Translation

• Translation


  1. Ribosome decodes mRNA created during transcription

  2. Ribosome turns mRNA code into a polypeptide
• Codon


  A chain of 3 nucleotides that encodes for a specific amino acid
• There are 20 different amino acids
• The genetic code is redundant
• Start codon


  AUG encodes for methionine (Met), the only start codon
• Stop codons


  • UAA
  • UAG
  • UGA
• Translating mRNA into protein

  1. Read codons one at a time

  2. Assemble amino acids in the right sequence

  3. Occurs in the 5' to 3' direction of the mRNA

  4. With the help of transfer RNA (tRNA)
• tRNA

  Folded RNA molecule with an anticodon loop complementary to the mRNA codon, carries a specific amino acid
• The 3rd nucleotide in the anticodon is often less important than the first two
• The 3rd nucleotide is in the "wobble" position, allowing fewer tRNA to be needed
• Phases of Translation

  1. Initiation

  2. Elongation

  3. Termination
• Ribosome

  • Complex of proteins and RNA
  • Has 3 binding sites for tRNA
• Initiation

  1. tRNA carrying met attaches to small ribosomal subunit

  2. Complex binds to mRNA cap and travels to start codon

  3. Binds to large ribosomal subunit to begin translation
• Elongation

  1. tRNA matching codon binds to A site
• Termination

  1. Stop codon enters A site

  2. Release factor binds and facilitates hydrolysis to free polypeptide chain

  3. Complex separates to be reused
• Post-translational modifications

  • Amino acids may be removed
  • Polypeptide can be divided into pieces
  • Multiple polypeptides can join to form larger proteins
  • Polypeptides fold into 3D structure with chaperone proteins
  • Specific amino acid sequences can target proteins to organelles
• Polysome

  Multiple ribosomes reading the same mRNA, allowing more protein to be made simultaneously