Nucleic Acids 2 - Video Tutorials & Practice Problems
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Nucleic Acids 2
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the nucleotide monomers in a nuclear kassid polymer are connected by phosphor Oh di Ester Bonds. And you can see these here in our diagrams of DNA and RNA. Here is a phosphor digester bond in DNA. And here is a phosphor Oh, digester bond in RNA. Now, the individual strands of DNA are actually said to be anti parallel to each other. And the way I like to think of this is if you if you look at a strand of DNA and generally speaking, we read the code of DNA in the five prime to three prime direction. So if you look at a strand of DNA, you noticed that at one end we have the five prime end and the five prime end is going to have a phosphate group on it. And the other end, we have a three prime end, and that's going tohave a hydroxyl group on it. So if you notice on the other side, the three prime end and the five prime end are actually flipped from the other strand. So if you think about this as an arrow on arrow that points from five prime 23 prime, you can see If I draw my arrows on the two sides of DNA, they're going in opposite directions. They're parallel to each other, but they're pointing in opposite directions. And that's what we mean when we say anti parallel. That's basically the definition of the term. So two parallel lines pointing in opposite directions, just like we have here in DNA. If you kind of think of the three prime and five prime ends is different ends of a narrow again, it's important to remember that your five prime end is going to be the one with the phosphate group because the phosphate group is attached to the five prime carbon of your pantos sugar. Likewise, your three prime end is going to be the one with the hydroxy group because that hydroxy group is attached to the three prime carbon. So that's where these ends in this naming convention is coming from. It's just coming from the carbon numbers. Now RNA is actually less stable than DNA, and that's because at high pH, this to prime hydroxyl group is reactive and it can actually react with the phosphate group next door. Basically, and what you'll wind up with is some kind of structure that looks like this. You know, I'm kind of half drawing my sugar here, but it'll basically react with itself. And of course, that's bad. It can cause TheStreet rand to break in half or whatever. So because of this, DNA has been favored as the genetic information storage molecule, because again it's more stable than RNA. And it's more stable because it's missing that hydroxy group on the three on the two prime carbon. So, um, there are some. It's thought that very early life forms would have used RNA as their genetic storage. But over the course of evolution, DNA has been like, vastly favored by comparison toe RNA because basically, all life uses DNA. You know, some viruses used RNA Aziz their genetic information storage, but that begs the question of our viruses even alive. And you know, that's a whole other tangents. So the main point is DNA much more stable, And it's all because of, uh, the absence of that to prime hydroxyl group now nucleotides and you click assets. It's important to remember they have this property where they absorb light. They absorb the maximum amount of light at nanometers, the wavelength 260 nanometers. This is important because proteins absorb light maximally at 280 nanometers. And you might remember it's actually the amino acids trip to fan and tire scene that are doing the most absorption of light at that frequent or at that wavelength. So this is important because it allows biochemists a really easy way to test for Thebes presence or absence of DNA versus protein. So if you're trying Thio, isolate something from a cell, for example, you're trying to isolate its DNA. You can go through the various steps and then test your sample by running it through a spectra for Tom Attar and seeing what wavelengths it absorbs. If it's on Lee absorbing it to 60 and it's not absorbing it to 80 then you know you probably got rid of the proteins, and you just have your new click assets there. So, uh, small property. But important facts to know. Now the two strands of DNA are said to be complementary, and basically that means that the code on one strand complements the code on the other strand, and this is due to specificity of base pairing And basically what that means is there are specific rules that guide base pairing and those rules, more or less are what we see below here. So we have in on the left. Here we have an ad ning that is binding Thio a thigh mean and on the right. You can see that we have a guanine and I'm just gonna take myself out of the image so you can see this better. We have a guanine binding Teoh, a cytosine. Now, a couple things to note here. First, admin and thiamine are forming to H bonds, whereas guanine and cytosine are forming three H bonds. Now this is important because it's going to result in, um, strands of DNA with higher GC composition, having a higher melting temperature or just generally being harder to separate. And it literally comes from the fact that they're going to have mawr hydrogen bonds holding the strands of DNA together, making them stick together stronger. Other important thing to note is wow. While RNA is generally in in the areas we're going to be exploring, RNA is generally going to be single stranded, its bases. It can be double stranded. First of all, but its bases will also, um, form these bonds with DNA during gene expression and during the transcription process. So what I'm getting to hear is that when that happens, Adnan will actually be binding with your Ozil, just like we see it with finding here. And let me put this in parentheses. That's not what we're. That's not what this image is, but that's what happens. So adding also binds with your Ozil in the same way we see it happening with signing. Of course the differences, the presence of this methyl group there, um, but yeah. So the basic story is because there are these specific base pairing rules. If you have one strand of DNA or just a single stranded piece of RNA, you can very easily determine what the complementary strand is. And in fact, that's how genetic information is transmitted in the South. So a little thought experiment here. What percentage of DNA is going to be made up of pure ings and what percentage is going to be made up of pyre? Imagenes? Well, let's think about this for a second. Add Ning always pairs with timing right and guanine always pairs with cytosine. Now add ning and guanine or both. Pure ings and thigh mean and cytosine are both pie remedies, so that means that we're always going tohave a pure ing bind. Teoh a pie remedy, meaning that DNA is going to be 50% pure ing's and 50% pi remedies means. And this is actually very important because it means that the width of the double stranded DNA is going to be consistent. It's always going toe, have the same width, and you can see that, Um, that's determined by the fact that we have the two ring structure and the one ring structure always binding to each other. And that's the width will be the same regardless, if it's a GC pair or an 80 pair. So always gonna be 50% hearings, 50% pyre imagenes. Now let's do another little thought. Experiment here. If a piece of double stranded DNA has 35% add ning and 15% cytosine. How much time in and guanine is it gonna have? Um, sort of in a similar line of thinking to the last problem. If there's 35% Adnan, then that means there has to be 35% timing right, because Adnan is always going to be bound tooth. I mean likewise. If there's 15% cytosine, then there's gonna have to be 15% guanine because cytosine always binds to guanine, so you're gonna find those in equal amounts. Now let's flip the page and talk a little bit more about these ideas we just explored.