What is the primary structure of a protein?

• A. Nucleotides linked by peptide bonds
• B. Amino acids joined by peptide bonds
• C. Fatty acids linked by ester bonds
• D. Sugars linked by glycosidic bonds

Answer: (B)

Proteins are built from amino acids, and the primary structure is the exact linear sequence of those amino acids in the polypeptide chain. These amino acids are connected by peptide bonds, formed when the carboxyl group of one amino acid reacts with the amino group of the next in a condensation reaction. This specific order of amino acids sets the foundation for how the chain will fold into higher levels of structure, influenced by the properties of the side chains. The other options describe different macromolecules: nucleic acids are made from nucleotides linked by phosphodiester bonds, lipids from fatty acids linked by ester bonds, and carbohydrates from sugars linked by glycosidic bonds.