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Steps of the translation process.
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Initiation.
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The small ribosomal subunit binds to the initiator tRNA carrying the initiator amino acid methionine.
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This complex then attaches to the cap structure at the five prime end of an mRNA and scans for the start codon AUG.
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The process is mediated by several initiation factors at the start codon.
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The large ribosomal subunit joins the complex and all initiation factors are released.
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The ribosome has three sites.
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The A site is the entry site for new tRNA charged with amino acid or aminoacyl tRNA.
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The P site is occupied by peptidyl tRNA, the tRNA that carries the growing polypeptide chain.
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The E site is the exit site for the tRNA after it is done delivering the amino acid.
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The initiator tRNA is positioned in the P site.
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Elongation.
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A new tRNA carrying an amino acid enters the A site of the ribosome.
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On a ribosome.
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The anticodon of the incoming tRNA is matched against the mRNA codon positions in the A site.
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During this proofreading, tRNA with incorrect anticodons are rejected.
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And replaced by new tRNA that are again checked.
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When the right aminoacyl tRNA enters the A site, a peptide bond is made between the two now adjacent amino acids.
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As the peptide bond is formed, the tRNA in the P site releases the amino acids.
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Onto the tRNA in the A site and becomes empty.
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At the same time, the ribosome moves one triplet forward on the mRNA.
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As a result, the empty tRNA is now in the E site.
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And the peptidyl tRNA is in the P site.
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The A site is now unoccupied and is ready to accept a new tRNA.
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The cycle is repeated for each codon on the mRNA.
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Termination.
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Termination happens when one of the three stop codons is positioned in the A site.
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No tRNA can fit in the A site at that point as there are no tRNA that match that sequence.
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Instead, these codons are recognized by a protein.
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A release factor.
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Binding of the release factor catalyzes the cleavage of the bond between the polypeptide and the tRNA.
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The polypeptide is released from the ribosome.
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The ribosome is disassociated into subunits and is ready for a new round of translation.
