 Tertiary structure of protein is actually the tertiary level of organization of protein molecules. So, the overall three-dimensional arrangement of all the atoms in a protein molecule is referred to as the protein's tertiary structure. So, if a polypeptide chain, it assumes a fully folded and three-dimensional structure that is a tertiary structure of protein. Here you can see this is single polypeptide chain and it has assumed the tertiary structure by folding back and forth and it assumes fully functional three-dimensional structure. Here in this structure, you can easily see the disulfide bonds have been formed between the different amino acids of the same polypeptide chain. As you know, in a secondary structure, the secondary structure includes the local aspect of confirmation, but the tertiary structure includes longer range of amino acid sequences. For example, the amino acids that are far apart in the polypeptide chain may interact within the completely folded structure of a protein. For example, if a protein contains 16 amino acid at position 5 and the second 16 amino acid may be present at position 90, so they are very far from each other. But when the polypeptide will fold into its tertiary structure, the 90 number 16 may come close to the 16 number 5 and they both may form disulfide linkage. So, in this way, the number of weak interactions and covalent interactions are made between quite far amino acid residues. Interacting segments of a polypeptide chain in a tertiary structure, they are held in their characteristic tertiary positions by different kind of weak interactions and sometimes by covalent bonds as well. So, as you know, this tertiary structure is fully folded confirmation and this fully folded confirmation is also a stable confirmation like alpha helices and beta sheets. And how these segments are remained at their place and they remain at their place through the weak interactions as well as covalent interactions. As I mentioned, the far located 16 bonds, 16 amino acids can make disulfide linkages. Similarly, very far located amino acids can make hydrogen bonds, hydrophobic interactions, ionic interactions and some covalent interactions. So, in this way, a stable three dimensional structure is formed which is tertiary structure of proteins. Some very large polypeptide chains usually fold into two or more global clusters and these clusters are called domains which the presence of these domains often give these proteins a bi or multi-lobal appearance. So, there is a fully folded protein molecule and in this molecule there are many maybe two or more smaller clusters of polypeptide chains and these smaller clusters are called domains. Like here in this pictures you can see this is a protein and this is at its tertiary level of organization. In this protein you can clearly see one domain is this, other domain is this and the third domain is this. So, this whole molecule contains three domains and it appeared that this protein is trilobular. Three lobes are present in this molecule. Similarly, in this molecule you can see this is a three dimensional structure of a protein which is at its tertiary level of organization and in this protein although it is tightly folded and it is hard to distinguish domains but even then on close observations you can find the domains. One is this, other is this and third one is this. So, these three domains are also present in this protein molecule. So, in this way the larger proteins they make domains and these domains are further made up of some super secondary structures which are also called as motif.