IGCSE Biology Paper-4: Specimen Questions with Answers 259 - 259 of 279

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DNA controls cell function by controlling the production of proteins.

Question 259 (2 of 2 Based on Passage)


Describe in Detail


Describe the role of mRNA in protein synthesis.



Formation of protein from mRNA is called translation is also known as polypeptide synthesis or protein synthesis.

  • Activation of Amino Acids: An amino acid combines with its specific aminoacyl tRNA synthetase enzyme (AA-activating enzyme) in the presence of ATP to form aminoacyl adenylate enzyme complex (AA-AMP-E) . Pyrophosphate is released. Amino acid present in the complex is activated amino acid. It can attach to CCA or 3 ′ end of its specific tRNA to form aminoacyl or AA-tRNA .

  • Initiation: It is accomplished with the help of initiation factors. Prokaryotes have three initiation factors – , and . Eukaryotes have nine initiation factors – , , , , , , , , , mRNA attaches itself to smaller subunit of ribosome with its cap encountering 3 ′ end of 18 S rRNA (16S RNA in prokaryotes) . It requires ( in prokaryotes) . The initiation codon AUG or GUG comes to lie over P-site. It produces 40S – mRNA complex. P-site now attracts met tRNA (depending upon initiation codon) . The anticodon of tRNA (UAC or AUG) comes to lie opposite initiation codon. Initiation factor eIF3 ( in prokaryotes) and GTP are required. It gives rise to . Methionine is nonformylated (tRNA ) in eukaryotic cytoplasm and formylated (tRNA ) in case of prokaryotes. The larger subunit of ribosome now attaches to complex to form 80S mRNA -tRNA complex. Initiation factors and (A, B and C) are required in eukaryotes and IF1 in prokaryotes. is essential for union of the two subunits of ribosomes. A-site becomes operational. Second codon of mRNA lies over it.
  • Elongation/chain formation: A new AA-tRNA comes to lie over the A site codon by means of GTP and elongation factor eEF1 in eukaryotes, EF-Tu and EF-Ts in prokaryotes. Peptide bond is established between carboxyl group of amino acid of P-site and amino group of amino acid at A-site with the help of enzyme peptidyl transferase/synthetase.
    • Connection between tRNA and amino acid of P-site and A-site tRNA comes to bear a dipeptydl. Freed tRNA of P-site slips away. By means of (translocase in eukaryotes and EF-G in prokaryotes) and GTP, ribosome moves in relation to mRNA so that peptidyl carrying tRNA comes to lie on P-site and a new codon is exposed at A-site. Incorporation of an amino acid in polypeptide chain thus requires one ATP and two GTP molecules. Peptide formation and translocation continue uninterrupted till the whole m-RNA code is translated into polypeptide. In a polyribosome, when a few ribosomes are helping in translation of same mRNA code, the ribosome nearest the 5′ end of mRNA carries the smallest polypeptide and the one towards the 3 ′ end the longest. Of course, ultimately the whole polypeptide is formed by each.
  • Termination: Polypeptide synthesis stops when a nonsense or termination codon [UAA, (ochre) , UAG (Amber) or UGA (opal) ] reaches A-site. It does not attract any AA-tRNA, P-site tRNA separates from its amino acid in the presence of release factor in eukaryotes (RF1for UAG and UAA, for UAA and UGA in prokaryotes) . The completed polypeptide is released, mRNA and ribosome separate. The two subunits of ribosome also dissociate with the help of dissociation factor.
  • Modification: Formylated methionine present at the beginning of polypeptide in prokaryotes and organelles is either deformylated (enzyme deformylase) or removed from chain (enzyme exopeptidase) . Initially the polypeptide is elongated having only primary structure. As soon as the polypeptide comes out the groove of larger ribosome sub-unit, it forms -helix (secondary structure) which coils further forming a few linkages (tertiary structure) . Two or more polypeptides may get associated to become -pleated which then coil to produce tertiary and quaternary structure.
Recycling of Translational Components

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