Levels of Protein Structure

Steps:

• Primary structure is the linear sequence of amino acids in a polypeptide chain, determined by mRNA translation.
• Tertiary structure arises from interactions like hydrogen bonds and disulfide bridges folding the chain into a 3D shape.
• Quaternary structure forms when multiple polypeptide chains assemble into a functional protein.
• Evaluate options against these definitions to identify the accurate description.

Why D is correct:

• It precisely matches biochemical definitions: primary as amino acid sequence from mRNA, tertiary from side-chain interactions, and quaternary from multiple polypeptides.

Why the others are wrong:

• A: Misdefines primary as "number" of amino acids; tertiary not "cross all amino acids"; quaternary confuses polypeptides with the protein itself.
• B: Primary not directly "encoded by DNA" (it's via mRNA); quaternary vaguely mentions "two types" without specifying polypeptides.
• C: Primary not "result of mRNA molecule"; tertiary garbles interactions (e.g., "ionic by disulfide"); quaternary incomplete and erroneous.

Final answer: D