Translation 

▪ Translation is the process of synthesis of protein from mRNA with the help of ribosome. 

▪ A translational unit in mRNA from 5' → 3' comprises of a start codon, region coding for a polypeptide, a stop codon and untranslated regions (UTRs) at both 5'-end and 3'-end for efficient process. 

▪ There are three stages of protein synthesis: 

i. Initiation 

• Assembly of ribosome on mRNA. 

• Activation of amino acids and its delivery to tRNA 

ii. Elongation 

• Repeated cycle of amino acid delivery. 

• Peptide bond formation and movement along the mRNA called translocation. 

iii. Termination 

• The release of a polypeptide chain. 

i. Initiation 

• In prokaryotes, initiation requires the large and small ribosome subunits, the mRNA, initiation tRNA and three initiation factors (IFs). 

• Activation of amino acid: Amino acids become activated by binding with aminoacyl tRNA synthetase enzyme in the presence of ATP. 

• Transfer of amino acid to tRNA: The AA–AMP–Enzyme complex formed reacts with specific tRNA to form aminoacyl-tRNA complex. AA–AMP– Enzyme complex + tRNA → AA–tRNA + AMP + Enzyme. 

• The cap region of mRNA binds to the smaller subunit of ribosome. 

• The ribosome has two sites, A-site and P-site. 

• The smaller subunit first binds to the initiator mRNA and then binds to the larger subunit so that initiation codon (AUG) lies on the P-site. 

• The initiation tRNA, i.e., methionyl tRNA then binds to the P-site. 

ii. Elongation of polypeptide chain 

• Another charged aminoacyl tRNA complex binds to the A-site of the ribosome at the second codon. 

• A peptide bond is formed between carboxyl group (—COOH) of amino acid at P-site and amino group (—NH) of amino acid at A-site by the enzyme peptidyl transferase. 

• The ribosome slides over mRNA from codon to codon in the 5′→3′ direction. 

• According to the sequence of codons, amino acids are attached to one another by peptide bonds and a polypeptide chain is formed. 

iii. Termination of polypeptide 

• When the A-site of ribosome reaches a termination codon which does not code for any amino acid, no charged tRNA binds to the A-site. 

• Dissociation of polypeptide from ribosome takes place, which is catalysed by a ‘release factor’. 

• There are three termination codons namely UGA, UAG and UAA.