Deoxyribonucleic acid (DNA) is found in the cells of all living things, except certain viruses, and contains the instructions for how to create proteins and other molecules necessary for cellular function. Ribonucleic acid (RNA) helps create these proteins and molecules by copying the genetic code contained in the DNA. There are different types of RNA, including messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA). A cistron, or structural gene, is a sequence of genetic material in either DNA or RNA that contains the genetic code needed to make either RNA molecules, or polypeptides, which can be a protein, or serve as building blocks for proteins. In genetics, the term cistron has often been replaced with the terms intron and exon, which refer to two different types of genetic sequences that can be contained within a structural gene.
Cistrons received their name from the cis-trans test originally used to determine what functions specific sections of genetic material had in various biochemical reactions. The word cistron was then applied to a specific gene that was responsible for creating a certain protein or polypeptide. Later, the meaning of the term was broadened to also include genes that contained the genetic code for creating various kinds of RNA molecules. Cistron can refer to a genetic sequence in both DNA or RNA. A DNA cistron is the genetic code on the gene itself, while an RNA cistron refers to that same genetic sequence when it has been copied, or transcribed, by RNA.
In 1978, biochemist Walter Gilbert suggested in a research article that the term cistron should be replaced by the terms intron and exon. Intron, a word derived from the term "intragenic regions," are non-coding segments of genetic material, meaning they do not contain instructions, or code, for creating molecules such as RNA or proteins. These segments, sometimes called junk DNA, are removed from the genetic material when the RNA is copying the DNA code to create proteins and various types of RNA. Exons, a word derived from the term "expressed regions," are the genetic sequences that do contain instructions for how to create new proteins or RNA molecules.
Most cistrons contain alternating sequences of exons and introns. When the code of a DNA cistron is copied by RNA to produce a new molecule, the introns are sliced away in a process called cis-splicing. The remaining exons are then joined together in a process called trans-splicing, resulting in a molecule of mRNA, rRNA, or tRNA.