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Molecular Basis of Inheritance — BIOLOGY STD 12 Science — Question

Gujarat BoardEnglish MediumSTD 12 ScienceBIOLOGYMolecular Basis of Inheritance5 Marks
Question
Explain the process of protein synthesis from processed mRNA.
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Answer

Translation:
  • Translation is the process of synthesis of protein from mRNA with the help of ribosome.
  • A translational unit in mRNA from 5'→3' comprises of a start codon, region coding for a polypeptide, a stop codon and untranslated regions (UTRs) at both 5'-end and 3'-end for efficient process.
There are three stages of protein synthesis:
  1. Initiation:
  • Assembly of ribosome on mRNA.
  • Activation of amino acids and its delivery to tRNA.
  1. Elongation:
  • Repeated cycle of amino acid delivery.
  • Peptide bond formation and movement along the mRNA called translocation.
  1. Termination:
  • The release of a polypeptide chain.
  1. Initiation:
  • In prokaryotes, initiation requires the large and small ribosome subunits, the mRNA, initiation tRNA and three initiation factors (IFs).
  • Activation of amino acid: Amino acids become activated by binding with aminoacyl tRNA synthetase enzyme in the presence of ATP.
  • Transfer of amino acid to tRNA: The AA - AMP - Enzyme complex formed reacts with specific tRNA to form aminoacyl-tRNA complex. AA - AMP - Enzyme complex + tRNA → AA - tRNA + AMP + Enzyme.
  • The cap region of mRNA binds to the smaller subunit of ribosome.
  • The ribosome has two sites, A-site and P-site.
  • The smaller subunit first binds to the initiator mRNA and then binds to the larger subunit so that initiation codon (AUG) lies on the P-site.
  • The initiation tRNA, i.e., methionyl tRNA then binds to the P-site.
  1. Elongation of polypeptide chain:
  • Another charged aminoacyl tRNA complex binds to the A-site of the ribosome at the second codon.
  • A peptide bond is formed between carboxyl group (-COOH) of amino acid at P-site and amino group (-NH) of amino acid at A-site by the enzyme peptidyl transferase.
  • The ribosome slides over mRNA from codon to codon in the 5′ → 3′ direction.
  • According to the sequence of codons, amino acids are attached to one another by peptide bonds and a polypeptide chain is formed.
  1. Termination of polypeptide:
  • When the A-site of ribosome reaches a termination codon which does not code for any amino acid, no charged tRNA binds to the A-site.
  • Dissociation of polypeptide from ribosome takes place, which is catalysed by a 'release factor'.
  • There are three termination codons namely UGA, UAG and UAA.

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