How many nucleotides code for one amino acid (the length of a codon)?

How many nucleotides code for one amino acid (the length of a codon)?

• A. 4
• B. 3
• C. 2
• D. 1

Answer: (B)

Three nucleotides make up a codon. In the genetic code, translation reads mRNA in triplets, and each triplet specifies one amino acid (or a stop signal). With four different nucleotides, there are 4^3 = 64 possible codons, which is enough to cover all 20 amino acids plus stop signals. If a codon were only two nucleotides long, there would be 4^2 = 16 possible codons—far too few to uniquely code for every amino acid and stop instruction. If it were one nucleotide long, there would only be 4 possibilities. So the length of a codon is three nucleotides.

Three nucleotides make up a codon. In the genetic code, translation reads mRNA in triplets, and each triplet specifies one amino acid (or a stop signal). With four different nucleotides, there are 4^3 = 64 possible codons, which is enough to cover all 20 amino acids plus stop signals. If a codon were only two nucleotides long, there would be 4^2 = 16 possible codons—far too few to uniquely code for every amino acid and stop instruction. If it were one nucleotide long, there would only be 4 possibilities. So the length of a codon is three nucleotides.