In this video, we'll learn some tricks to help us remember the structures of different pyrimidines. Here, first of all, when we say pyrimidine, this is the general structure of pyrimidine. The 3 pyrimidines that we have are just modifications of this original structure, and it all begins with Uracil. Uracil 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, which is not allowed. Nitrogen ideally wants to make 3 bonds. To do that, it would have to be connected to a hydrogen. So it would have a connection to the 2 carbons and then the 3rd bond would be to the hydrogen. We encounter the same issue here with this bottom nitrogen. We can't have a double bond like we have here because then this carbonyl carbon would be making 5 bonds; carbon can only have up to 4. So, for nitrogen to reach its ideal number of 3 bonds, it would be connected to a hydrogen. This represents the structure of uracil. 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 is very similar in structure to uracil; the only difference is the presence of a methyl group. The methyl group would be located here, and we'd still have our 2 carbonyls here and here, and our nitrogens would still have a hydrogen attached. This is thymine.
Cytosine is a little trickier, but just remember, we substitute a cyamine group. So, understanding the structure of cytosine: we would still have our carbonyl here and a hydrogen on this nitrogen here. But now, we retain this double bond just like this nitrogen here possesses a double bond, achieving its 3 bonds, so it does not need an additional hydrogen. Noting the cyamine, "amine" group pronounces the same as an amine, which is a -NH2 group. So rather than having 2 carbonyls, we have 1 carbonyl and 1 NH2 group here. This represents our cytosine. Remember, this is the starting structure of pyrimidine. The 3 pyrimidines are merely modifications of it. It all starts with uracil, and from there, you can adapt it to create thymine or cytosine. This is the key to remembering the structures of these different types of nitrogenous bases.
