The genetic code embodied in the DNA molecule, a code we describe in Section 2.1, consists of strings of symbols, each of which is one of the four letters T, C, A, or G. Find by enumeration the number of different codewords or strings using these letters and having length 3 or less.
for the total of 84 possibilities.
We can verify it by counting:
\[4^1 + 4^2 + 4^3 = 84\]