iGenetics 3rd Edition by Peter Russell

Exercise 7

Suppose that, at stage A in the evolution of the genetic code, only the first two nucleotides in the coding triplets led to unique differences and that any nucleotide could occupy the third position. Then, suppose there was a stage B in which differences in meaning arose, depending on whether a purine ( A or G ) or pyrimidine ( C or U ) was present at the third position. Without reference to the number of amino acids or the multiplicity of tRNA molecules, how many triplets of different meaning can be constructed out of the code at stage A? at stage B?