Editing Cistron (section)

== 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.