Which statement defines a codon?

• A. A codon is a triplet of nucleotides that codes for an amino acid or stop signal
• B. A codon is a single nucleotide
• C. A codon is a group of four nucleotides
• D. A codon is a protein found in the ribosome

Answer: (A)

Codons are three-nucleotide units in messenger RNA that specify the amino acid to be added next or signal to stop translation. In the ribosome, the mRNA is read in groups of three nucleotides, and each triplet corresponds to one amino acid (or a stop signal), giving rise to the sequence of amino acids in a protein. There are 64 possible codons (4 bases x 4 bases x 4 bases), which cover the 20 amino acids plus stop signals; some amino acids are encoded by more than one codon, which is why there’s redundancy in the genetic code. The statement is correct because a codon must be a triplet of nucleotides, not a single nucleotide or a four-nucleotide group, and it is part of the RNA sequence that directs protein synthesis rather than a component like a ribosomal protein.