· Translation refers to the process of polymerisation of amino acids to form a polypeptide
· In this process, amino acids become joined together by peptide bonds, to form polypeptides.
· The formation of peptide bonds requires energy and hence in the first phase, the amino acids are activated in the presence of ATP
(A) Activation of amino acid
¾ In this process, a particular amino acid becomes activated and linked to 3’’end of their cognate (specific) tRNA–a process commonly called as charging of tRNA or aminoacylation of tRNA
¾ The reaction is catalysed by the enzyme amino–acyl-tRNA synthetase.
(B) Initiation of polypeptide synthesis
---- The ribosome, in its inactive state exits as two subunits – a large subunit and a small
subunit. (The ribosome consists of structural RNAs and about 80 different proteins.)
¾ When the small subunit encounters the mRNA, translation begins.
¾ The mRNA binds to the small subunit of ribosome, following base pair rule between the bases of mRNA and those on rRNA; it is catalysed by ‘initiation factors’.
¾ There are two sites on the larger subunit, the P-site and the A-site
¾ The small subunit (with the mRNA) attaches to the large subunit in such a way that the initiation codon (AUG) comes on the P-site.
¾ The anticodon UAC of initiator tRNA (methionyl tRNA) carrying amino acid methionine(AUG) undergoes pairing with the initiation codon AUG of mRNA through hydrogen bonds at P site .
tRNA- the adapter molecule
(C) Elongation of polypeptide chain
¾ A second tRNA charged with an appropriate amino acid binds to the A-site of the ribosome.
¾ A peptide (CO-NH) bond is formed between the carboxyl group of methionine and the amino group of the second amino acid (The ribosome also acts as a catalyst (23S rRNA in bacteria is the enzyme- ribozyme) for the formation of peptide bond.
¾ The ribosome moves from codon to codon along the RNA in the 5’-3’ direction.
¾ Amino acids are added one by one in the sequence of the codons and become joined together to form a polypeptide.
(D) Termination of polypeptide synthesis
¾ When one of the termination codons comes at the A-site, it does not code for any amino acid and there is no tRNA molecule for it.
¾ As a result, the polypeptide synthesis (or elongation of polypeptide) stops.
¾ At the end, a release factor binds to the stop codon, terminating translation and releasing the complete polypeptide from the ribosome.
¾ A translational unit in mRNA is the sequence of RNA that is flanked by the start codon (AUG) and the stop codon and codes for a polypeptide. An mRNA also has some additional sequences that are not translated and are referred as untranslated regions (UTR). The UTRs are present at both 5' -end (before start codon) and at 3' -end (after stop codon). They are required for efficient translation process.