Amino acids join together by condensation reactions. The -OH from the carboxyl group of one amino acid combines with an -H from the amino group of another, and a molecule of water is removed.The covalent bond that forms between the carbon of the carboxyl group and the nitrogen of the amino group is called a peptide bond, -CONH-. Two amino acids joined in this way form a dipeptide.Further condensation reactions add more amino acids to form a polypeptide. In cells this happens on ribosomes during translation. A polypeptide always has a free amino group at one end (the N-terminal) and a free carboxyl group at the other end (the C-terminal).Because the order matters, two different dipeptides can be made from the same two amino acids. For example, gly-ala and ala-gly are different molecules.Peptide bonds can be broken by hydrolysis, which adds a water molecule back. In the body this is catalysed by protease enzymes.