Translation. During translation process, proteins are made by the ribosomes on mRNA strand. The main steps are :

1. Activation of amino acid. 2. Transfer of activated amino acid to tRNA. 3. Initiation of synthesis. 4. Elongation of polypeptide chain.

5. Termination of chain.

1. Activation of amino acid. In the presence of an ATP, an amino acid combines with a specific enzyme called aminoacyl tRNA synthetase. Mg⁺⁺ is required. It produces amino acyl adenylate synthetase.

2. Transfer of activated amino acid to tRNA (Charging of tRNA). The

activated amino acid is transferred to its specific tRNA. A high energy ester bond is formed between carboxylic group of amino acid and 3’ hydroxylic group of terminal adenosine tRNA. tRNA has two selection sites, one for recognizing the specific aminoacyl tRNA synthetase and the other anticodon for recognizing the codon on mRNA.

3. Initiation of synthesis. mRNA becomes attached to smaller subunit. The various initiation factors required are IF-1, IF-2 and IF-3. In eukaryotes, no IF-2 is found. The attachment is such that initiation codon of mRNA (AUG or GUG) comes to he at the P-site.

Aminoacyl t-RNA complex specific for the initiation codon (methionine-tRNA, valine tRNA and formylated methionine tRNA in prokaryotes) reaches the P-site. Anticodon (UAC of met tRNA) establishes temporary hydrogen bonds with codon of mRNA The codon-anticodon reaction occurs in the presence of GTP and IF. Now the larger subunit of ribosome combines with mRNA-met tRNA complex. The A-site is exposed as such.

4. Elongation. (Polypeptide chain formation). An amino acyl tRNA complex reaches the A-site and attaches to mRNA codon next to initiation codon. It requires GTP and elongation factor (EF). A peptide bond is established between NH₂ group of /RNA attached at A-site with the carboxyl group (—COOH) on the P-site in the presence of an enzyme peptidyl transferase.

In the meanwhile, the link between tRNA and amino acid at P-site breaks and free tRNA slips away. The /RNA at the A-site bears a dipeptide.

â–² Fig. 2.12. Steps of Translation.

In the next step, the t-RNA at the A-site (bearing dipeptide) is pulled to the P-site along with mRNA. In this process, the mRNA moves by one triplet thus exposing the new codon at the A-site. This process is carried by an enzyme translocase and energy from GTP and termed translocase and energy form GTP and termed translocation. Another amino acyl tRNA complex reaches the A-site and bond is formed. Only one by one codon of mRNA are exposed at A-site and get decoded through incorporation of amino acid in the peptide chain which elongates and lie in the groove of larger subunit.

5. Termination. Synthesis of polypeptide terminates when a stop signal (UAA, UAG or UGA) reaches the A-site. The P-site tRNA is hydrolysed and completed polypeptide is released in presence of releasing factors (RF). The two subunits of ribosomes also dissociates.

In case of free cytoplasmic polyribosomes, the polypeptides are released into cytoplasm when they are employed for synthesis of more cytoplasm, enzyme and components of cell organelles.