A three-nucleotide base sequence on DNA and mRNA that codes for a single amino acid?

• A. Triplet
• B. Base pair
• C. Gene
• D. Codon

Answer: (D)

Amino acids are specified by groups of three nucleotides. In the messenger RNA that the ribosome reads during protein synthesis, each three-nucleotide unit is called a codon, and this codon determines which amino acid will be added to the growing protein (with special codons for start and stop signals). The DNA sequence that is transcribed into this RNA has a corresponding three-nucleotide unit, often called a triplet, but the term codon is the standard way to describe the unit in the RNA that codes for a single amino acid. So the three-nucleotide unit that directly codes for one amino acid in the RNA is a codon. The other terms don’t fit as precisely: a base pair is about pairing between DNA strands, a gene is a larger functional unit, and a triplet is a broader label for the DNA sequence rather than the RNA coding unit.