Protein Synthesis And The Proteins

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Protein synthesis starts when the mRNA binds to a small ribosomal subunit and the first codon (AUG) is positioned correctly for the initiation of protein synthesis. The AUG codon then base-pairs with the anticodon of tRNA carrying methionine. This methionine eventually becomes the first Amino Acid in the polypeptide chain. Now a large ribosomal subunit binds to the complex and the reaction of protein synthesis itself can begin. (Fig. 3) A ribosome has two sites where tRNA can bind. As a result, of the events outlined above, the initiation codon, AUG, on the mRNA molecule is positioned at the first of these sites on the ribosome, the peptidyl (P) site. The mRNA codon for the second Amino Acid is lined up with the second site, the aminoacyl (A) site. From this point, the polypeptide chain elongates by a cycle of three steps: The first step is the binding of the next tRNA to an anticodon complementary to the next mRNA codon. The amino Acid carried by this trNA will be the next amino acid in the polypeptide chain. The second step is the peptidyl transferase reaction, which results in the peptide bond formation. The peptidyl transferase enzyme, which catalyses the reaction, is an integral part of the large ribosomal unit. This enzyme attaches the first Amino Acid to the second one by a peptide bond, so that the first tRNA is now empty and the second is holding both amino acids. The third step in the cycle moves the ribosome along mRNA. This brings the next codon to the
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