This chain folds into a unique 3D structure to form a protein. The side chains distinguish the AA's from each other. There are 20 different kinds of AA's. They can be classified as either charged or uncharged, polar or non-polar, or hydrophobic or hydrophilic. Probably the most structuraly complex molecules known.
Polypeptide backbone has many different amino acid side chains attached to it.
Many of the covalent bonds allow free rotation, the chain can fold in an enormous number of ways. There are a huge number of 3D shapes a protein can be in due to the folding of the polypeptide chain. The nitrogen containing rings are referred to as bases. Most fundamental role is the storage and retrieval of biological information. Genetic information directs the synthesis of proteins.