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Translation: Elongation and Termination
Following initiation, the ribosome moves on to elongation. During elongation, the next amino acid in line is brought into the A-site of the ribosome by the appropriate transfer RNA molecule. Now the ribosome contains two different aminoacyl tRNA molecules (also known as charged tRNA molecules), one in the P-site and one in the A-site. An enzyme called peptidyl transferase then creates a peptide bond between the two amino acids. The amino acid that is attached to the tRNA in the P-site then breaks off from the tRNA. At this point, the tRNA in the P-site no longer contains the amino acid and the growing polypeptide chain is attached entirely to the tRNA in the A-site. Now the ribosome is ready to move along the mRNA, in the 5' to 3' direction. As the ribosome moves three nucleotides towards the 3' end, the uncharged tRNA that was initially in the P-site moves into the E-site, which is the site that is responsible for expelling the tRNA from the ribosome. At the same time, the charged tRNA that carries the polypeptide chain moves from the A-site and into the P-site. This process by which the ribosome moves along the mRNA is known as translocation. Once translocation occurs, the cell can repeat the entire process to add more amino acids. Termination takes place when the ribosomes A-site reaches a special sequence of nucleotides known as the stop codon. The stop codon corresponds to either UAG, UAA or UGA. Once this sequence is reaches in the A-site, a special group of proteins called the release factor bind to the A-site and this causes the polypeptide chain to break off. Once the polypetide breaks off, the entire ribosome then dissociates. This final stage of translation is known as termination.
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