iGen3 06-17
Elongation of polypeptide

Think of the order of sites as EPA. (1) The fMet-tRNA initiates the process in the P site; subsequent charged tRNAs enter through the A site. (2) The fMet is cleaved trom the tRNA in the P site, and linked by a peptide bond to the amino acid in the A site. (3) The tRNA in the P sites moves to the E site, and the tRNA in the A site with the di-peptide moves to the P site. (4) The uncharged tRNA is released. (5) A new charged tRNA enters the A site: note that at this point the P site contains a peptidyl bond, and A site an aminoacyl bond. (Last) The dipeptide is cleaved from the tRNA in the P site, and linked by a peptide to the amino acid in the A site (as in step 2 above). The cycle continues in this manner.

Note that transfer of the amino acid from the first tRNA to the second tRNA may at first seem counter-intuitive. This however ensures that the original amino terminus of the first amino acid always remains unmodified: thus the polypeptide grows in the amino carboxy (NC).

Figure after 2010 PJ Russell, iGenetics 3rd ed.; all text material 2013 by Steven M. Carr