 You know during translation, the ribosome works in a cyclic way that all the changes, all the processes take place in cycles. So the ribosomes cycle of aminoacyl, tRNA binding, peptide bond formation and translocation continues until one of the stop codon enters the A site. So this process goes on as soon as any stop codon comes to the A site, the first site in the ribosome. When it comes there, this process stops. It was initially postulated that there would be one or more chain terminating tRNAs that would recognize these codons. However, this is not the case. Instead, stop codons are recognized by the proteins called release factors that activate the hydrolysis of the polypeptide from the peptidyl tRNA. There are two classes of release factors, class one and class two. Class one release factors recognize the stop codons and trigger hydrolysis of the peptide chain from the tRNA in the P site. You know the polypeptide is attached at the P site of the tRNA. So if this polypeptide is released or cut from the P site, it will be released. Prokaryotes have two class one release factors called RF1 and RF2. RF1 recognizes the stop codon UAG and RF2 recognizes the stop codon UGA. While the third stop codon, which is UAA, this is recognized by both the RF1 and RF2. In eukaryotic cells, there is a single class one release factor, which is called ERF1 that recognizes all the three stop codons. Class two release factors stimulate the dissociation of the class one factors from the ribosome after release of the polypeptide chain. So class one release factors will first bind the ribosome and release the polypeptide. And now who will release them? To release them, class two release factors will bind and they will release them. Prokaryotes and eukaryotes have only one class two factor called RF3 and ERF3 respectively. So in prokaryotes, this is RF3 and in eukaryotes, this is ERF3. Like EFG, IF2 and EFTU, class two release factors are regulated by GTP binding and hydrolysis. So you will remember these EFG and EFTU and IF2 are associated with GTP and GTP hydrolysis. So class two release factors are also dependent on GTP hydrolysis. How do release factors recognize stop codons? Because release factors are composed entirely of proteins. So protein RNA interaction must mediate stop codon recognition. Because release factors are proteins and ribosomes are RNA. So if protein and RNA interaction occurs, then this recognition can occur. Experiments in which short coding regions were genetically swapped between RF1 and RF2. Because these RF1 and RF2 have different stop codon specificities. They identified a three amino acid sequence that is critical for release factor specificity. Research of these three amino acids between RF1 and RF2 swaps the stop codon specificity of two complexes. For this reason, this three amino acid sequence is called a peptide anticodon and must interact with and recognize stop codon. A 3D structure of RF1 bound to the ribosome confirms that RF1 binds to the A site of the ribosome. In this structure the peptide anticodon is located very near the anticodon. But it is likely that there are additional protein regions that contribute to codon recognition. So there are some additional regions that contribute to codon recognition. Origin of class 1 release factors that stimulates polypeptide release has also been identified. All class 1 factors share a conserved three amino acid sequence and these three amino acid sequences glycine, glycine, glutamine. And it is essential for polypeptide release. Moreover, the structure of RF1 bound to the ribosome confirms that the GGQ motif is located in close proximity to the peptidyl transferase center. It remains unclear whether the GGQ motif is directly involved in the release of polypeptide from the peptidyl tRNA or it induces a change in the peptidyl transferase center that allows the center itself to catalyze the hydrolysis of polypeptide chain.