A three-nucleotide sequence of DNA or mRNA that specifies an amino acid or termination signal is called a:

• A. Codon
• B. Anticodon
• C. Exon
• D. Intron

Answer: (A)

A three-nucleotide sequence that encodes a single amino acid or a stop signal is called a codon. During translation, the ribosome reads these mRNA codons three bases at a time and adds the corresponding amino acid to the growing protein chain, with specific codons that signal termination. The same triplet concept applies to DNA in that a DNA triplet is transcribed into the matching mRNA codon (with thymine replaced by uracil in RNA). An anticodon is the three-nucleotide region on tRNA that pairs with a codon to deliver the correct amino acid. Exons are the coding portions of a gene, while introns are noncoding segments removed during RNA processing. So the described unit is the codon.