is where we have lots of As and Ts. Okay, so... A and T's are weak bonds, and they're in good positions, if you like, for the start of replication. They easily melt, they fall apart, and allow other enzymes to get in there and interact with the DNA. So you can see here in this diagram, we've got the sugar.

So these are these pentose sugars. And so you've got those pentose sugars that are connected to form the chain. The same on this side. And one very important point here is that the two strands are anti-parallel. So you can see this strand on the left is running from 5' at the bottom to 3' at the top.

And on the right, it's the opposite way around. It runs from 5' at the top to 3' at the bottom. So the other thing to point out here is that if you look at the structure here, that you get this larger gap here.

So a groove, this is what's known as the major groove. And then here we have what's known as the minor groove. And these are quite important because this is the point at which interaction with the DNA can occur.

And so there are lots and lots of different things that will interact with the major groove. They get access to the different bases in the middle here. There are also some that will interact with the minor groove.

But by and large, most things will interact with the major groove because there's more access to the bases at that point. So let's have a look at the basic unit of a nucleic acid. So that's the nucleotide.

And so what we have here is we have a pentose sugar, and we have these carbons that are shown as 1' to 5'. So you've got the 1' carbon here, the 2' carbon here, 3' here, 4' here, and 5' here. And so the 5' carbon has a phosphate group attached. And that gives DNA...

and RNA its negative charge. In fact, it's this phosphate that is where the two bases will connect. So the 3' carbon has a hydroxyl group attached.

The 1' carbon, this has the purine or pyrimidine base attached to it at this point here. And the 2' carbon, well that depends on whether it's DNA or RNA. So if we have OH, the sugar is ribose. And if we have H here...

then it's deoxyribose. So ribose for RNA, deoxyribose for DNA. And that's the difference here in that unit, that basic unit.

So at this point here we have the purine or primidine bases. So let's have a look at the structure of those and how they work. So we've got two types of bases. We have purines, so adenine and guanine.

And you can see they have this double nitrogenous ring structure. And then the primidines, well, they have a single ring structure, and they include cytosine and thymine and uracil. So the purines are adenine and guanine.

The primidines in DNA are cytosine and thymine, but you don't get thymine in RNA, and instead you get uracil. So effectively, you can think of T and U as interchangeable. If you're looking at RNA, well, a T will be a U. And of course, if you're looking the opposite way, well, a U will be a T. So those nitrogenous bases are attached at this point here to the pentose sugar.

And if we go back, you can see that attachment here at this point. So that's the point of attachment. And then, of course, we have this base pairing between these nitrogenous bases.

So adenine here will pair with thymine. And then you have guanine here, which will base pair with cytosine. So you always get that same pairing. A with T and G with C. And so you can see the bonds.

So adenine binds with bonds with thymine here. And these are the two points at which you get those hydrogen bonds. So you get those two hydrogen bonds. And for guanine and cytosine, you get three hydrogen bonds. So that's this point here.

OK, we can just flick back. And you can see those bonds here. So here is C, the nitrogenous base C, that's bound here with G, with those three hydrogen bonds, and A with T, with just those two bonds. So the key difference between DNA and RNA is effectively this two prime carbon position here.

What's attached to that? So if it's ribose, you have this OH group. If it's deoxyribose, so DNA, you get this H.

Okay, so this is the key difference between the two, the presence of a hydrogen at the 2' carbon in deoxyribose versus a hydroxyl group at the 2' carbon in the ribose sugar. Now, there's some terminology here that I guess we should go through that sort of complicates sort of how we understand these bases. So we've got nucleoside with an S.

This is just the sugar. and the base. Okay, so the sugar, the pentose sugar, and the base. And then we have the nucleotide, which is the sugar, the pentose sugar, the nitrogenous base, and the phosphocrypt.

Okay, so nucleoside and nucleotide. And so this gives you the nomenclature of nucleosides and nucleotides. So if we look at the base, if the base is adenine, then the nucleoside is adenosine.

And if we look at the nucleotides, well in RNA, that is AMP, and in DNA, it's DNA. AMP. So for guanine, the nucleoside is guanosine, and the nucleotide in RNA is GMP, and in DNA it's DGMP. For cytosine, the nucleoside is cytidine. In RNA, it's CMP, and in DNA, DCMP.

Thymine, the nucleoside is thymidine. And so that doesn't exist in RNA. So we just get that in DNA, and that's DTMP. And then for uracil, the nucleoside is uridine, and the nucleotide in RNA is UMP. And again, that doesn't exist in DNA.

And so these are the nucleotides. Now they also exist in di- and triphosphate forms. So you're probably more familiar with hearing about dNTPs, or DATP, CTP, GTP, and so on.

