How many nitrogen bases make up a codon?

• A. Two
• B. Four
• C. Three nitrogen bases
• D. One

Answer: (C)

A codon is the three-nucleotide unit that specifies a single amino acid during protein synthesis. Because there are four different nitrogen bases, triplets yield 4^3 = 64 possible codons, which is enough to encode the 20 amino acids plus stop signals. If a codon were only two bases long, there would be 4^2 = 16 possibilities, which isn’t enough to uniquely specify all amino acids and termination. A one-base codon would have only 4 possibilities. Therefore, the number of nitrogen bases in a codon is three.