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  Elongation of polypeptide chain

Elongation of polypeptide chain:

Elongation consider to sequential addition of amino acids to methionine, according to the sequence of codon in the mRNA. It includes the following steps:

1. The second codon in the mRNA is identified and according to the recognition, the amino acyl tRNA consisting of the corresponding anticodon moves to the 70s ribosome and that fits in the A-site. At this time the anticodon of tRNA base pairs along with the second codon of mRNA.

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                                                       Diagram: The second tRNA fits in A site of 70s ribosome

2. A peptide bond is created among the carboxyl group (- COOH) of 1st amino acid of site P and the amino group (-NH2) of 2nd amino acid of A-site. The peptide bond connects two amino acids to create a dipeptide. The bonding is catalysed via the enzyme peptidyl transferase that is exists in 50s ribosomal subunit.

3. Later than the formation of peptide bond, the methionine and tRNA are separated via an enzyme known as tRNA deacylase.

4. After that the dissociated tRNA is released from P-site into the cytoplasm for additional amino acylation.

5. Here the ribosome carries on the mRNA in the 5' to 3' direction that is why the first codon goes out of ribosome, the 2nd codon comes to take place in the P-site from A-site and the 3rd codon comes to take place in the A-site. Concurrently, the second tRNA is shifted from A-site to P-site. Within all these events, the movement of ribosome, the release of first tRNA (transfer RNA) from P-site and shifting of 2nd tRNA (transfer RNA) from A-site to P-site comprise translocation.

Translocation is catalyzed via the enzyme translocase.

6. The third codon is identified and the amino acyl tRNA (transfer RNA) consisting of the equivalent anticodon moves to the 70s ribosome and fits within the A-site. The anticodon base pairs along with the codon. A peptide bond is created among the site-A’s third amino acid and the dipeptide’s second amino acid exist in the P-site. So a tripeptide is created.

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                                                           Diagram: The ribosome moves in the 5' to 3' direction

7. The amino acids are added one after one according to the codon in the mRNA and therefore the tripeptide is transformed into polypeptide chain. The polypeptide chain elongates via the adding up of more and more amino acids.

8. The polypeptide chain’s elongation is brought about through a number of protein factors known as elongation factors.

 

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