Then we moved on to the secondary structure and we said that secondary structure is the spatial arrangement of those amino acids. It's the interaction of those amino acids that are found in close proximity on that polypeptide chain. And we said that secondary structure consists of these regular patterns we call alpha helixes, betappleated sheets, beta turns and omega loops. And finally, in the previous lecture we discussed tertiary structure and we said that tertiary structure is the interaction, it's the spatial arrangement of those amino acids that are found far away on that polypeptide chain. In this lecture we're going to focus on the final level of protein structure. So all proteins contain primary structure.