Which description best defines the primary structure of a protein?

• A. Three-dimensional arrangement of atoms
• B. Linear sequence of amino acids bound by peptide bonds
• C. Folding into helices and sheets
• D. Assembly of multiple subunits (quaternary structure)

Answer: (B)

The main idea here is that the primary structure is the exact linear order of amino acids in a polypeptide, linked together by peptide bonds. This sequence, recorded by genetic information, dictates how the chain will later fold into its functional form. The description mentioning a three-dimensional arrangement of atoms points to higher levels of structure, such as tertiary or quaternary structure, not the primary level. The notion of folding into helices and sheets describes secondary structure—local patterns like alpha helices and beta sheets. The idea of assembling multiple subunits refers to quaternary structure, which is about how several polypeptide chains come together. So, the linear sequence of amino acids bound by peptide bonds best defines the primary structure.