Final answer:
Each domain in a protein is formed from a sequence of amino acids that folds independently into a 3D structure.
Step-by-step explanation:
Each domain is formed from a sequence of amino acids in the polypeptide chain that is folded into a 3D structure independently of the rest of the protein. The primary structure of a polypeptide is its amino acid sequence. Interactions between amino acids near each other in the sequence cause the polypeptide to fold into secondary structures, such as alpha helix and beta-pleated sheet. Tertiary structures form when non-covalent interactions between amino acids at some distance from each other in the primary sequence cause the polypeptide to fold further into a complex 3-dimensional structure.