Translation, the ribosome and antibiotic specificity

• Translation


  1. mRNA exported out of nucleus via nuclear pores

  2. mRNA bound by proteins in cytoplasm
• Eukaryotic mRNA


  • 5' end has cap, 3' end has poly A tail
  • Coding sequence is continuous, no introns

  
• Untranslated regions


  • 5' UTR controls translation rate
  • 3' UTR controls mRNA stability
• Coding sequence


  Starts with start codon, ends with stop codon
• Eukaryotic mRNAs and proteins are co-linear
• Genetic code


  • Triplet code, unambiguous, degenerate
  • Start codon AUG codes for methionine

  
• Ribosome


  • Large and small subunits, contains rRNAs and proteins
• tRNA


  Single strand folded into cloverleaf shape, has anticodon and amino acid attachment site

  
• Protein synthesis


  1. Charging of tRNAs with amino acids

  2. Initiation of polypeptide synthesis

  3. Elongation of polypeptide synthesis

  4. Termination of polypeptide synthesis

  
• Charging of tRNAs


  Aminoacyl tRNA transferase enzyme attaches correct amino acid to each tRNA

  
• Initiation of polypeptide synthesis

  1. Start at AUG codon, methionine tRNA binds to P site

  2. Amino-acyl-tRNA binds to A site, peptide bond forms

  3. Ribosome moves 3 bases, spent tRNA exits via E site

  
• Peptide bond formation


  Catalysed by RNA within ribosome, sped up by elongation factors
• Termination


  Stop codon recognized by release factor, polypeptide released, ribosome dissociates
• Polysome


  Multiple ribosomes on single mRNA strand
• Differences between eukaryotic and prokaryotic protein synthesis

  • Bacterial mRNA lacks cap
  • Bacterial ribosomes recognize Shine Dalgarno sequence
  • Bacterial start codon AUG codes for N-formyl methionine
  • Bacterial ribosomes are smaller and simpler

  
• Antibiotics inhibit bacterial protein synthesis but not viral protein synthesis