In the genetic code, how many codons code for amino acids and how many act as stop signals?

• A. Sixty codons code for amino acids; four stop codons.
• B. Sixty-four codons code for amino acids; zero stop codons.
• C. Sixty-one codons code for amino acids; three stop codons.
• D. Fifty codons code for amino acids; five stop codons.

Answer: (C)

Codons are triplets of nucleotides, and with four bases there are 4^3 = 64 possible codons. Of these, three stop codons signal the end of protein synthesis, so the remaining codons—61 in total—code for amino acids. The stop codons are UAA, UAG, and UGA. The fact that there are 61 amino-acid codons but only 20 amino acids reflects redundancy in the genetic code, where multiple codons can specify the same amino acid. This is why the correct balance is 61 for amino acids and 3 as stop signals.