Which level of protein organization is described as the initial description of a protein based on the type, number, and order of amino acids in the chain?

• A. Secondary structure
• B. Primary structure
• C. Tertiary structure
• D. Quaternary structure

Answer: (B)

The main concept here is that a protein’s primary structure is its linear sequence of amino acids. This level is defined by the type, number, and order of amino acids in the chain, connected by peptide bonds. The sequence is encoded by the gene and serves as the starting point that determines how the protein will fold into higher levels of structure. Secondary structure involves local folding into motifs like alpha helices and beta sheets formed by hydrogen bonds along the backbone. Tertiary structure is the overall three-dimensional shape of a single polypeptide, stabilized by interactions among side chains. Quaternary structure describes the arrangement of multiple polypeptide subunits in a protein. Since the description specifies the initial description based on the amino acid sequence, it corresponds to the primary structure.