Once the mRNA leaves the nucleus a ribosome attaches to the start codon (triplet of bases) which is always AUG.A tRNA molecule with the complementary anticodon UAC bound to the amino acid methionine attaches to the mRNA. Complementary base pairing and hydrogen bonding occurs between the codon of the mRNA and anticodon of the tRNA.Another tRNA whose anticodon is complementary to the next codon binds to the mRNA by base-pairing. Each tRNA molecule is attached to a specific amino acid.The large sub unit of the ribosome acts as an enzyme peptidyl transferase and uses energy from the hydrolysis of ATP to join the two amino acids together with a peptide bond.The ribosome moves along the mRNA strand as the first tRNA molecule no longer bound to its amino acid detaches from the mRNA. Another amino acid bound tRNA molecule with an anticodon complementary to the next codon binds to the mRNA at the next codon and the process of joining amino acids together and the ribosome moving along the mRNA to the next codon continues so that about 15 amino acids are joined every second.The peptide chain grows until a stop codon is reached. The ribosome detaches.Up to 50 ribosomes may be bound to one mRNA chain at a time moving along the mRNA behind each other.