What is a codon?

• A. A sequence of two nucleotides in DNA that codes for a specific amino acid.
• B. A sequence of three nucleotides in DNA that codes for a specific amino acid.
• C. A single nucleotide that codes for a protein.
• D. A sequence of three nucleotides in DNA that codes for a stop signal.

Answer: (B)

A codon is a sequence of three nucleotides that specifies which amino acid will be added next during protein synthesis. In the process of translation, the ribosome reads the mRNA in triplets, and each triplet corresponds to a particular amino acid (or a stop signal in some cases). This three-nucleotide code explains why the genetic information is read in groups of three and why there are 64 possible codons for 20 amino acids.

The reason this option is the best is that three nucleotides are required to uniquely specify an amino acid, whereas two nucleotides are too few and a single nucleotide cannot encode a full amino acid. Stop signals are also codons but are a special subset, not the general rule for coding amino acids.