The mRNA binds to a ribosome at the start codon. (1) The ribosome has a space for a tRNA molecule with a three base anticodon to bind to the complementary codon on the mRNA.(2) The tRNA brings the specific amino acid (3) which starts the polypeptide chain. Then the ribosome moves along to the next codon and binds another tRNA (4). The amino acids from the two tRNA molecules form peptide bonds by a condensation reaction to form the polypeptide (5) and this is repeated until the ribosome reaches a 'stop' codon.(6)