What does primary structure describe?

• A. The sequence order and composition of amino acids.
• B. The 3D arrangement due to R-group interactions.
• C. The regular patterns of the backbone.
• D. The arrangement of multiple polypeptides.

Answer: (D)

Primary structure is the linear sequence of amino acids in a single polypeptide, defined by the exact order and identity of residues linked by peptide bonds. This sequence sets the stage for how the chain can fold and interact later on, and even small changes can alter folding and function. The option describing the arrangement of multiple polypeptides refers to quaternary structure, not primary. Other options describe higher levels of structure: the regular backbone patterns that form secondary structures like alpha helices and beta sheets, and the 3D arrangement of a single chain driven by side-chain interactions that defines tertiary structure.