Elongation of the chain requires the participation of certain factors called elongation factors (EF). These are EF-Tu, EF-TS and EF-G in prokaryotes and EF-1 and EF-2 in eukaryotes.
After the first amino acid tRNA complex has moved from the A site of the ribosome to the P site, a second AA. tRNA complex how occupies the A site. The codons are recognized enzymatically.
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This requires the participation of Ef-Tu, GTP and Mg. Peptide linkage takes place between the two amino acids and the AA. tRNA complex migrates to the P site rendering the A site vacant. Now a third amino acid tRNA complex occupies the A site and the process continues.
The shifting of the AA. tRNA complex from A site (releasing site) is called translocation. At the P site the tRNA is released from the ribosome while A site has a peptide chain liked to a tRNA which has brought in the last amino acid of the peptide chain. During translocation, the ribosome moves relative to the mRNA in the 5′-3′ direction.
Image Source: mmbr.asm.org
Chain termination:
The elongation of the chain continues by the alternate occupation and vacancy at the A site by the AA. tRNA complexes until a termination codon (UAA, UAG or UGA) reaches the ribosome’s. The termination codon sends signals to the ribosomes to attach release factors (RF-1, RF-2 and RF-3 in prokaryotes and RF in eukaryotes).
The release factors interact with the enzyme peptidyl transferees (which catalyses the formation of peptide bond) resulting in the hydrolysis of the bond between the polypeptide chain and the tRNA (linked to the last amino acid of the peptide chain at the site) and the former is released from the ribosome. Other events that occur at the termination of the polypeptide chain are –
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(i) Release factors get disassociated from the ribosome’s due to the hydrolysis of GTP.
(ii) tRNA is also released.
(iii) Ribosomal sub units (30s and 50s) get disunited.
(iv) The polypeptide chain is processed by the removal of the methionine residue from the chain. Depending upon the circumstances the chain may get cleaved into two or more pieces.
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In eukaryotes, the polypeptide chain passes through a tunnel between the two subunits of the ribosome into the cavity of the cistema of the endoplasmic reticulum.