Name: Lac Operon
Uploaded: 2019-07-19
Duration: 9 min 11 s
Description: A lac operon is a group of genes with a single promoter The genes in the operon encode proteins that allow the bacteria to use lactose as an energy source. L...

Question

With proper diagram describe the process of protein synthesis.

Accepted Answer

Process of Protein Synthesis: The process of protein synthesis could be divided into three steps: Initiation, Chain elongation and Termination. In the beginning, protein synthesis requires the activation of amino acid and the binding of activated amino acid with tRNA.
Activation of amino acid: This process requires energy from ATP to form an amino acid-AMP complex mediated by aminoacyl-tRNA synthetase enzyme.

Binding of amino acid with tRNA: The amino acid-AMP now binds with specific tRNA at the amino acid attachment site of tRNA by aminoacyl-tRNA synthetase enzyme.

1. Initiation:

During the initiation of protein synthesis in prokaryotes the 30S ribosomal subunit binds with the 5' end of the mRNA at a specific site. The special base sequence of the mRNA at the 5' end is 5 AGGAGGU 3' which forms complementary base pairing with 16S rRNA found at the 30S subunit.

In a prokaryotic cell the 30S ribosomal subunit, the initiation codon AUG of mRNA, a special tRNA carrying f met (formyl methionine) amino acid and the initiation factors (IF) form a complex known as initiation complex.

The 50S ribosomal subunit joins with the initiation complex and a full protein-synthesizing apparatus or unit is formed.

The first tRNA, the f met tRNA - f met carrying formyl methionine amino acid, binds at the 'P' site of the ribosome and its anticodon 'UAC' pairs with complementary 'AUG' codon of mRNA.

The mRNA is lying in between the two subunits of a ribosome.

The protein synthesis proceeds from the 5' end to the 3' end of the mRNA.

2. Chain elongation:

There is the binding of an aminoacyl-tRNA to the '4' site of the ribosome.

The amino acid (formyl methionine) present at the 'P' site is detached from its tRNA and now is attached to the amino acid present at the 'A' site by the formation of a peptide bond between them catalyzed by peptidyl transferase enzyme.

This is followed by the process of translocation or shifting of ribosomal subunits along the mRNA towards the 3' end to the next codon of mRNA, keeping mRNA and tRNA fixed in their respective positions. The process of translocation is catalyzed by the translocase enzyme.

Translocation results in the change of position of tRNA of 'P' site to 'E' site and tRNA of'A' site to 'P' site.

Now 'A' site is lying vacant. A charged tRNA with appropriate anticodon may bind with codon of mRNA and may occupy the 'A' site and this requires elongation factor (EF).

The whole process of chain elongation is repeated several times until a stop codon or nonsense codon appears at the 'A' site for termination of protein synthesis.

There are two elongations or EF-Tu and EF-Ts found similar in both prokaryotes and eukaryotes.

3. Termination:

The process of chain termination is signalled when any of the three chain termination codons (UAA, UAG or UGA) or nonsense codons enters the 'A' site on the ribosome.

In presence of the nonsense codon, the release factors (RF) are produced in the 'A' site.

By the effect of release factors all the protein-synthesizing components like ribosomal subunits, mRNA and tRNA are dissociated and the polypeptide is released from tRNA into the cytoplasm.

The polypeptide, thus formed, is transformed from its primary structure into the secondary structure and then into the tertiary structure and finally reaches the functional quarternary structure.

In prokaryotes, there are two release factors namely RFI and RF2. RF1 recognizes UAA and UAG terminating codons and RF2 recognizes UAA and UGA. In eukaryotes, eRF recognizes all three terminating codons.

With proper diagram describe the process of protein synthesis.

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