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  • Post last modified:July 29, 2021
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Protein synthesisTranslation


Protein synthesis or translation is the mechanism by which the triplet base sequence of an mRNA guides the linking of a specific sequence of amino acids to form a polypeptide (protein) or ribosomes. Protein synthesis is the process by which biological cells generate new proteins.

Protein synthesis, Translation steps and process
peptide synthesis: translation

Process of protein synthesis:-


Protein synthesis occurs over the ribosomes. Each ribosome has two unequal subunits small and large. 


The larger subunit of ribosomes has a groove for pushing out the newly formed polypeptideand protecting the same from cellular enzymes. 


The smaller subunit of ribosome fits first over the larger one like a cap but leaves a tunnel for mRNA. 


The two subunits come together only at the time of protein synthesis. The phenomenon is called association. Mg+2  is essential for it.


Ribosomes usually form helical groups during active protein synthesis, known as polyribosomes or polysomes. 


There are three reactive sites in ribosome


(I)P-site (peptidyl transfer or donor sites.) 
(II)A-site (aminoacyl or acceptor site) and 
(III)E or exit site. 


P-site is contributed by both ribosomal subunits. A- site and E-site are largely confined to the larger subunit of the ribosome.


It is messenger RNAwhich brings coded information from DNAand takes part in its translation by bringing amino acids in a particular sequence during the synthesis of the polypeptide. The codons of mRNA are recognized by anticodon of tRNAs

The synthesis of a polypeptide can be considered in terms of:

Initiation, Elongation and Termination


These fundamental processes also have additional stages like 
Activation of amino acids before their incorporation into polypeptide and  
Post translational processing of the completed polypeptide. 

Both these processes play important roles in ensuring the proper function of the protein product. 

For the synthesis of a polypeptide, the carboxyl group of each amino acid must be activated to facilitate the formation of peptide bond.

Initiation:-

Amino acids are activated in the presence of ATP and linked to their cognate tRNA- A process commonly called as charging of tRNA or aminoacylation of tRNA to be more specific. 

The translation of mRNA begins with the formation of the initiation complex. Initiation factors are designated as IFs in prokaryotes and eIFs in eukaryotes.

Elongation:-


Elongation requires the initiation complex, aminoacyl-tRNAs, elongation factors, and GTP

During the elongation 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 tRNAanticodon. 


The ribosome moves from codon to codon along the mRNA.

Termination:-

Termination is signaled by the presence of one of three termination codons in the mRNA (UAA, UAG, UGA), immediately following the final coded amino acid. 

Three release factors RF-1, RF-2, and RF-3 contribute in the release of the free polypeptide and last tRNA now uncharged, from the ribosomes. And thus completes the translation or protein synthesis.