MOLECULAR GENETICS 11

 Translation

• Translation refers to the process of polymerization of amino acids to form a polypeptide.

• The order and sequence of amino acids are defined by the sequence of bases in the mRNA and the amino acids are joined by a bond which is known as a peptide bond.

• Formation of a peptide bond requires energy and thus amino acids are activated in the presence of ATP and linked to their cognate tRNA by the process of charging of tRNA or aminoacylation of tRNA.

• If charged tRNAs are brought close enough, a peptide bond forms which is enhanced by the presence of a catalyst such as ribosome.

• Ribosome in its inactive state exists as two subunits; a large subunit and a small subunit.

• There are two sites in the large subunit, for subsequent amino acids to bind to and thus become close enough to each other for the formation of a peptide bond.

• A translational unit in mRNA is the sequence of RNA that is flanked by the start codon (AUG) and the stop codon and codes for a polypeptide.

• An mRNA also has some additional sequences that are not translated and are referred as untranslated regions (UTR).

• The UTRs are present at both 5'-end (before start codon) and at 3'-end (after stop codon) which are required for efficient translation process.

• After activation of amino acids, translation starts with its three steps-
• Initiation
• Elongation
• Termination

 

• Initiation-
• For initiation, the ribosome binds to the mRNA at the start codon (AUG) that is recognised only by the initiator tRNA.

• Elongation-
  • During elongation stage, complexes composed of an amino acid linked to tRNA, sequentially bind to the appropriate codon in mRNA by forming complementary base pairs with the tRNA anticodon.
  
  
  • The ribosome moves from codon to codon along the mRNA.
  
  
  
  
  
  • Amino acids are added one by one, translated into Polypeptide sequences dictated by DNA and represented by mRNA.
  
  
• Termination-
• At the end, a release factor binds to the stop codon, terminating translation and releasing the complete polypeptide from the ribosome.