Advanced Placement (AP) Biology Practice Exam

What describes the primary structure of a protein?

1. A specific 3D shape of the entire molecule

2. A sequence of amino acids

3. Localized folding patterns

4. The arrangement of multiple polypeptide chains

The correct answer is: A sequence of amino acids

The primary structure of a protein is accurately described as a sequence of amino acids. This sequence is determined by the genetic code and dictates how the protein will fold and function. Each amino acid is linked to the next by peptide bonds, forming a polypeptide chain. The specific order of these amino acids is crucial because even a single substitution can significantly impact the protein's structure and function, potentially leading to various diseases or malfunctions. In contrast to primary structure, the other levels of protein structure include secondary, tertiary, and quaternary structures. Secondary structure refers to localized folding patterns such as alpha helices and beta sheets. Tertiary structure refers to the overall three-dimensional shape of a single polypeptide chain, while quaternary structure describes the arrangement and interaction of multiple polypeptide chains within a protein. Understanding these distinctions is important in the study of proteins and their functions in biological systems.