Nucleic Acids 4

the double helix can actually take three different forms, and the one that's most common and was the structure predicted by Watson. Crick is the B form. But let's go ahead and talk about all of the forms. So here, below in the image we have our A, B and Z form, and I'm sure the B looks the most familiar to you. Now it's worth noting that the A and B forms are both right handed, Helix sees, but the A form is mawr condensed or more squished down than the B form. So actually, in this image that we have here, uh, each each strand has the same number of bases, and as you can see the A form it occupies less space than the B form. It's a little more condensed, and it turns out the double stranded RNA actually takes the A form. And, as I said, most DNA is found in the B. For most double stranded DNA is found in the B form. The Z form is a left handed helix, unlike a and B, and the Z form is generally found near regulatory sequences, and basically you'll have what are called busy junctions which are areas where the helix switches between the B and the Z forms, and more or less what happens is the nucleotides flip out from the strands. Remember the whole sin anti confirmation thing. The nucleotides flip out from the strands, and they cause the actual helix to switch direction from left handed to right handed. And this actually, um, puts a abrupt change in the shape of the DNA. And this is really important because this sort of noticeable shape difference is used by cells is exploited by cells to determine where regulatory sequences are. And you'll see this pattern. Ah, lot in biology where basically, um, you know, the actual physical shape and structure of DNA is used to identify various things now. Also important to note about the structure of these different forms is what they look like. If you sort of were thio, stare down them, so these images below are basically if you were looking the draw, they can. I hear if you were looking down on the structure and if you were looking down in the A structure, you'd see that there's actually an opening in the center of it. It has a hollow space inside of it. Whereas there's no opening in the B and the Z forms right here. No opening in those structures. And, um, that is going to be pretty important in terms of our DNA, because single strands of DNA are held together, not Onley by hydrogen bonds, but also by these hydrophobic stacking forces and thes stacking forces result from the fact that the bases are actually pretty hydrophobic. And as you can see in, let me actually change colors here. As you can see in the center here, all of our bases, which are, uh, those blue shapes we see are mostly blue. There's some other colors in there, but those mostly blue shapes we see in the center they're all stacked on top of each other, right? So they're having these hydrophobic stacking forces between them that actually help, uh, maintain that structure that be formed structure of DNA. So zooming out on that image a little bit more if you think about the outside, we have all those phosphate groups, right, and we actually have because DNA is found in cells, it's in an aqueous environment. So there's water around DNA that helps stabilize its shape. And then there's those hydrogen phobic stacking forces inside DNA toe also help stabilize its shape. All right, let's flip the page.