And these are the things that we use if we're setting up a PCR reaction. We would get the dNTPs and use as the building blocks for those reactions. So you've got the monophosphate form, so DAMP. is the monophosphate form, DADP is the diphosphate form, and DATP is the triphosphate form. So that's the nomenclature of nucleosides and nucleotides.

And as I say, if you're setting up a PCR reaction, then you need the triphosphate form of those nucleotides to utilize them in molecular biology, to set up a PCR, for example. So we've already talked about the complementary base pairing and you can see that in a little bit more detail on this slide. So you've got T with A with these two hydrogen bonds.

And then you have C with G with these three hydrogen bonds. And you can see that if we have a T on one strand, then on the other strand we have an A. That's the complementary base pair.

If we have a C on one strand, then on the other strand we have an A. we have a G. If we have an A on the left strand, then we're going to have a T on the right strand. And then at the bottom here, we have a G on one strand, then we have a C on the other. Okay, so that's the complementary base pairing with those two hydrogen bonds between A's and T's and those three hydrogen bonds between the C's and the G's.

And as we said, already the strands are anti-parallel so you can see effectively on the left, you can see that we have the 5' end here at the top and the 3' end at the bottom. And so it's the phosphate group at that 5' end, shown in this sort of pink colour. And then the opposite strand, well, it's the opposite way around.

It has 5' at the bottom and 3' at the top. So at the 5' end, we've got this free phosphate group that you can see here. And that's not involving forming a bond with another nucleotide. All of these other phosphate groups are involved in forming a bond with the next nucleotide down in the chain.

And then, and it's exactly the same on the other strand. So we have this free phosphate group here at the 5' end, and then everything else, all of these other phosphate groups are involved in forming a bond with the next in the chain. And then at the three prime end, either here or here, we have this free hydroxyl group. So when new nucleotides come in, well, they're added to the 3' end. We'll talk about that more in just a moment, but effectively, a chain is always extended at the 3' end.

You don't add anything new to the 5' end. What you do is you add new bases to the 3' end. So this is what that structure looks like. So this is a diagram showing that structure.

So in the sort of grey, blue colourish, this is the phosphodiester backbone. So this is the sugar backbone. and you can see the two strands coiling over one another. So this one goes on top here and then underneath and then back out and the strands underneath and then on top.

And you then get, as you saw previously, you get that major groove here in red and you get the minor groove here in yellow. And in the middle here, in this sort of orange and red colour, these are the bases. and you You can see they're stacked effectively on top of one another, a bit like a pack of cards.

You get one base on top of the other going all the way up through that double helical structure. And you can see there's lots and lots of access here. For replication, effectively, you can get lots of access to those bases in the major groove.

It's much more difficult to get access in the minor groove. And you do have enzymes that will interact with both, but predominantly with that major groove. So DNA is a right-handed helix.

And you can sort of see that here. So it has this right-handed coil. and so there are actually three forms of DNA that we have listed here and the one really to sort of be concerned with is is B so B is the standard form of DNA and say it has this right handed helix.

And if we look at some of the numbers here, well, between the turns, well, you get 10 base pairs per turn. And the distance between base pairs is 3.4. And the diameter of the helix is 90. And if we compare that to other forms, well, the A form, okay, this is quite similar to the B form.

This is, again, it's a right-handed helix. But instead of having 10 bases per term, we have 11. The distance between the base pairs is a bit smaller, and the diameter of the helix... is a bit wider. So effectively the A form is very similar to the B form, the standard B form, but it's effectively squashed down. So you get less distance between the base pairs, you get a fatter helix, and there are 11 base pairs per turn.

And then the Z form, well that's a little bit more complicated, that's a left-handed helix, but it zigzags. rather than coiling elegantly. And so the one to be predominantly concerned about is this B form, which is what you find in the natural states. You get a lot of A form when you're looking at interactions between DNA and RNA. In the lab, presumably also, that occurs naturally.

And the Z form you find when you have lots of repetitive structures. So let's have a look at that B form of DNA. So you've got this very large major groove here and the narrow minor groove.

And it's that major groove that's recognised by DNA binding protein. So that's your access point, your predominant. access point to the DNA molecule with some interaction in the minor groove and you can see that there are there are 10 base pairs per turn and you've got 3.4 Armstrong here is the is the nucleotide, and you've got 19 across.

That's the width. Now, if we can compare that to the A form, you can see that it's just a bit squashed. In fact, that B form has been squashed down to form this A form. And you get these 11 base pairs per term.

The nucleotide, it's 2.6, as opposed to 3.4 here. And then you've got this width here of 23. as opposed to 19. So it's just a bit fatter. It's squashed down. It's still coiling in the same direction, but it's squashed down. It's compressed.

