Cistrons

Cistron is a historical term used in genetics to denote a functional hereditary unit, roughly equivalent to what is now commonly called a gene. The concept emphasizes that a specific genetic unit is responsible for producing a particular product or phenotype. The term arose in the early era of molecular genetics, with Seymour Benzer introducing the idea to distinguish the functional unit of heredity from the surrounding chromosomal region, and to separate it from the mutation site, or muton.

In prokaryotes, many genes are organized into operons, and a single polycistronic mRNA can contain several coding segments. Each segment typically corresponds to a cistron, coding for a distinct polypeptide. Thus, an operon may include multiple cistrons, each contributing to a different protein. In contrast, most eukaryotic genes are monocistronic, typically encoding a single polypeptide product, which contributed to the shift away from the term in common usage.

Historically, the cistron concept was closely linked to the one gene-one enzyme hypothesis proposed by Beadle and Tatum, which argued that each gene directs the synthesis of a single enzyme. Today, the term is largely of historical interest, and modern genetics generally uses the term gene to describe the functional hereditary unit, incorporating broader understandings of gene structure, regulation, and expression.

See also: gene, muton, operon, one-gene-one-enzyme hypothesis.