Determine which of the following statements describes a tertiary structure of a protein.

• A. Three polypeptide chains interact to form a biologically active protein. 
• B. Hydrogen bonds form between adjacent segments of the backbones of the same protein to form its creased structure.
• C. Nonpolar side chains are repelled by water and move to the interior of the protein. 
• D. Amino acids react in a condensation reaction to form a peptide bond. 

Answer: (C)

Verified step by step guidance

1. Understand the levels of protein structure: Primary structure is the sequence of amino acids, secondary structure involves local folding patterns like alpha-helices and beta-sheets, tertiary structure is the overall 3D shape of a single polypeptide chain, and quaternary structure involves the assembly of multiple polypeptide chains.
2. Identify the key feature of tertiary structure: It involves the overall 3D folding of a single polypeptide chain, which is stabilized by various interactions, including hydrophobic interactions, hydrogen bonds, ionic bonds, and disulfide bridges.
3. Analyze the given statements: The first statement describes quaternary structure as it involves multiple polypeptide chains. The second statement describes secondary structure due to hydrogen bonding within the same protein. The fourth statement describes the formation of peptide bonds, which is related to primary structure.
4. Focus on the third statement: 'Nonpolar side chains are repelled by water and move to the interior of the protein.' This describes a key aspect of tertiary structure, where hydrophobic (nonpolar) side chains tend to cluster away from the aqueous environment, contributing to the protein's 3D shape.
5. Conclude that the correct description of tertiary structure is the movement of nonpolar side chains to the protein's interior, as this is a defining characteristic of how proteins achieve their functional 3D conformation.