Answer:
There are three stages of protein synthesis:
1. Initiation:
Assembly of ribosomes on mRNA In prokaryotes, initiation needs the large and small ribosome subunits, the mRNA, initiation tRNA and three initiation factors (IFs); IF1, IF2 and IF3.
Activation of Amino Acid These amino acids have 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 AAtRNA + AMP + Enzyme
The cap region of the mRNA binds to the smaller subunit of ribosome on that site.
The ribosome has two sites, A- site and P- site.
The smaller subunit first binds the initiator tRNA then 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.
2. Elongation: Another charged aminoacyl tRNA complex binds to the A- site of the ribosome.
Peptide bond formation and movement along with the mRNA is defined as translocation. A peptide bond is formed between carboxyl group (-COOH) of amino acid at the 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’ to 3’ direction.
According to the codon sequence, amino acids are attached to one another with the peptide bonds and then a polypeptide chain is formed.
3. Termination: 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 the ribosome takes place which is catalyzed by a release factor. There are three termination codon:
UGA, UAG and UAA
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