And this is a form that you get by double-stranded RNA. And if you get RNA-DNA hybrids, its significance in nature isn't all that clear, but it's something that we see commonly in the lab. And then there's this Z form of DNA, and this is left-handed instead of right-handed.

It's sort of zigzag DNA. It has this very narrow diameter, 18, and it's effectively stretched. So you've got 12...

base pairs per term, 3.7 is the distance for a nucleotide, and it's just this stretched form of DNA. And you find it when you get repeated DNA sequences like this. So you've got GC, GC, GC, GC. So these sort of dinucleotide repeat systems. And it's something that you see in vitro, and you'd expect it to also form transiently in vivo during gene transcription.

So if you're producing RNA... you'd expect this to form. Now, one very important property of DNA is that you can denature it and renature it. And so, effectively, we go from this double-stranded DNA molecule here to a single-stranded molecule by denaturing.

Okay, and back to denature, all we need to do is apply heat. So if we were doing this in something like a PCR reaction in the lab, then we'd heat to sort of 94, 96 degrees C, and effectively the two strands would fall apart. But we can also get them to re-anneal. by simply cooling the thing down.

And this is a technique that we use if we want to put primers on, if we want to do a PCR reaction, anything like that, then effectively we would denature the DNA by heating to separate the strands out, and we would then add a primer. primer and that would allow us to replicate. So this is a really important property that we can heat to denature, we can separate the strands, I'll then form these random coils that you see here, we can also then go back the other way simply by cooling, they should re-anneal. So as it says at the bottom here, you know, this is important in genetic techniques. And of course, it's also the process by which DNA is replicated.

It's a very important process, both in the lab and in vivo. So RNA is a single-stranded molecule. And it's very similar to a single strand of DNA, but as we mentioned earlier, it contains uracil instead of thymine, and it has that ribose sugar instead of deoxyribose.

And there are three important types of RNA in cells. So there's messenger RNA, or mRNA. So this is RNA that is copied from DNA. It's a set of instructions, effectively, to produce a protein.

So the messenger RNA is copied from DNA and used to make that protein. And it's effectively an intermediate state between DNA and protein. so you carry out what you'll have is you'll have the messenger RNA is produced in the nucleus it will then move across into the cytoplasm where it will go to a ribosome and that gives the genetic code the instructions the building blocks if you like for making that particular protein so ribosomal RNA or rRNA these are really important genes And effectively what they do, they don't encode protein, but they coil upon themselves to form a secondary and a tertiary structure. So they're quite long molecules, and they just, the single strand of...

effectively calls on itself and that process of coiling will produce a structure a little bit like a protein will produce a structure and this is a functional RNA component so it effectively is the translational machinery in the cells okay so there's no protein involved in that it's just ribosomal RNA so you've probably heard of you know things like the small sub unit and large sub unit ribosomal RNA genes Sometimes they're called 18S and 28S genes. So the ribosomal RNA genes, they're present across all life, and they effectively form those ribosomes, the site of protein synthesis in the cell. And then we have tRNA here. And so these are the transfer RNA molecules, and they're involved in the process of translation. And what they do is they bring amino acids to the ribosomes during translation.

And so rRNA and tRNA, they sort of form similar structures, but we'll have a look at tRNA. tRNA has this much simpler structure, so these are very short molecules. And so RNA is single-stranded, so unlike DNA is single-stranded, but you can see what happens is you get the single strand here that effectively coils on itself. to form this cloverleaf structure. And you can see that clearly here.

So this is the two-dimensional representation of that cloverleaf. And you can see what happens is that you have complementary base pairing. like on either sides of these stems you see here so we've got some initial start here we've got this stem then we have a loop around here in this sort of ready pinky color and then we have the stem on this side and the stems are held together by standard Watson and Crick base pairing so C with G C with G G with C G with C a with you that's the same And here we've got G with C, A with U, G with C, G with C, G with C.

And so that's what forms the structure. Effectively, you have a single-stranded RNA molecule that calls on itself to form that 2D structure. So this is the tRNA.

If we were looking at ribosomal RNA genes, well, effectively, it's a similar array. but it's much, much longer and much more complicated. And so this is the tRNA. Its job is to bring in amino acids to the ribosome. And this point here is where those amino acids are attached onto those tRNA molecules.

And so this is the 2D structure. And if we look at that in 3D, well, this is what it looks like. You have that 3D structure of that here.

But effectively, it's a single strand that calls upon itself to form that structure. There's no encoding for proteins there, effectively. It's a functional molecule on its own, as is the ribosomal RNA.

Okay, so we'll break now for 10 minutes. And then after the break, we'll talk about replication. Thank you.

Transcript for:8. DNA and RNA

Transcript for:
8. DNA and RNA