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The amino acids are joined to form proteins by peptide bonds. The formation of peptide bonds requires a good amount of energy. Therefore, in the first phase of translation for protein synthesis, the amino acids are activated in the presence of ATP and linked to their cognate tRNA (transfer RNA). This process is called as charging of the tRNA or aminoacylation of tRNA. If two such charged tRNA are brought close enough, the formation of peptide bond between them is favored energetically. This function occurs inside the , as it contains two sites for subsequent amino acids to bind to and thus be close enough for bonding.
Protein Synthesis - Elmhurst College
The process of polymerization of amino acids to form a polypeptide is called as Translation. It is the second and final step of protein synthesis. The order into which the amino acids are arranged is defined by the bases in mRNA (messenger). Ribosome is the cellular factory responsible for the protein synthesis. The ribosome consists of structural RNAs and about 80 different . It is in inactive stage and exists as two subunits, one large and other small. The synthesis of begins when the small subunit encounters an mRNA. The ribosome also acts as a catalyst for the formation of bonds.
9. As the ribosome moves by two codons, next round of protein synthesis is initiated by the attachment of a new ribosome. Thus, at a time, a single mRNA is found to be attached with many ribosomes with their polypeptides of different length, (shortest polypeptide at the 5′ end of the mRNA and longest at the 3′ end), called polysomes.
Fig. 8.15 Peptide bond formation in growing polypeptide.
10. Ultimately, the A-site of ribosome is occupied by the termination codon (UAA,UAG or UGA) at the 3′ end of mRNA, which is not recognized by any tRNA. Thus, the termination of the protein synthesis is helped by the release factors RFl, RF2 and RF3 (in eukaryotes eRF1), which release the newly synthesized polypeptide chain from the P-site (Fig. 8.16).