How many codons encode amino acids, and how many are stop codons?

How many codons encode amino acids, and how many are stop codons?

• A. 64 sense codons; 3 stop codons (UAA, UAG, UGA).
• B. 61 sense codons; 3 stop codons (UAA, UAG, UGA).
• C. 60 sense codons; 4 stop codons (UAA, UAG, UGA, UGA).
• D. 61 sense codons; 2 stop codons (UAA, UAG).

Answer: (B)

Three-nucleotide codons give 64 possible combinations. Three of these function as signals to end translation, so they are stop codons. The remaining codons encode amino acids, totaling 61 sense codons. The standard stop codons are UAA, UAG, and UGA. (Note that AUG, the start signal, also encodes methionine, so it’s counted among sense codons.) Therefore, the correct counts are 61 codons that encode amino acids and 3 stop codons.

Three-nucleotide codons give 64 possible combinations. Three of these function as signals to end translation, so they are stop codons. The remaining codons encode amino acids, totaling 61 sense codons. The standard stop codons are UAA, UAG, and UGA. (Note that AUG, the start signal, also encodes methionine, so it’s counted among sense codons.) Therefore, the correct counts are 61 codons that encode amino acids and 3 stop codons.