Some major stages of Protein Synthesis are: (a) Activation of amino acids, (b) Transfer of amino acid to tRNA, (c) Initiation of polypeptide chain, (d) Chain Termination, (e) Protein translocation
The major steps are:
(a) Activation of amino acids:The step requires enzymes called amino acyI RNA synthesizes. Due to this reaction amino acid (AA) and adenosine triphosphate (ATP), mediated by above enzyme, amino acyl – AMP – enzyme complex is formed
(b) Transfer of amino acid to tRNA:
The AA – AMP – enzyme complex formed reacts with specific tRNA. Thus amino acid is transferred to tRNA. As a result the enzyme and AMP are liberated.
(c) Initiation of polypeptide chain:
Charged tRNA shifts to ribosome. Ribosome is the site where the protein synthesis occurs. The mRNA binds to SOS sub-unit of ribosome of 70S type.Each amino acid is coded for three letters word of nucleic acid. The initiation of polypeptide chain in prokaryotes is always brought about by the amino acid methionine which is regularly coded by the codon AUG but rarely also by GUG (for valine) as also initiating codon.
(d) Chain Termination:
The termination of polypeptide is signalled by one of the three terminal triplets (codons) in the mRNA. The three terminal codons are UAG (Amber), UAA (Ochre) and UGA (Opal). They are also called stop signals.
(e) Protein translocation:
Two classes of poly ribosomes have been identified (i) Free poly ribosomes and membrane bound poly ribosomes. For free ribosomes, termination of protein synthesis leads to the release of completed protein into cytoplasm. For membrane bound poly ribosomes, polypeptide chain which grows on mRNA is inserted into the lumen of ER membrane.