Translation: Mechanism of Protein Synthesis | B.Sc Notes

Translation: Mechanism of Protein Synthesis | B.Sc Notes

The genetic information in m-RNA (nucleotides) molecules is translated into the amino acid sequences of polypeptides according to the specifications of the genetic code.

In other words, the process of conversion of the genetic information present in the form of nucleotide sequence in DNA or transcribed m-RNA into co-linear sequence of amino-acid constituting a functional polypeptide based on the specification as governed the genetic code is called Translation.

Components of Translation:

1. Amino-acid : All amino acids involves in the finished protein must be present at the time of protein synthesis.
2. Ribosome : Ribosomes are the site of protein synthesis. They are large complexes of protein and r-RNA. In human, they consist of two sub-units; one large (60S) and one small (40S).
3. t-RNA : These are called adaptor molecules that carries specific amino acid to the m-RNA as per specific codon sequences. Minimum of 20 t-RNA are required, one for each amino-acid but some amino-acids having more than one codon require more than one t-RNA. 30 to 50 t-RNA may occur.
4. Aminoacyl t-RNA synthetase : This enzyme catalyse the attachment of amino-acid at the A-residue present at the 3’ end of t-RNA forming corresponding aminoacyl t-RNA.
5. m-RNA : It carries code for protein to be synthesized.
6. Protein factors : These are soluble proteins that are required for chain initiation (IF₁, IF₂, IF₃), chain elongation (EF factor), termination (these are called released protein factor).
7. ATP and GTP : They are required as source of energy.

Mechanism of Translation :

1. Chain Initiation

The small unit of ribosome (prokaryotes= 30S) specifically binds with the Shine Dalgarno Sequence before the chain initiation codon at the 5’ end of the RNA.

This binding is facilitated due to complementary base pairing between 3’ end of 16S r-RNA and 5’ end of m-RNA.

The GTP molecule hydrolyses the initiation factor IF-1, IF-2 and IF-3 facilitate the association of large unit of ribosome containing E, P and A site of enzyme peptidyl transferase, this results in formation of translation initiation complex.

• • A site- It is the site of large ribosome where newly coming aminoacyl t-RNA binds with the specific action.
  • P site- It is called as peptidyl site. It is the binding site of initiator t-RNA (formylmethionyl t-RNA in prokaryotes and methionyl t-RNA in eukaryotes).
  • E site- It is called as exit site that contains the uncharged/discharged t-RNA that leave after the formation of peptide bond.

2. Chain Elongation

Elongation is facilitated by the formation of peptide bond between the carboxyl group of one amino-acid with the amino group of the newly incoming amino-acid catalyzed by enzyme peptidyl transferase assisted by soluble proteins called elongation factors.

The newly incoming aminoacyl t-RNA containing specific amino-acid enters the A-site of ribosome.

The amino acid present on the P-site move to the A-site and forms peptide bond with new amino-acid.

The ribosome moves one codon forward and the next aminoacyl t-RNA recognized by the next codon, lands on the A-site and similarly forms peptide bond with the previous amino-acid present on the P-site.

This process continues till the entire polypeptide is synthesized and during this process the discharged t-RNA are subsequently released from the E-site.

3. Termination

When one of the three chain termination/ stop codon (UAA, UAG, UGA) enters A-site of the ribosome. These codons are not recognized by any aminoacyl t-RNA but are recognized by release factors (RF_s) which catalyse the addition of water molecule at the carboxyl group of terminal amino-acid of polypeptide chain which result in release of free polypeptide chain ribosomal complexes and dissociation of m-RNA.

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