The two ends of a polypeptide chain
Free amino group vs. free carboxyl group
Every polypeptide chain has two distinct ends. The N-terminus carries the free amino group (–NH₂) of the first amino acid, and the C-terminus carries the free carboxyl group (–COOH) of the last one. Ribosomes build proteins from the N-terminus to the C-terminus, adding each new amino acid to the carboxyl end of the growing chain. Amino acid sequences are therefore written and read in this direction, usually starting with methionine (M), which the start codon encodes. The ends of a protein often carry signals, for example a sequence that directs the protein into an organelle.