Protein Structure Levels Steps:

• Recall four levels: primary (amino acid sequence), secondary (local folding like helices via H-bonds), tertiary (overall 3D shape from side-chain interactions), quaternary (assembly of multiple polypeptides).
• Examine options for mismatched descriptions of these levels.
• Identify fragments in D that align with quaternary (linking polypeptides) while others mix levels incorrectly.
• Confirm D's final clause matches quaternary definition.

Why D is correct:

• Quaternary structure forms when multiple polypeptide chains (subunits) link via non-covalent interactions, as stated in D's last part.

Why the others are wrong:

• A: Mixes primary (amino acid count) with secondary (right-handed spiral) and tertiary (cross-linking side chains), plus quaternary (subunits).
• B: Confuses primary (amino acid order from mRNA) with secondary (alpha-helix via H-bonds) and tertiary (folding by H-bonds).
• C: Jumbles translation (mRNA to amino acids), peptide bonds (H-bonds), tertiary (ionic/hydrophilic interactions), and incomplete quaternary reference.

Final answer: D