Translation or protein synthesis is a process during which the genetic information (which is stored in the sequence of nucleotides in an mRNA molecule) is translated, following the dictations of the genetic code, into the sequence of amino acids in the polypeptide.

 

Protein synthesis occurs over the ribosomes. Each ribosome has two unequal parts, small subunit and large subunit. The two subunits come together only at the time of protein formation.

 

There are three reactive sites in a ribosome – P-site (peptidyl transfer or donor site.) A-site (amino-acyl or acceptor site) and E or exit site.

 

The codons of mRNA are recognised by anticodons of tRNAs. tRNA can pick up only a specific amino acid. Each tRNA has an area for coming in contact with ribosome (T  C arm) and the enzyme amino acyl tRNA synthetase (DHU arm) that helps in combining amino acid to its particular tRNA. The enzyme is specific for each amino acid.

 

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. mRNA also has some additional sequences that are not translated and are referred as untranslated regions (UTR). The UTRs are present at both 5ï‚¢-end (before start codon) and at 3ï‚¢-end (after stop codon). They are required for efficient translation process.

 

The synthesis of polypeptide can be considered in terms of initiation, elongation, and termination stages. These fundamental processes have additional stages : activation of amino acids before their incorporation into polypeptide and the post-translational processing of the completely synthesised polypeptide.

 

(i) Activation of amino acids and charging of tRNA : The carboxyl group of each amino acid must be activated to facilitate formation of a peptide bond. Amino acids are activated in the presence of ATP and linked to their cognate tRNA. This process is commonly called as charging of tRNA or amino-acylation of tRNA.

 

(ii) Initiation : Initiation of protein synthesis is accomplished with the help of initiation factor which are 3 (IF2, IF3, IF1) in prokaryotes and 9 in eukaryotes (eIF2, eIF3, eIF1, eIF4A, eIF4B, eIF4C, eIF4D, eIF5, eIF6). The ribosome binds to the mRNA at the start codon (AUG) that is recognised only by the initiator tRNA. A polypeptide chain forms as tRNAs deliver amino acids to the ribosome. Large ribosomal subunit binds the initiation complex forming two (A and P) binding site for tRNA molecules. The first site is P site or peptidyl site which is occupied by tRNAmet. The second site is A or amino-acyl site and is positioned over the second codon.

 

Elongation : During this stage, complexes composed of an amino acid linked to tRNA, sequentially bind to the appropriate codon in mRNA by forming complementary base pairs with the tRNA anticodon. The ribosome moves from codon to codon along the mRNA. Amino acids are added one by one, translated into polypeptide sequences dictated by DNA and represented by mRNA. The enzyme peptidyl synthetase catalyses the formation of peptide bond between the carboxylic group of amino acid at P-site and amino group of amino acid at A-site. Enzyme translocase brings about the movement of mRNA by one codon. After about 30 codons of mRNA get translated, a new ribosome join at 5ï‚¢ - end of mRNA to synthesise another polypeptide. As many as 5-20 ribosome may join the same mRNA depending upon its length.

 

Termination : It is signalled by the presence of one of three termination codons in the mRNA (UAA, UAG, UGA). Three release factors RF-1, RF-2 and RF-3 contribute in release of the polypeptide and last tRNA, now uncharged, from the ribosome.