Two individual amino acids can be linked to form a larger molecule, with the loss of a water molecule as a by-product of the reaction. Here is a textual representation of such a reaction.

The newly created C-N bond between the two separate amino acids is called a peptide bond. The term 'peptide bond' implies the existence of the peptide group which is commonly written in text as -CONH- .

Two molecules linked by a peptide bond become what is called a dipeptide. A chain of molecules linked by peptide bonds is called a polypeptide.

A protein is made up of one or more polypeptide chains, each of which consists of amino acids which have been mentioned earlier.

Instead of writing out complex formulae, sequences of amino acids are commonly written using the previously mentioned three- or one-letter codes eg ala-gly-val-leu-phe (3 letter) or AGVLF (1 letter).
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