The individual building blocks that peptides and proteins are assembled from — like letters in an alphabet that spell out different molecules.Twenty standard amino acids are encoded by the human genome. Each contains an amino group (NH₂), a carboxyl group (COOH), and a variable side chain that determines its chemical properties — whether it’s acidic, basic, hydrophobic, or polar. Peptide bonds form between the carboxyl group of one amino acid and the amino group of the next, releasing water. The sequence and chemistry of the side chains determine the three-dimensional structure and function of the finished peptide.