Hi. In this video, I'm going to be talking about DNA structure. So most of this is going to be a review from bio from your intro bio classes that I do just want to go over in case you've forgotten or, you know, just don't necessarily want to go back and review yourself, which is totally fine. So, first, DNA stands for deoxyribonucleic acid, and has 3 main components. These components include a phosphate, a sugar, which for DNA is deoxyribose, and this is different from RNA, which has ribose. The difference between the two sugars is what is at the 2^{nd} carbon position, so in deoxyribose it's a hydrogen, and in ribose it's a hydroxyl group, and these are present at the 2^{nd }carbon, and actually this is what gives DNA and RNA very different qualities, It's just this sugar here.

And then finally, there's a nitrogenous base, so these are the 3 components, 1, 2, 3, and the base could be adenine, guanine, guanine, thymine, and you may see there are two names for these, for different combinations of these three components. You may see nucleosides and nucleotides and wonder how they're different. Nucleosides contain the base and the sugar, where nucleotides contain the base, sugar, and phosphate group. So in case you come across these two different names, that's what makes them different. So here's an example of a nucleotide. You have your phosphate group, you have your sugar, and you have your base. And those are the 3 components that make up a nucleotide.

Now the nitrogenous bases, so this part here, fall into 2 categories. You have purines, these are bases with double rings, so this is what's shown here, purine, you have 1 ring, 2 rings here, and this is adenine or guanine. And then the second class is pyrimidines, and these have a single ring and include cytosine and thymine. It's important to know that purines always pair with pyrimidine, so you have one double ring and one single ring in every nucleotide base pair. And then there's this fancy guy named Chargaff, and he studied the ratio of all of these nitrogenous bases in organisms, and he found that the ratios were that A pairs with T and C pairs with G, and these are the correct pairings. So here we have, adenine and thymine, and these pair together, and we have guanine and cytosine, these pair together. These 2 are the purines, these 2 are the pyrimidines, and a purine always pairs with the pyrimidine.

Now, there are 2 types of bonds that create DNA. The first is the phosphodiester bonds, and these connect the nucleotides together in a single strand. So this is the backbone. This is one strand, how is connected together through these phosphodiester bonds. Hydrogen bonds are the second type of bond, and these connect the complementary strands together, and that's what is referred to, the second strand that has the inverted nucleotide sequence on it. Right? Because if you have TTCG, then you have in order for these to pair, you have to have the opposite. Right? So here you have hydrogen bonds, you have your first strand, you have your complementary strand over here, and, phosphodiester bonds are here. Phosphodiester bonds connect this strand together. Now, hydrogen bonds actually differ depending on pairing it is. So if it's a GC pairing, there are 3 hydrogen bonds, and if it's an AT pairing, there are 2, and this actually makes DNA with more GC bands actually stronger than DNA with more AT bands, because there's an extra hydrogen bond for every GC baron. Now, we say that the complementary strands here are antiparallel, right, because it means that the nucleotides are inverted. So we give them, sides based on the position of the carbon. So here we have the 5^{th} prime and the 3^{rd} prime side, and this pairs with the 5^{th} prime and the 3^{rd} prime side over here. And this refers to the position of the carbon on the sugar, this 5^{th} prime or 3^{rd} prime. It's the 5^{th} prime carbon or the 3^{rd} prime. And this is orientation. So if one strand is 5^{th} prime to 3^{rd} prime, the second complementary strand will be inverted, and that makes it antiparallel. It'll run 3^{rd} prime to 5^{th} prime, and when they come together like this, these two strands, they form a double helix, and the double helix has a major groove, which has more base pairings in it, and a smaller minor groove. So here we have our double helix. The hydrogen bonds here are forming between the bases. You can see them here with a T and A. There's 2. 1, 2. With a G and a C, there's 3. 1, 2, 3. Now, the backbone that's curling around here, these are the phosphodiester bonds. And, you have your major group, which is bigger. You have your minor group, which is smaller. And I'm trying to think any of these differences. If you have one going 5^{th} prime, 5^{th} prime, let's follow this around, to 3^{rd} prime, then you have 5^{th} prime to 3^{rd} prime. And that makes them antiparallel, and the complementary is the recurring to the, you know, the actual nucleic acids being complementary to each other and allowing them to bind together. So that's the overview of DNA, what it consists of, its structure, how it all binds together, and what we all call the different sides of everything. So with that, let's now move on.