Polypeptides are chains of amino acids. Proteins are made up of one or more polypeptide molecules. The amino acids are linked covalently by peptide bonds. The picture below shows how three amino acids are linked by peptide bonds into a tripeptide.
Figure 2.9.1: A tripeptide generated from glycine, alanine and cystein amino acids
One end of every polypeptide, called the amino terminal or N-terminal, has a free amino group. The other end, with its free carboxyl group, is called the carboxyl terminal or C-terminal. The schematic below shows the N-terminal at the upper left and the C-terminal at the lower right.
Figure 2.9.2: A longer peptide.
The sequence of amino acids in a polypeptide is dictated by the codons in the messenger RNA (mRNA) molecules from which the polypeptide was translated. The sequence of codons in the mRNA was, in turn, dictated by the sequence of codons in the DNA from which the mRNA was transcribed. Proteins are made up of one or more polypeptide molecules.