The ideas we just talked about on the last page are more or less encapsulated by one of Chargaff's rules, which are a series of rules about the composition of DNA that were used by Watson and Crick to help determine DNA structure. Now there were a number of different rules Chargaff developed. However, we're really just going to focus on the one that deals with the base composition of DNA and it's more or less summarized by the statement that in a double-stranded molecule of DNA, the amount of adenine equals the amount of thymine and the amount of guanine equals the amount of cytosine. Likewise, the amount of purines will be equal to the amount of pyrimidines. And again, these are the ideas we were just talking about on the previous page. Watson and Crick didn't just use Chargaff's rules to determine DNA structure though. They also used an x-ray crystallography image from Rosalind Franklin, pictured right here. And the image they used, you can see right next to her, was called photo 51. I love this name. It's like super mysterious sounding, right? Like area 51, photo 51. It's great. And basically, what this image showed Watson and Crick is that DNA had a simple structure and it had substituents that were about 3.4Å apart and of course, those substituents are the bases we now know about. That is an important number to know guys, 3.4Å apart, make sure you memorize that. Alright. So the structure of DNA that Watson and Crick came up with is called a double helix. The double helix contains 2 grooves in it actually. A major groove and a minor groove and I'm going to trace those out for you so that you can, maybe see them a little better here. So, the minor groove is this smaller one in this opening right here and it kinda snakes along this way in the molecule. The major groove, on the other hand, And you can actually before we go there, you can actually see the minor groove right here. It's this space. The major groove, on the other hand, is this big open area here and you can see it wraps around the molecule like that and it's pictured over here as this big open area like that. So, the main point of these grooves is this is how stuff interacts with DNA. It binds into DNA generally using the major groove though sometimes the minor groove is used by certain specific proteins. But obviously, lots of proteins and enzymes need to bind DNA to carry out gene expression and to regulate gene expression. Now, it's important to note that the structural bonds of DNA are all single bonds. And that makes the molecule pretty flexible. And if you think about it, the nucleotides are also with the exception of some double bonds in the bases of course and the phosphate groups, the nucleotides are made of many single bonds as well and this means that they're also quite flexible and actually, nucleotides can have 2 conformations that you can see right here. There is the synform and the antiform. And basically, that is just a flip flop of the base on this single around on that single bond between these positions. However, in our DNA, the anti-form is generally the conformation seen. Syn is much less common. Also, that is the worst arrow I've ever drawn in my life. So, I'm redrawing it. Boom. Anti-form is more common. Alright. Now let's flip the page.
Review 1: Nucleic Acids, Lipids, & Membranes
Nucleic Acids 3