In this beetle, we'll learn some tricks to help us remember the structures of different pyrimidines. So here, first of all, when we say pyrimidine, this is the general structure of pyrimidine. The 3 pyrimidines that we have are just modifications to this original structure, and it all begins with Uracil. Uracil here has its 2 nitrogens just like pyrimidine does, but it also has 2 carbonyl groups. So we have a double bond O here and a double bond O here. Now, here, we had a double bond, but we can no longer have a double bond here because then this carbon would be making 5 bonds. So that's not allowed. But nitrogen ideally wants to make 3 bonds. To do that, it would have to be connected to a hydrogen. So, it'd have a connection to the 2 carbons and then the 3rd bond will be to the hydrogen. We went into the same issue here with this bottom nitrogen. We can't have a double bond like we have here because then this carbonyl carbon will be making 5 bonds. Carbon can only go up to 4. So for nitrogen to reach its 3 bonds, its ideal number of bonds, it'd be connected to a hydrogen. So this represents the structure of uracil. So just remember, we have our 2 carbonyls here and then each of the nitrogens to make their 3rd bond connects to a hydrogen.

Now, here uracil, the other two pyrimidines are just modifications of this, thymine, and cytosine. Thymine and uracil are very similar in structure, the only difference is that there's a methyl group involved. The methyl group would be right here, and then we'd still have our 2 carbonyls here and here, and our nitrogens would still have an H each. So this is thymine.

Cytosine is a little bit trickier. But just remember, we're going to say, cytosine is cytosine amine group. So when we see amine group, kind of have a moment like, oh gosh. Here it goes. Si amine group. So what does this mean? Well, we're going to still have our carbonyl here. We're going to still have an H on this nitrogen here. But now, we're still going to possess this double bond just like this nitrogen here possesses a double bond. It's making its 3 bonds, so it doesn't need an H on it. And then, cytosine amine. Amin. Right? It's pronounced the same way as an amine, the functional group amine, which we know is an NH₂ group. So instead of having 2 carbonyls, we have 1 carbonyl and 1 NH₂ group right here. So this represents our cytosine. So just remember, this is the starting structure of pyrimidine. The 3 pyrimidines are just modifications of it. It all starts with uracil and from there, from uracil, you can adapt it to give us thymine or to give us cytosine. This is the key to remembering the structures of these different types of nitrogenous bases.