Primary Protein Structure Definition

Steps:

• Identify the question asks for the correct description of protein structures, focusing on standard levels (primary, secondary, etc.).
• Parse each garbled option to match biological definitions: primary (amino acid sequence), secondary (coiling/sheets), tertiary (3D folding).
• Eliminate options with mismatches, like confusing levels or incorrect bonds.
• Select the option accurately defining primary structure via mRNA coding.

Why D is correct:

• Primary structure is the linear sequence of amino acids in a polypeptide, directly coded by the mRNA sequence during translation (central dogma of molecular biology).

Why the others are wrong:

• A: Mixes primary with secondary elements like cross-links, which are tertiary.
• B: Confuses primary sequence with secondary coiling and beta-sheets.
• C: Describes tertiary folding via hydrogen/disulfide bonds, not mRNA directly.

Final answer: D