Question

Describe the mechanism of translation with the help of neat and labelled diagram.

Answer 1

Definition:
The translation is the mechanism in which codons of mRNA are translated and specific amino acids in a sequence form a polypeptide on ribosomes. The process of translation requires amino acids, mRNA, tRNA, ribosomes, ATP, Mg⁺⁺ ions, enzymes, elongation, translocation, and release factors.

1. About 20 different types of amino acids available in the cytoplasm are known to form proteins.
2. DNA controls the synthesis of proteins having amino acids in a specific sequence. This control is possible through the transcription of mRNA. The genetic code is specific for a particular amino acid.
3. RNAs serve as intermediate molecules between DNA and protein.
4. Ribosomes serve as sites for protein synthesis. Each ribosome consists of large and small subunits. These subunits occur separately in the cytoplasm. Only during protein synthesis, in presence of Mg⁺⁺ ions, these two subunits get associated together. Mechanism of translation (Synthesis of polypeptide chain): It involves three steps initiation, elongation, and termination:

• Initiation:

1. Activation of amino acids is essential before translation initiates.
2. Amino acid is activated by utilizing energy from the ATP molecule. This amino acid binds with the amino acid binding site of tRNA and forms of tRNA- amino acid complex.
3. A small subunit of ribosome attaches to the mRNA at the 5’ end.
4. Initiator codon, AUG is present on mRNA which initiates the process of protein synthesis.
5. Initiator charged tRNA (with activated amino acid methionine) binds with initiation codon (AUG) by its anticodon (UAC) through hydrogen bonds.
6. It carries activated amino acid methionine (in eukaryotes) or formyl methionine (in prokaryotes).
7. It occupies the P site of the ribosome and the A- the site is vacant.
8. Now the large subunit of ribosome joins with the smaller subunit that requires Mg⁺⁺ ions.

• Elongation:
  During this process, activated amino acids are added one by one to the first amino acid (methionine).
  The addition of Amino acid occurs in 3 Step cycle -

1. Codon recognition - Amino acyl tRNA molecule enters the ribosome at A-site. Anticodon binds with the codon by hydrogen bonds.
2. Amino acid on the first initiator tRNA at P-site and amino acid on tRNA at A-site join by a peptide bond. Here enzyme Ribozyme acts as a catalyst. At this time the first tRNA at the ‘P’ site is kicked off.
3. TranslocationThe tRNA at A-site carrying a dipeptide at A-site moves to the P-site. This process is called translocation. In translocation, both the subunits of the ribosome move along in relation to tRNA and mRNA. Hence, tRNA carrying dipeptide now gets positioned at the ‘P’ site of the ribosome, making the ‘A’ site vacant. At this site, the next charged tRNA molecule carrying amino acid will be received. During this process, the first uncharged tRNA is discharged from E-site. This process of arrival of tRNA- amino acid complex, formation of the peptide bond, ribosomal translocation, and removal of the previous tRNA, are repeated. As ribosomes move over the mRNA, all the codons on mRNA are exposed one by one for translation.

• Termination and release of polypeptide:

1. Towards the 3’ end of mRNA, there is a stop codon (UAA/ UAG/ UGA). It is exposed at the A-site.
2. It is not read and joined by anticodon of any tRNA.
3. The release factor binds to the stop codon, thereby terminating the translation process.
4. The polypeptide is now released in the cytoplasm.
5. Two subunits of ribosome dissociate and the last tRNA is set free in the cytoplasm.
6. mRNA also has some additional sequences that are not translated and are referred to as untranslated regions (UTR).
7. The UTRs are present at both 5’-end (before start codon) and at 3’-end (after stop codon). They are required for an efficient translation process.
8. Finally, mRNA is also released in the cytoplasm. It gets denatured by nucleases immediately. Hence mRNA is short-lived.