Hi in this video we're gonna be talking about the structure and function of D. N. A. So we've already reviewed this a little bit and I'm sure you've gone over most of this in your intro classes. But we're just going to hit at home one more time so that we can all be on the same page both in this uh this current topic on this in the next few topics that we're gonna be talking about. So nucleotides are the building blocks of D. N. A. So there are four bases. These are A. C. T. And G. Um or I. C. G. And T. How I have them ordered. Um And they're divided into two classes. So the perimeter means have one carbon ring and the pure rings have to carbon rings. So if we just take a second to look at this um you can see here there's one carbon ring. So this is a primitive primitive mean? And this is two carbon rings. So this one here is appearing and I have them up here. The pairings are A. And G. And the perimeter things R. C. And T. I don't know if you can see that. Let me disappear for a second. So yeah, the perimeter here or the one ring. The earrings are the two rings. So come back, let's talk about the charge offs rules. And these are rules that say which nucleotides pair with each other. So a pairs with T. And C. Pairs with G. And so this is each time these pair together it's called a base pair base pair and they pair through either two or three hydrogen bonds. So we disappear again. So you can see here that when a pairs with T. You have two hydrogen bonds. But when G pairs with C. You have three hydrogen bonds. And each one of these is called a base pair. And so due to the size and to the chemical nature of each base appearing can only pair with the permitting. So here we have appearing of G. And permitting of C. We have appearing of a. Or yeah pairing of a pairing of the primitive thing of T. And so this is how this has to happen. So these individual bonds are eventually linked together to form the backbone of D. N. A. And this is created between bonds between sugar and phosphate groups on adjacent nucleotides. So the five prime phosphate group binds to the three prime hydroxyl group and the neighboring molecule. So this gives it a directionality. So you always have the five prime here and the three prime here and their binding you know this way. And so there's this direction here. And of course it's it's this way on the other side. But there is directionality to a linear strand. So each kind of each portion of the D. N. A strand has a directionality which becomes really important. And so because there are these charged phosphate groups um like I said here, the bonds between sugars and phosphate groups hydroxyl groups on the sugar. Um they're charged. And so this gives them this makes them polar. Which means polar, they can interact with water really easily. So if we're looking at the D. N. A. Strand here, you can see that there's a five prime end three prime end on each side. So each strand. So there's two strands here, one strand to strand each has directionality. And um you can see here that you have this these base pairs here between the bases and you have the sugar phosphate backbone. See the prostate here and the sugars here and that these bonds form these um these sugar phosphate backbones that allow the nucleotides to face into each other. And so when this structure is formed this actually folds into a double helix. And that's the most energetically favorable form that it can have. And so the DNA double helix is created through bonds between two linear strands of D. N. A. So that's what you see here. But we can look at it as a double helix. So the sugar phosphate backbone forms the outside edges with the bases facing the center. So you can see this is exactly what is shown here and that's exactly what it's gonna be shown down here in this example with the phosphate backbone here circling around and then you have the bases in the center. And so for every helical turn there are 10 base pairs and you don't necessarily need to know these numbers. It's just sort of if you're thinking about the D. N. A. Helix. Um let's just put it into some kind of contacts contacts. So there are 10 bases for helical turn and one helical turn adds 3.4 nan meters um to link of DNA, which means there's 0.34 nanometers per nucleotide. Since there's 10 nucleotides. Like I said you don't need to memorize those numbers. But if you're sort of thinking about it in the context of reality, this is this is the reality of it. 3.4 nanometers for every 10 bases which is a helical term. Okay and so we turned the bases or each strand in the DNA double helix as complementary because they bind together. And then other confirmations they wouldn't be able to bind together. So complimentary means that A. And the t binding together in the sea and the G. Buying together. So each strand is complimentary. So if there's a T. On one strand there's going to be an A. On another. There's a C. On one strand and there's a G. On the other that's complementary but it also means that it runs anti parallel which refers to the directionality that I've been hitting over and over again. And that is if you have the five prime and three prime here. Well then the five prime and three prime have to be down here. So these directionality. These are what we mean when we do when we say anti parallel and then um just the structure of a DNA double helix actually has to um a groups, it has a major groove which you can see here is just bigger and then the minor groove which is smaller and that's just that's just how the DNA double helix forms. So you've probably been familiar with at least most of this just from your intro classes. But there's a new sort of layer that we're going to add on to this um in cell biology. And that is that there are actually three types of DNA double helix is here. So we are really familiar with, you know this type here. This is what we've seen intro classes high school but there's actually three types and they're labeled B. A. And C. And so um the B. Is the most common. This is the one you're most familiar with. A. Is rare and so is easy, is rare but it's it's also short A. Is also shorter and it's also considered a right handed helix. Which what that means is if you're just looking down on the helix, you know what way is it turning? Um And that Z. Is a left handed helix. Um And really the significance of Z is completely unknown. So if we're just sort of gonna look at this real fast, you can see this is what this is the normal one. So this is B. This is the one you're familiar with back out of the way, just in case and then you have A. Which here, you can see a shorter and you have Z here, and you can see that it's kind of hard to sort of imagine these, he lucy's, what does it mean to be a right hand and left hand? But you can see when you look at these images, that there are really significant differences in their structure, both when presented in this format, but also when presented at the cross section in this format here. So, with all of that, all of that review and a little bit of new stuff. Now, let's move on.
2
Problem
Problem
Chargoff's rules state that…
A
A purine always pairs with a purine
B
A pairs with C and T pairs with G
C
A pairs with T and C pairs with G
3
Problem
Problem
The two complementary DNA strands that make up the double helix run parallel to each other.
A
True
B
False
4
Problem
Problem
Which of the following is not a purine nucleotide?
