Final answer:
Proteins have four levels of structure: primary, secondary, tertiary, and quaternary. The primary structure is the sequence of amino acids, the secondary structure is the organized sections of the chain, the tertiary structure is the overall folded 3D structure, and the quaternary structure refers to the arrangement of multiple chains.
Step-by-step explanation:
The four levels of protein structure are primary, secondary, tertiary, and quaternary.
The primary structure refers to the sequence of amino acids in a polypeptide chain.
The secondary structure is the organized sections of the polypeptide chain, such as alpha helices and beta sheets.
The tertiary structure is the overall folded 3D structure of the entire polypeptide chain, held together by various interactions.
The quaternary structure describes how multiple polypeptide chains come together to form a larger protein.