Amino acids can be polymerized with the loss of to form a polypeptide chain, as show on the right. This chain is the backbone of all protein molecules.

We will see in Chapter 22 the ways in which a polypeptide chain can fold into a three-dimensional protein molecule.

The order of side groups, here symbolised by R-, determines what protein the chain will form, and how it will fold.

Twenty different side chains are coded in the genetic machinery of living organisms; the order or sequence of these side chains in the protein-to-be is specified by the order in which purine and pyrimidine bases appear along the chain sequence in DNA.