Chapter 6 - Translation

• Translation: nucleotides into amino acids, 64 codons, Redundancy, 3 reading frames
• Redundancy: more than 1 codon can code for the same amino acid
• Start codon?

  AUG (Methionine)
• Stop codons?

  UAA, UAG, UGA
• tRNA: reads codon and brings in right amino acid; adapter molecule, 80 nt, 31 tRNAs, 4 helical regions, Wobble effect
• Wobble Effect: 1 tRNA molecule can recognize multiple codons; Modified Inosine can pair to with multiple nucleotides
• Synthesis of tRNA - Made by Pol III; Modified in nucleus; Trimmed, Introns spliced, modification of nucleotides, Inosine; Over 50 other modification
• Binding of amino acid to tRNA

  1. Amino acid binds site in aminoacyl tRNA synthetase (ATP used)
  2. tRNA binds site in aminoacyl tRNA synthetase (AMP released)
  3. High energy ester bond forms between amino acid and tRNA
  4. Aminoacyl tRNA
• Specificity of aminoacyl tRNA Synthetase

  • Highest affinity amino acid will bind
  • After binding, amino acid forced into second position
  • Reads anti-codon on tRNA
• Ribosomes decode message

  • more than 50 ribosomal proteins
  • Several rRNA
  • Assembled in nucleolus
  • Large and small subunits
• Ribosome - Large subunit

  • 5S, 5.8S, 28S rRNA
  • Peptide bonds formed
• Ribosome - Small subunit : 18S rRNA, Matches codons
• Steps in Translation

  1. Initiation
  2. Elongation: Codon Recognition, Peptide Bond Formation, Translocation
  3. Termination
• Elongation Steps?

  Codon Recognition, Peptide Bond Formation, Translocation
• Translation - Initiation

  • Pre-Initiation complex
  • Initiator tRNA
  • Leaky Scanning
  • eIF2 mediated binding of initiator tRNA to small ribosomal subunit
  • eIFs leave
  • Large Ribosomal subunit comes in
• Large Ribosomal Subunit - Peptidyl Site (P), Aminoacyl Site (A), Exit Site (E)
• tRNA must enter through A site in translation except for initiator tRNA
• Initiator tRNA - only one that can bind small ribosomal subunit in absence of large ribosomal subunit
• Elongation - Codon Recognition

  1. Codon in A site is read
  2. Appropriate tRNA with amino acid comes into A site
  3. Anti-codon
• Anti-codon is complementary to codon, H bonds
• Elongation - Peptide Bond Formation

  1. High energy ester bond between amino acid and tRNA in P site is broken
  2. Energy is released
  3. Energy used to create peptide bond between amino acids in P and A sites
  4. Peptidyl transferase
  5. tRNA in P site no longer attached to amino acid

  
• Elongation - Translocation

  1. mRNA moves through ribosome by 3 nucleotides (1 codon)
  2. tRNA in E site leaves ribosome
  3. tRNA in P site moves to E site
  4. tRNA in A site with peptide chain moves to P site
  5. Cycle repeats
• Translation - Termination

  1. Elongation continues until stop codon is reached in A site
  2. Release factor binds in A site
  3. Peptidyl transferase catalyzes addition of water
  4. Complex disassembles
  5. Protein is released
• Translational Inhibitors - Puromycin and Cycloheximide
• Puromycin: Pre-mature release of polypeptide
• Cycloheximide: Blocks translocation from A to P site