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Protein structure is the three-dimensional arrangement of atoms in an amino acid-chain molecule. Proteins are polymers – specifically polypeptides – formed from sequences of amino acids, the monomers of the polymer. A single amino acid monomer may also be called a residue indicating a repeating unit of a polymer. A monopeptide has one amino acid. A dipeptide has two amino acids. A tripeptide has three amino acids. A tetrapeptide has four amino acids. The complete structure of a protein can be described at four different levels of complexity: primary, secondary, tertiary, and quaternary structure.