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 promoter, the RNA-coding sequence, and the terminator. However, new evidence now calls into question even this definition. Recent research suggests that much of the genome is transcribed into RNA, although it is unclear what, if anything, much of this RNA does. What is certain is that the process of transcription is more complex than formerly thought, and defining a gene as a sequence that is transcribed into an RNA molecule is not as straightforward as formerly thought. It seems that the more we learn about the nature of genetic information, the more elusive the definition of a gene becomes.