What is a codon?

• A. A sequence of three nucleotides that codes for an amino acid
• B. A sequence of two nucleotides
• C. A protein
• D. A ribosome

Answer: (A)

A codon is a sequence of three nucleotides on mRNA that specifies which amino acid should be added next during protein synthesis. The genetic code uses triplets because four nucleotides raised to the third power give 64 possible codons, enough to encode all 20 amino acids plus signals to start or stop translation. During translation, the ribosome reads these codons three at a time and, with the help of tRNA, brings in the corresponding amino acids to build the protein. Some amino acids are encoded by more than one codon, which is why there are multiple codons for the same amino acid. The start codon AUG signals the beginning of translation and also codes for methionine. The other options aren’t codons because a codon isn’t a protein, isn’t a ribosome, and isn’t just two nucleotides.