The double helix can actually take 3 different forms. And the one that's most common and was the structure predicted by Watson and 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 helices but the A form is more condensed or more squished down than the B form. So actually, in this image that we have here, each strand has the same number of bases and as you can see, the A form occupies less space than the B form. It's a little more condensed. And it turns out that double stranded RNA actually takes the A form. And as I said, most DNA is found in the B form. 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 the BZ 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 syn/anti conformation 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 puts an 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 a lot in biology where basically, 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 were to, stare down them. So these images below are if you were looking I'm going to draw like an eye here. 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, that is going to be pretty important in terms of our DNA because single strands of DNA are held together not only by hydrogen bonds but also by these hydrophobic stacking forces. And these 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 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 maintain that structure, that B form 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, you know, 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 hydrophobic stacking forces inside DNA to also help stabilize its shape. Alright, let's flip the page.
Review 1: Nucleic Acids, Lipids, & Membranes
Nucleic Acids 4