 Alright, I think this is going to be a short video, thank God. There are many different ways of describing a protein molecule, and I don't know why this is taught to students, at least students at the beginning, but I'm only going to teach you one of the ways, because even that, I think, is a little bit of a waste, but I have to test you on a little bit of this. Usually the ways of describing a protein molecule are referred to as describing the primary structure of a protein molecule, and that is abbreviated with a number one and a little degree symbol that means primary, or describing the secondary structure of a protein molecule that's written with the number two and a degree symbol, tertiary structure and quaternary structure. Each of those phrases has a specific meaning. I am not going to expect you to know secondary, tertiary, or quaternary structure of protein molecules and what they mean. The only one that I want you to know is primary structure, and believe it or not, we have already talked about primary structure, we just haven't called it that. Primary structure is just a fancy way of saying the order that the amino acids are connected in in your protein molecule. So, for example, if we were on a plane ride together and I had you trapped in your seat and you had to listen to me for four hours, I might say, oh, you know what my favorite molecule is? It's this one, and let me tell you the amino acid sequence. It's alanine connected to alanine connected to leucine, to isoleucine, to proline, to methionine, and I went on like that for about three hours and I described the entire molecule to you. What I am describing to you is the order that the amino acids are connected in. What I am describing to you is the primary structure of my protein molecule. That's all primary structure means. The question is why would you ever want to do this? Why would you ever want to describe the primary structure of a protein molecule? If I just talk about the order that the amino acids are connected in, it does not tell you the 3D shape of the protein. It just says this one comes first, that one comes second, and you know, you talk for a while and you reach the end. It doesn't tell you anything about the 3D shape of the protein. That sounds like it's a bad thing, but sometimes it's a good thing to not do that. Where it's useful is it's useful to describe where mutations take place or where changes in the sequence takes place. Let's say when you have a disease. So for example, this here is the primary structure of the protein molecule called hemoglobin beta. This is the protein molecule that's involved in sickle cell disease. And this is the primary structure. Every letter here is the one letter abbreviation for one of the amino acids. This is the end terminus. That's where the protein starts. This is the C terminus. That's where the protein ends. This does not tell you anything about the 3D shape, but that's actually an advantage here because it doesn't... There's nothing really complicated. I don't have to look at a bunch of squiggles and spirals and things like that. I can just lay the protein molecule out like it's words in a book, and then I can say, oh, you know what? This is the amino acid that gets changed to a valine when people have sickle cell disease. So it's very clear to people when you show them the primary structure where certain changes take place. Oh, this takes place near the beginning, or I have a change that takes place over here. That's the whole purpose of primary structure is just to make it clear to people where the amino acids are from the front to the end, and maybe where certain changes take place. So I want you to know what primary structure means. So I want you to know what primary structure means. And that's the end of this video. Hopefully, show it.