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Molecular Basis of Inheritance - Notes | Class 12 | Part 7: Translation (Protein Synthesis)

Molecular Basis of Inheritance: Translation (Protein Synthesis)

Translation (Protein Synthesis)

  • It is the process of polymerisation of amino acids to form a polypeptide based on the sequence of codons in mRNA.
  • It takes place in ribosomes. Ribosome consists of structural RNAs and about 80 types of proteins.
  • Ribosome also acts as a catalyst for the formation of peptide bond between amino acids. E.g., 23S rRNA in bacteria is the enzyme- ribozyme.
  • Inactive ribosome exists as two subunits: a large subunit & a small subunit. Large subunit has two sites for subsequent amino acids to bind through peptide bond.
  • Formation of peptide bond needs energy from ATP. For this, amino acids are activated (amino acid + ATP) and linked to their cognate tRNA. It is called Charging (aminoacylation) of tRNA.

Steps of Translation

1. Initiation

  • In this, small subunit of ribosome binds to mRNA at the start codon (AUG).
  • Now large subunit binds to small subunit to form initiation complex.
  • Large subunit consists of aminoacyl tRNA binding site (A site) and peptidyl site (P site).
  • The initiator tRNA (which carries methionine) binds on P site. Its anticodon (UAC) recognises start codon AUG.

Process of Translation

2. Elongation

  • Second aminoacyl tRNA binds to the A site of ribosome. Its anticodon binds to the second codon on the mRNA and a peptide bond is formed between first and second amino acids in presence of peptidyl transferase.
  • First amino acid and its tRNA are broken. This tRNA is removed from P site and second tRNA from A site is pulled to P site along with mRNA. This is called translocation.
  • These processes are repeated for other codons in mRNA.
  • During translation, ribosome moves from codon to codon.

3. Termination

  • When a release factor binds to stop codon, the translation terminates.
  • The polypeptide and tRNA are released from the ribosomes.
  • The ribosome dissociates into large and small subunits.
  • A group of ribosomes associated with a single mRNA for translation is called a polyribosome (polysomes).
  • An mRNA has additional sequences that are not translated (untranslated regions or UTR). UTRs are present at both 5’-end (before start codon) and 3’-end (after stop codon). They are required for efficient translation process.

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