DNA contains instructions for cellular function, while RNA helps make proteins by copying genetic code. Cistrons are sequences of genetic material that make RNA or polypeptides. The term has been replaced by introns and exons, which refer to non-coding and coding segments of genetic material, respectively. Most cistrons contain alternating exons and introns, which are removed during cis-splicing to create mRNA, rRNA, or tRNA.
Deoxyribonucleic acid (DNA) is found in the cells of all living things, except some viruses, and contains instructions on how to make proteins and other molecules necessary for cellular function. Ribonucleic acid (RNA) helps make these proteins and molecules by copying the genetic code contained in DNA. There are several 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 DNA or RNA that contains the genetic code needed to make RNA molecules or polypeptides, which may be a protein or serve as the building block 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 may 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 responsible for making a particular protein or polypeptide. Subsequently, the meaning of the term was broadened to include genes that contained the genetic code for the creation of various types of RNA molecules. Cistron can refer to a genetic sequence in both DNA and RNA. A cistron DNA is the genetic code on the gene itself, while a cistron RNA refers to that same genetic sequence when it was copied, or transcribed, from RNA.
In 1978, biochemist Walter Gilbert suggested in a research paper that the term cistron should be replaced by the terms intron and exon. Introns, a word derived from the term “intragenic regions,” are non-coding segments of genetic material, meaning they contain no instructions or code for making molecules such as RNA or proteins. These segments, sometimes called junk DNA, are removed from the genetic material when RNA copies the DNA code to make proteins and various types of RNA. Exons, a word derived from the term “expressed regions,” are the genetic sequences that contain instructions on how to make new proteins or RNA molecules.
Most cistrons contain alternating sequences of exons and introns. When a cistron DNA code is copied from RNA to make a new molecule, the introns are cut off in a process called cis-splicing. The remaining exons are then joined in a process called trans-splicing, resulting in an mRNA, rRNA, or tRNA molecule.
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