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Molecular Basis of Inheritance — BIOLOGY STD 12 Science — Question

Maharashtra BoardEnglish MediumSTD 12 ScienceBIOLOGYMolecular Basis of Inheritance5 Marks
Question
Describe the process of translation in protein synthesis.
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Answer

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Translation involves the following steps:
1. Activation of amino acids and formation of charged t-RNA (t-RNA – amino acid complex):
i. In the presence of an enzyme amino acyl t-RNA synthetase, the amino acid is activated and then attached to the specific t-RNA molecule at 3’ end to form charged t-RNA (t-RNA – amino acid complex).ii. ATP is essential for the reaction.
2. Initiation of Polypeptide chain:
  • Small subunit of ribosome binds to the m-RNA at 5’ end.
  • Start codon is positioned properly at P-site.
  • Initiator t-RNA, (carrying amino acid methionine in eukaryotes or formyl methionine in prokaryotes) binds with initiation codon (AUG) of m-RNA, by its anticodon (UAC) through hydrogen bonds.
  • The large subunit of ribosome joins with the smaller subunit in the presence of $Mg^{++}.$
  • Thus, initiator charged t-RNA occupies the P-site and A – site is vacant.
3. Elongations of polypeptide chain:
Addition of amino acid occurs in 3 Step cycle-
i. Codon recognition.
Anticodon of second (and subsequent) amino acyl t-RNA molecule recognizes and binds with codon at A-site by hydrogen bonds.
ii. Peptide bond formation.
  1. Ribozyme catalyzes the peptide bond formation between amino acids on the initiator t-RNA at P-site and t-RNA at A-site.
  2. It takes less than 0.1 second for formation of peptide bond.
  3. Initiator t-RNA at ‘P’ site is then released from E-site.
iii. Translocation.
  1. Translocation is the process in which sequence of codons on m-RNA is decoded and accordingly amino acids are added in specific sequence to form a polypeptide on ribosomes.
  2. Due to this A’-site becomes vacant to receive next charged t-RNA molecule.
  3. The events like arrival of t-RNA – amino acid complex, formation of peptide bond, ribosomal translocation and release of previous t-RNA, are repeated.
  4. As ribosome move over the m-RNA, all the codons on m-RNA are exposed oiie by one for translation.
4. Termination and release of polypeptide:
When stop codon (UAA, UAG, UGA) gets exposed at the A-site, the release factor binds to the stop codon, thereby terminating the translation process
The polypeptide gets released in the cytoplasm.
Two subunits of ribosome dissociate and last t-RNA and m-RNA are released in the cytoplasm.
m-RNA gets denatured by nucleases immediately.

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