Cells Recognize Prokaryotic Molecules

Eventually, one of three termination codons (also called Stop codons) becomes positioned in the A site (Fig. 7). These are UAG, UAA and UGA. Unlike other codons, prokaryotic cells do not contain aminoacyl-tRNAs complementary to Stop codons. Instead, one of two release factors (RF1 and RF2) binds instead. RF1 recognizes UAA and UAG whereas RF2 recognizes UGA. A third release factor, RF3, is also needed to assist RF1 or RF2. Thus either RF1 + RF3 or RF2 + RF3 bind depending on the exact termination codon in the A site. RF1 (or RF2) binds at or near the A site whereas RF3/GTP binds elsewhere on the ribosome. The release factors cause the peptidyl transferase to transfer the polypeptide to a water molecule instead of to aminoacyl-tRNA, effectively cleaving the bond between the polypeptide and tRNA in the P site. The polypeptide, now leaves the ribosome, followed by the mRNA and free tRNA, and the ribosome dissociates into 30S and 50S subunits ready to start translation afresh. 

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