# Cistron

(Redirected from Polycistronic)

A cistron is an alternative term to a gene.[1] The term cistron is used to emphasize that genes exhibit a specific behavior in a cis-trans test; distinct positions (or loci) within a genome are cistronic.

## History

The words cistron and gene were coined before the advancing state of biology made it clear that the concepts they refer to are practically equivalent. The same historical naming practices are responsible for many of the synonyms in the life sciences.

The term cistron was coined by Seymour Benzer in an article entitled The elementary units of heredity.[2] The cistron was defined by an operational test applicable to most organisms that is sometimes referred to as a cis-trans test, but more often as a complementation test.

## Definition

For example, suppose a mutation at a chromosome position ${\displaystyle x}$ is responsible for a recessive trait in a diploid organism (where chromosomes come in pairs). We say that the mutation is recessive because the organism will exhibit the wild type phenotype (ordinary trait) unless both chromosomes of a pair have the mutation (homozygous mutation). Similarly, suppose a mutation at another position, ${\displaystyle y}$, is responsible for the same recessive trait. The positions ${\displaystyle x}$ and ${\displaystyle y}$ are said to be within the same cistron when an organism that has the mutation at ${\displaystyle x}$ on one chromosome and has the mutation at position ${\displaystyle y}$ on the paired chromosome exhibits the recessive trait even though the organism is not homozygous for either mutation. When instead the wild type trait is expressed, the positions are said to belong to distinct cistrons / genes. It's important to know that cistron localization has been studied for several years, and, depending of the organism, this gene can be found in different places. For example, the gene can be found in RNA phage and is related to the cistron coat in some particular organisms.[3] Some examples of the organisms that have been study are Tobacco, grapevine.

For example, an operon is a stretch of DNA that is transcribed to create a contiguous segment of RNA, but contains more than one cistron / gene. The operon is said to be polycistronic, whereas ordinary genes are said to be monocistronic.

## References

1. ^ Lewin B (2000). Genes VII. New York: Oxford University Press and Cell Press. p. 955. ISBN 0-19-879276-X.
2. ^ Benzer S (1957). "The elementary units of heredity". In McElroy WD, Glass B. The Chemical Basis of Heredity. Baltimore, Maryland: Johns Hopkins Press. pp. 70–93. also reprinted in Benzer S (1965). "The elementary units of heredity". In Taylor JH. Selected papers on Molecular Genetics. New York: Academic Press. pp. 451–477.
3. ^ Hindley J, Staples DH, Billeter MA, Weissmann C (November 1970). "Location of the coat cistron on the RNA of phage Q-beta". Proceedings of the National Academy of Sciences of the United States of America. 67 (3): 1180–7. JSTOR 60432. PMID 5274447.