Explanation
If a codon consisted of only two nucleotides, there would be 16 possible codons.
Since there are four possible nucleotides (A, C, G, and T), each position in a two-nucleotide codon could be filled by any of these nucleotides.
Therefore, to calculate the total number of possible codons,
We multiply the number of possibilities for the first position by the number of possibilities for the second position:
4 nucleotides * 4 nucleotides = 16 possible codons.