Three of the 64 codons, UAG, UAA, and UGA, do not specify any amino acid. When a translating ribosome encounters such a stop codon, no amino acid is inserted. Instead, one of two release factors binds to the stalled ribosome and causes the release of peptidyl‐tRNA. The release factors are codon‐specific. RF1 causes termination at UAA and UAG, while RF2 recognizes UAA and UGA. A third factor, RF3, is a GTPase and helps dissociate the complex. The ribosomal subunits are now separated from each other in preparation for the next initiation event. The binding of initiation factors to the 30S subunit helps this.
Protein synthesis is an energy‐intensive process. High‐energy phosphate bonds are expended for each peptide bond formed. One high‐energy bond is consumed when an amino acid is activated by its aminoacyl‐tRNA synthetase. Delivery of aminoacyl‐tRNA by EF‐Tu consumes one GTP per amino acid, and the translocation reaction consumes another.