There may be a question sequence about this chart below:
| DNA non-coding | A T G | G C G | A T C | A C C | G G G | T A G | G T G | A A C | T G A |
| :--- | :--- | :--- | :--- | :--- | :--- | :--- | :--- | :--- | :--- |
| DNA coding | | | | | | | | | |
| mRNA | | | | | | | | | |
Note: Introns are shaded
Scientists struggled to understand how four bases could code for 20 different amino acids. If one base coded for one amino acid, the cell could produce only four different kinds of amino acids (4ΒΉ). If two bases coded for each amino acid, there would be four possible choices (of nucleotides) for the first base and four possible choices for the second base. This would produce 4Β² or 16 possible amino acids. Mathematical logic indicates that at least three bases must code for each amino acid. This led scientists to ask: How can we determine whether this is true? Which combinations of bases code for each of the amino acids? To answer these questions, scientists manufactured different artificial mRNA strands. When placed in appropriate conditions, the strands could be used to produce polypeptides.
Assume a scientist made three artificial mRNA strands:
(x) 5' A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-3'
(y) 5' U-U-U-C-C-C-U-U-U-C-C-C- U-U-U-C-C-C-U-U-U-C-C-C-U-U-U- 3'
(z) 5' A-U-A-U-A-U-A-U-A-U-A-U-A-U-A-U-A-U-A-U-A-U-A-U- 3'
When she analyzes the polypeptides produced (assuming that a start codon and stop codon were also added to each strand, of course, and the conditions were correct to synthesize a polypeptide), she finds that:
(x) produces a polypeptide composed entirely of lysine
(y) produces a polypeptide that is 50% phenylalanine and 50% proline
(z) produces a polypeptide that is 50% isoleucine and 50% tyrosine
a) Do these results definitely support the "three-bases-per-codon" hypothesis or could it support a "four-bases-per-codon" hypothesis? Could the data suggest both? Explain briefly, given the data you see here.
b) Would you have constructed this experiment in exactly the same way (strands x, y and z)? What would you change about this experiment to find the genetic code of these amino acids?
c) This type of experiment was actually used to discover the mRNA nucleotide codons for each of the 20 amino acids. If you were doing these experiments, what sequences (you should come up with at least three) would you try next? Explain your logic.