iGen3 06-17
Stepwise elongation of polypeptide chains

    The diagram presents the messenger RNA (mRNA) molecule as "fixed", with the 5'3' direction oriented left to right". The Ribosome "moves" along the message from left to right, thus the mRNA is read 5'3' [or, the mRNA passes through the Ribosome in the 5'3' direction]. Note that the Ribosome has already encountered the Shine-Delgarno promoter sequence in the mRNA.

    Given this orientation, the diagram therefore shows the left-to-right order of the Exit, Peptidyl, & Aminoacyl sites as EPA. However, the functional order is APE, since successive tRNAs enter through the A site, pass to the P site, and exit from the E site.

(1) The fMet-tRNA that initiates the process is shown already in the P site. The next charged tRNA enter the A site.

(2) The fMet is cleaved from 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 from the E site.

(5) A new charged tRNA enters the A site. At this point, the P site contains a peptidyl bond, and the A site an aminoacyl bond.

(6) The di-peptide is cleaved from the tRNA in the P site, and linked by a peptide bond to the amino acid in the A site (as in step 2 above). This produces a tri-peptide in the A site.

The cycle continues in this manner. The tRNAs in the P and A sites shift to the E and P sites, the uncharged tRNA in the E site drops out, a new tRNA with the amino acid for the next codon enters the A sites, and so on.

    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 ©2016 by Steven M. Carr