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Proteins are the polymers of amino acids, linked together by the peptide bonds. When this results in the formation of a linear polypeptide chain then it is called the primary structure of a protein. The sequence of amino acids in a polypeptide chain is determined by the sequence of codons present in the mRNA.

The primary structure of the protein is the sequence of amino acids joined by peptide bonds in a linear fashion. When these polypeptide chains fold into helix or sheets then it is called the secondary structure of proteins. When the proteins fold into the three-dimensional structure due to side-chain interactions then it is called the tertiary structure of proteins. And when the protein is made up of many polypeptide chains or subunits, then it forms the quaternary structure.