Cracking the Genetic Code

Question 1

Marshall W. Nirenberg’s 1st Experiment

Answer 1

• designed a system to test which codons relate to which aas
• Johann Mattei assisted
• Their first experiment:
  1. Hard to make mRNA artificially, so they used cell-free extract to get all the important cellular components out
  2. They added an artificial mRNA of their specification to the mix
  3. They destroyed the existing DNA by adding DNAse
  4. They used polynycleotide phophorylase to make a string of Us or As (but that’s it)
  5. They used radioactivity to find out which protein was produced
  6. They added all 20 aas to different solutions of cell-free extract, but a different aa is labelled each time
  7. One tube will contain a radioactive protein when all the labelled aas are put in a single protein
• so they could narrow down which codon led to which aa, but they didn’t know exactly

Question 2

Nirenberg’s 2nd experiment

Answer 2

• Khorana knew how to deliberately synthesize DNA and RNA nucleotides
• so he could dictate the order of bases
  1. They started simple, and could eliminate codons that they knew didn’t work
  2. They discovered that codons could promote the binding of aa-tRNAs to ribosomes
  3. They supplied a single triplet, all the radioactively labelled aas and they could see which aa would be chosen by the ribosome to synthesize a protein
  4. The mix is thenpassed through a nitrocellulose filter
  5. The tRNAs paired with ribosome-bound mRNA stuck to the filter, wherase unbound tRNAs passed through it
  6. if the tRNA is attached to the radioactive aa, then we can easily identify it
  7. The filter was assayed to determine which aa was bound

Question 3

Robery Holley

Answer 3

• Identified tRNA and later mapped its structure

- The Genetic Code couldn’t have been cracked without it