AV

7.3 Translation

  • Conversion of sequence of bases on mRNA into sequence of amino acids in a polypeptide.
  • Polypeptides are synthesized by ribosomes and tRNA.

tRNA

  • Delivers the amino acid to the ribosome as the polypeptide chain elongates.
  • 70-90 nucleotides long.
  • Folds into a cloverleaf shape.
  • One arm of tRNA contains the anticodon, which is complementary to codon in mRNA.
  • The amino acids is attached to the opposite end of the tRNA molecule.

The Wobble Hypothesis

  • The anticodon only needs to match the first TWO bases in the codon, The third can be incorrect but still retrieve the correct amino acid.
  • Remember that multiple codons may encode for the same amino acid.

Aminoacylation

  • Amino acids are linked to tRNA in a process called aminoacylation.
  • This process is performed by the enzyme aminoacyl-tRNA synthetase.
  • There are 20 enzymes, one for each amino acid. The product is called an aminoacyl-tRNA

Ribosomes

  • Composed of 2 subunits: large ribosomal subunit and small ribosomal subunit, both composed of rRNA (ribosomal RNA)

  • mRNA moves through groove between subunits

  • Contains 3 binding sites

    • A (aminoacyl) site – aminoacyl tRNA containing next amino acid binds to MRNA
    • P (peptidyl) site – TRNA binds to ribosome
    • E (exit) site – tRNA leaves ribosome

3 stages of translation:

Initiation

  • Translation begins at the start codon (AUG) on mRNA
  • AUG is the codon for the amino acid methionine
  • The aminoacyl-TRNA for methionine (the initiator tRNA) binds the small ribosomal subunit and scans the mRNA for the start codon
  • When met-tRNA recognizes the start codon, the large ribosomal subunit binds
  • A reading frame is established such that the bases are read 3 at a time from start codon.

Elongation

  • Begins with initiator tRNA bound to P site and empty A site
  • Aminoacyl-tRNA with next amino acid binds to A site
  • Amino acid in P site is removed from its tRNA and a peptide bond is formed with the amino acid in the A site
  • Ribosome moves to next codon, all tRNA’s shift (P --> E, A --> P)
  • A site is now ready to accept next aminoacyl-tRNA. E site ejects its tRNA.

Termination

  • Occurs when stop codons enter the A site (UAA, UAG, UGA)

  • Protein release factor binds instead of aminoacyl-tRNA (there are no corresponding amino acids for stop codons)

  • Polypeptide is released, ribosomal subunits detach.

    Elongation of Translation