Translation

• Ribosomes

  • The site of translation (mRNA → polypeptide chain)
  • Consist of two subunits
  • Large (60S) and small (40S)
  • 5' cap of the modified mRNA binds to the ribosomes (clamped between the two subunits)
  • Ribosomes read the mRNA in the 5' to 3' direction
• Reading Frame

  • Ribosomes read the mRNA in triplets (codons)
  • A new amino acid is added for every codon
  • AUG is always the first (or start) codon to ensure the ribosomes are using the correct reading frame
• tRNA

  • Delivers the amino acids to the 

  ribosomes so that the polypeptide chains can be synthesized

  • Small, single-stranded nucleic acid that has a cloverleaf structure
  • Amino acids are attached to the 3' end with an ester bond 

  The opposite arm has an anticodon
• Anticodon

  • The anticodon recognizes the codon of the mRNA using complementary base pairing
  • Each tRNA, with its unique anticodon, carries a specific amino acid
  • Aminoacyl-tRNA

  Example:

  mRNA codon: 5' – UUC – 3' 

  tRNA anticodon: 3' – AAG – 5'
• Synthesis of Polypeptide Chain

  • The start codon (AUG) is the first codon recognized by the ribosomes
  • Ensures the correct reading frame is used
  • AUG encodes for the amino acid methionine
  • Every protein initially starts with methionine
  • The ribosomes have two sites for tRNA
  • A (acceptor) site
  • P (peptide) site
  • To initiate protein synthesis, the tRNA carrying methionine enters the P site
• Elongation of Polypeptide Chain

  • The next tRNA carrying the required amino acid enters the A site
  • The first amino acid (methionine) is bonded to the second amino acids (peptide bond)
  • The ribosomes shifts over (translocates) one codon
  • This causes the first tRNA to be released from the ribosomes and the second to move to the P site
  • The next aminoacyl-tRNA can then enter the A site
• Post Translational Modification

  • Once the polypeptide chain is released it can be modified
  • Sugars can be added (glycosylation)
  • Phosphates can be added (phosphorylation)
  • Enzymes may cleave the chain at specific places
  • Often the initial methionine is cleaved
  • Protein folding begins after the first 30-40 residues have been linked together