How many amino acids are specified by the genetic code, and how many stop signals exist?

• A. 20 amino acids; 3 stop codons.
• B. 21 amino acids; 2 stop codons.
• C. 20 amino acids; 1 stop codon.
• D. 22 amino acids; 3 stop codons.

Answer: (A)

The genetic code translates mRNA triplets into amino acids, and in the standard code there are twenty amino acids specified. Because codons are triplets from four nucleotides, there are 64 possible codons; sixty-one of these code for amino acids, while the remaining three do not code for any amino acid and instead act as stop signals to terminate translation. These three stop codons are UAA, UAG, and UGA. While some organisms incorporate rare amino acids like selenocysteine or pyrrolysine in special contexts, the conventional count remains twenty amino acids with three stop codons. So the statement describing twenty amino acids and three stop signals is the correct one.