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.

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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.

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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.

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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

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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.

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