Process of Translation
The process in which sequence of nucleotides in mRNA is translated into the sequence of amino acids of a polypeptide chain is called as translation. It involves three steps viz- initiation, elongation & termination.
(A) Initiation
Initiation of polypeptide chain begins with the codon AUG The codon AUG stands for methionine, hence, the formation of polypeptide begins with methionine amino acid.
- Initiation of polypeptide chain in prokaryotes : In prokaryotes, the initiation is brought about by formylated methionine. The small subunit of ribosome (30S) combines to the 5′ end of mRNA with codon (AUG). There is formation of complex by 30S ribosome, mRNA, formyl methionine & tRNAf. met i.e. (30S mRNA) f.met-tRNAf met. Now, the large subunit (50S) combines to form 70S
ribosome & the translation begins. Initiation factors, IF 1,IF2&IF3 and GTP promote the initiation process.
- Initiation of polypeptide chain in eukaryotes : Eukaryotes per-form the initiation step by a mechanism similar to that used by prokaryotes, but ordinary methionine amino acid, instead of formyl methionine, plays the initiation role. The ordinary’ methionine bound to tRNA met.
(B) Elongation:
The elongation of the polypeptide chain begins after the formation of 70S mRNA f.met-tRNAfmet complex. The ribosomes translate an mRNA in the 5′ → 3′ direction. Hence, the protein or polypeptide grows in the amino (N) to carboxyl
- direction – i.e. N – terminal amino acid is added first, and the C-terminal amino acid is added last.
There are two binding sites on the larger subunit of ribosome for aminoacyl- tRNA viz – P- site (Peptidyl) & A – site (aminoacyl). The tRNAfinet is in the P- site. The second codon in mRNA is in the A – site, hence the second aminoacyl – tRNA will bindto this site. This binding requires a protein elongation factor (EF – Tp) & GTP.
Next, the first peptide bond is formed, an enzyme peptidyl transferase (found in the larger subunit) transfer the f. methionine from its tRNAfinet in the P- site to the aminoacyl – tRNA in the A – site. This assembly is called as dipeptidyl – tRNA. The tRNA at P- site is said to be decylated. In the next step, the deacylated tRNA in the P- site leaves the ribosome. The dipeptidyl – tRNA in the A-site moves into the P- site of the larger sub – unit. This process is called as translocation and it requires GTP & c factor called EF- G. No the ribosome shifts along mRNA in 5′ ® 3′ direction, so that the next codon on mRNA is available at A- site. The process then repeats to add another amino acid.
As one ribosome moves along the length of mRNA, the initiation point on mRNA becomes free which can form initiation complex with another ribosome. In this way, 3 to 5 ribosomes get attached to a single mRNA molecule, each with a polypeptide chain under formation.
(C) Termination:
There are three terminating codons in the genetic code viz- UAA, UAG & UGA which do not code for amino acid, hence are also called as nonsense codons. Termination of polypeptide chain occurs when the ribosome carrying polypeptidyl- tRNA reaches any one of the terminating codon. However, releasing factors RF1, RF2 & RF3 are required for chain termination in prokaryotes. In eukaryotes, single releasing factor, eRF, causes termination. The free ribosome also dissociates into two subunit with the help of dissociation factor. It is observed that if any one of the terminating codon occurs in the middle of the message, premature termination takes place.
Modification of released polypetide:
The just release polypeptide is linear and has primary structure. The formyl group from the first amino acid is removed by an enzyme deformylase. It may lose some amino acids either from N-terminal or C-terminal or from both terminals with the help of exopentidase enzyme. The reamining polypeptide may gets coiled or folded to from secondary or tertiary structure.
