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

Passage

DNA controls cell function by controlling the production of proteins.

Question 259 (2 of 2 Based on Passage)

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Describe in Detail

Essay▾

Describe the role of mRNA in protein synthesis.

Explanation

Translation

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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