Once the DNA double helix had been
discovered, the next big challenge was to work out how the four letters of DNA
could code for each of the twenty amino acids that make protein. The first question was how many DNA
letters coded for each amino acid. If it was one DNA letter for one amino acid, then you
could only code for a maximum of four amino acids. Two letters in every possible combination could
code for up to sixteen amino acids; still not enough. But three DNA letters provide more than enough
combinations to code for all twenty amino acids. So three was the answer; it was a triplet code.