Two amino acids can be linked by a CONDENSATION reaction (removal of a water molecule)

This creates a PEPTIDE BOND between the Carbon atom in the acid group and the Nitrogen atom in the amine group

The resulting molecule is called a DIPEPTIDE

A chain of amino acids can be built up in this way - it is called a POLYPEPTIDE

A protein may contain just one polypeptide or may have two or more chains that interact