A codon is defined as which of the following?

• A. A sequence of three sequential nucleotides that encodes an amino acid
• B. A sequence of two nucleotides
• C. A sequence of four nucleotides
• D. A molecule carrying amino acids

Answer: (A)

Understanding how the genetic code is read during translation, a codon is a sequence of three nucleotides that specifies one amino acid (or a stop signal) to be added to the growing polypeptide. The triplet nature of codons provides 64 possible codes, enough to cover all 20 amino acids plus stop signals. A sequence of two nucleotides would only give 16 possibilities, which is far too few to uniquely identify amino acids. A sequence of four nucleotides would yield 256 possibilities, which isn’t how the standard code is arranged and would complicate the reading frame. A molecule carrying amino acids describes tRNA, not a codon. So the description that matches is a sequence of three sequential nucleotides that encodes an amino acid.