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Gene Expression Part 1: Reading Genes to Make Proteins

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Peptide Bond Formation and Protein Building

A ribosome “workbench” has catalytic tools to go along with it’s ‘P’ and ‘A’ “vice sites”. The ribosome has enzymatic functions that allow it to break and form bonds. The ribosome will break the bond that binds the amino acid (met) to the tRNA at the ‘P’ site. Simultaneously the ribosome forms a peptide bond between the two amino acids that were brought in by the tRNAs. The tRNA at the ‘A’ site will now have two amino acids connected to it. The close proximity of these amino acids at the ribosome sites provides the opportunity for peptide bond formation (Fig. 19). Peptide bonds always bind the acid end of one amino acid with the amino end of the next (Fig. 6). Peptide bonds are strong covalent bonds which keep the amino acids connected during and after translation. The tRNA at the ‘P’ site has now accomplished its task by bringing in the first amino acid. This tRNA will then move off the ribosome (Fig. 19). The first two amino acids in the protein have now been put together.

 The synthesis of our protein is far from over. With the ‘P’ site now open, the ribosome will again shift one codon in the 5’ to 3’ direction and open up the ‘A’ site. A tRNA can now come into this site if it has the anticodon that is complementary to the next codon on the mRNA (Figs. 20,21). The ribosome will then form the peptide bond between the third and second amino acids and kick off tRNA number two (Fig. 21). The third tRNA now binds a chain of three amino acids. The ribosome shifts in preparation for tRNA number 4. The process continues as long as the ribosome encounters the proper tRNAs bringing in amino acids.

Fig.20: Ribosome holds tRNAs that complement the second and third codon. Fig.21: Ribosome will form the peptide bond, release the tRNA from the ’A’ site and move to the next codon.

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