Dna Translation

• Genetic code


  The code that determines how the nucleotide sequence specifies the amino acid sequence

  
• Genetic code


  • Degenerate - multiple codons code for the same amino acid
  • Nonoverlapping - a single nucleotide is not included in multiple codons
  • Universality - near universal specification of the same amino acids in all organisms

  
• Codon


  A set of three nucleotides that encodes an amino acid

  
• Codons


  • 64 possible codons
  • 3 stop codons (terminate translation)
  • 61 sense codons (code for amino acids)
  • One sense codon acts as translation start codon

  
• Nucleotide sequence can be read in 3 different reading frames, each translating a different amino acid sequence

  
• Translation


  The process of assembling amino acids into proteins
• Translation


  • Only mRNAs are translated into proteins
  • Takes place in ribosomes
  • Includes a series of RNA-RNA interactions

  
• Steps in translation


  1. Binding of amino acids to tRNAs

  2. Initiation of translation

  3. Elongation

  4. Termination

  
• tRNA charging


  The binding of an amino acid to its specific tRNA

  
• tRNA


  • Each tRNA is specific for an amino acid
  • tRNAs have a CCA sequence at the 3' end where the amino acid attaches

  
• Aminoacyl-tRNA synthetase


  The enzyme that recognises a specific amino acid and the tRNA that accepts it

  
• RNA charging requires energy from ATP
• Initiation of translation


  Assembly of the necessary components (mRNA, ribosomal subunits, initiation factors, initiator tRNA) at the ribosome

  
• Bacterial initiation


  • Shine-Dalgarno sequence in mRNA pairs with 16S rRNA to position the ribosome over the start codon
  • Initiator tRNA (fMet-tRNA) binds to the start codon with the help of initiation factors

  
• Eukaryotic initiation


  • Cap binding complex helps export mRNA from nucleus to cytoplasm
  • Initiator tRNA (Met-tRNA) binds to the start codon with the help of initiation factors
  • Kozak sequence around the start codon aids identification

  
• Eukaryotic mRNAs lack Shine-Dalgarno sequences
• Eukaryotic initiation


  • Proteins that bind to the mRNA 5' cap interact with proteins that bind to the poly(A) tail, forming a closed loop that enhances ribosome binding

  
• Elongation of translation


  1. Binding of charged tRNA to the A site

  2. Formation of peptide bond between amino acids in the P and A sites

  3. Translocation of the ribosome along the mRNA

  
• Ribosome sites


  • Exit (E) site
  • Peptidyl (P) site
  • Aminoacyl (A) site

  
• Elongation factors


  EF-Tu, EF-Ts, EF-G - proteins that facilitate the elongation process
• Termination of translation


  1. Ribosome encounters a stop codon

  2. Release factors (RF1, RF2, RF3) bind to the ribosome and promote the release of the polypeptide chain

  
• Eukaryotic termination


  • Only two release factors: eRF1 and eRF2
• The genetic code is the same in bacteria and eukaryotes, allowing for genetic engineering

  
• Transcription and translation occur simultaneously in bacteria, but separately in eukaryotes due to the nuclear envelope

  
• There are significant differences in the size and composition of ribosomal subunits between prokaryotes and eukaryotes