What defines the tertiary structure of a protein?

The tertiary structure of a protein refers to its final three-dimensional shape, which is crucial for its functionality. This structure is formed through the folding and twisting of the polypeptide chain, driven by interactions among the side chains of the amino acids, such as hydrogen bonds, ionic bonds, hydrophobic interactions, and disulfide bridges. The tertiary structure determines how the protein interacts with other molecules, including substrates and other proteins, influencing its biological activity.

The other options relate to different aspects of protein structure or function. The sequence of amino acids primarily pertains to the primary structure, which simply indicates the order of amino acids in the polypeptide chain. Multiple polypeptide chains contribute to the quaternary structure, which involves the assembly of multiple protein subunits but does not define the tertiary structure of an individual polypeptide. Interaction with DNA could refer to specific biological functions or interactions of proteins but does not describe the structural arrangement of the protein itself. Thus, the defining characteristic of the tertiary structure is indeed its final three-dimensional shape.