MoBio Cracking the Genetic Code Chapter 3
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Figure 3-E-2. An approach used by Marshall Nirenberg and his colleagues to crack the genetic code.

  1. Synthesize a trinucleotide (e.g. UUU) which mimics a codon in mRNA.
  2. Prepare various types of aminoacyl-tRNA, e.g., Thr-tRNA, Phe-tRNA, Lys-tRNA, etc.
  3. Radioactively label an aminoacyl-tRNA (e.g. Phe-tRNA) which might contain the anticodon for the synthesized trinucleotide.
  4. Place the trinucleotide, aminoacyl-tRNA and ribosome on a nitrocellulose filter.

Individual trinucleotide and aminoacyl-tRNA can pass through the filter, but the ribosome is too big to pass through. Therefore, if the labeled aminoacyl-tRNA contains the anticodon for the trinucleotide, it will bind to the trinucleotide and ribosome on the filter. In this case, the radioactivity can be detected on the filter and the amino acid in the labeled aminoacyl-tRNA is likely to be encoded by the trinucleotide. If no radioactivity was detected, the trinucleotide is unlikely to be the codon of the amino acid. Most of the 64 possible codons can be determined by repeating this procedure for different trinucleotides and labellings.