Protein Structure Levels Defined

Steps:

• Recall primary structure as the linear sequence of amino acids in a polypeptide chain, determined by mRNA.
• Identify secondary structure as local folding patterns like alpha helices or beta sheets, stabilized by hydrogen bonds between backbone atoms.
• Note quaternary structure involves multiple polypeptide chains associating to form a functional protein.
• Evaluate options against these definitions to find the accurate description.

Why D is correct:

• It precisely matches standard biochemical definitions: primary as amino acid sequence from mRNA, secondary via hydrogen bonds in the chain, and quaternary as multi-subunit assembly.

Why the others are wrong:

• A: Misdefines secondary as left-handed spiral (it's right-handed alpha helix) and quaternary as subunit (it's multi-subunit complex).
• B: Errs on primary (it's in protein, not DNA) and quaternary (no "two types of peptide").
• C: Confuses primary with mRNA process and secondary with oxygen atoms; quaternary lacks sense.

Final answer: D