How does the presence of introns affect our concept of a gene? To define a gene as a sequence of nucleotides that encodes amino acids in a protein no longer seems appropriate because this definition excludes introns, which do not specify amino acids. This definition also excludes the nucleotides that encode the 5′ and 3′ ends of an mRNA molecule, which, as we will see, are required for translation but do not encode amino acids. And defining a gene in these terms excludes sequences that encode rRNA, tRNA, and other RNAs that do not encode proteins. Given our current understanding of DNA structure and function, we need a more precise definition of a gene.
Many geneticists have broadened the concept of a gene to include all sequences in DNA that are transcribed into a single RNA molecule. Defined this way, a gene includes all exons, introns, and those sequences at the beginning and end of the RNA that are not translated into a protein. This definition also includes DNA sequences that encode rRNAs, tRNAs, and other types of nonmessenger RNA. Some geneticists have expanded the definition of a gene even further to include the entire transcription unit—
The discovery of introns forced a reevaluation of the definition of the gene. Today, a gene is often defined as a DNA sequence that encodes an RNA molecule or the entire DNA sequence required to transcribe and encode an RNA molecule.