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{{Short description|Region of DNA equaling a gene as defined by complementation test}}
{{More citations needed|date=February 2021}}

A '''cistron''' is a region of [[DNA]] that is conceptually equivalent to some definitions of a [[gene]], such that the terms are synonymous from certain viewpoints,<ref>{{cite book | last = Lewin | first = Benjamin | name-list-style = vanc | authorlink = Benjamin Lewin | title = Genes VII | year = 2000 | publisher = Oxford University Press and Cell Press | location = New York | isbn = 0-19-879276-X | page = [https://archive.org/details/genesvii00lewi/page/955 955] | url-access = registration | url = https://archive.org/details/genesvii00lewi/page/955 }}</ref> especially with regard to [[gene#Definitions|the molecular gene as contrasted with the Mendelian gene]]. The question of which scope of a subset of DNA (that is, how large a segment of DNA) constitutes a [[unit of selection]] is the question that governs whether cistrons are the same thing as genes<!--as further explained in the article body below; lede summarizes body-->. The word ''cistron'' is used to emphasize that molecular genes exhibit a specific behavior in a [[complementation test]] (cis-trans test); distinct positions (or [[locus (genetics)|loci]]) within a [[genome]] are '''cistronic'''.

== History ==

The words ''cistron'' and ''gene'' were coined before the advancing state of [[biology]] made it clear to many people that the concepts they refer to, at least in some senses of the word ''gene'', are either equivalent or nearly so. The same historical naming practices are responsible for many of the [[synonym]]s in the life sciences.

The term ''cistron'' was coined by [[Seymour Benzer]] in an article entitled ''The elementary units of heredity''.<ref>{{cite book | vauthors = Benzer S | date = 1957 | chapter = The elementary units of heredity | title = The Chemical Basis of Heredity | url = https://archive.org/details/symposiumonchemi00symp | url-access = registration | veditors =  McElroy WD, Glass B | pages = [https://archive.org/details/symposiumonchemi00symp/page/70 70–93] | publisher = Johns Hopkins Press | location = Baltimore, Maryland }} also reprinted in {{cite book | vauthors = Benzer S | date = 1965 | chapter = The elementary units of heredity | title = Selected papers on Molecular Genetics | url = https://archive.org/details/selectedpaperson0000tayl | url-access = limited | veditors = Taylor JH | pages = [https://archive.org/details/selectedpaperson0000tayl/page/451 451]–477 | publisher = Academic Press | location = New York }}</ref> 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]]. 

[[Richard Dawkins]] in his influential book ''[[The Selfish Gene]]'' argues ''against'' the cistron being the [[unit of selection]] and against it being the best [[gene#Definitions|definition of a gene]]. (He also argues against [[group selection]].) He does not argue against the existence of cistrons, or their being elementary, but rather against the idea that natural selection selects them; he argues that it used to, back in earlier eras of life's development, but not anymore. He defines a gene as a larger unit, which others may now call [[gene cluster]]s, as the unit of selection. He also defines replicators, more general than cistrons and genes, in this [[gene-centered view of evolution]].

== Definition ==
Defining a Cistron as a segment of DNA coding for a polypeptide, the structural gene in a transcription unit could be said as monocistronic (mostly in eukaryotes) or polycistronic (mostly in bacteria and prokaryotes). For example, suppose a [[mutation]] at a chromosome position <math>x</math> is responsible for a change in [[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, <math>y</math>, is responsible for the same recessive trait. The positions <math>x</math> and <math>y</math> are said to be within the same cistron when an organism that has the mutation at <math>x</math> on one chromosome and has the mutation at position <math>y</math> 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. Or simply put, mutations on the same cistrons will not complement; as opposed to mutations on different cistrons may complement (see Benzer's T4 bacteriophage experiments [[T4 rII system]]).

For example, an [[operon]] is a stretch of DNA that is [[transcription (genetics)|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 ==
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[[Category:Genes]]
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