Cracking the code

After the structure of DNA and a model for how it replicates had been laid out, the next big milestone was to crack the genetic code—that is, explain how DNA chains containing just four different nucleotides could direct the synthesis of all cellular proteins. Proteins are the workhorses of the cell, catalyzing its myriad biochemical reactions and making up a good part of the cell’s structural components, such as organelles and membranes.

How could the four different nucleotides in DNA possibly code for the 20 different amino acids found in proteins? Clearly, the genetic code must have more than one or even two nucleotides: Even a 2-nucleotide code would, at most, be able to specify 16 different amino acids (4 nucleotides combined 2 at a time)—not enough, given the 20 known amino acids. The code would have to have a minimum of three nucleotides, which would allow 64 distinct combinations. A flurry of experiments in the 1960s finally “cracked” the full code, determining the combination of 3 nucleotides that specified each of the 20 amino acids.