How many nucleotides code for one amino acid?

• A. Two
• B. Three
• C. Four
• D. Five

Answer: (B)

In the genetic code, instructions are read in blocks called codons, and each codon is three nucleotides long. That triplet specifies a particular amino acid (or a start/stop signal) during translation. With four nucleotides, three-position combinations yield 64 possible Codons, which is plenty to cover the 20 amino acids and the stop signals, though many amino acids are defined by more than one codon (degeneracy). This is why the unit that codes for one amino acid is a triplet. If codons were two nucleotides long, there would only be 16 possibilities—not enough to assign unique codons to all amino acids plus stops; if they were four nucleotides long, the code would be organized differently, which isn’t how the standard code is arranged.