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Acistron is a region ofDNA that is conceptually equivalent to some definitions of agene, such that the terms are synonymous from certain viewpoints,[1] especially with regard tothe 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 aunit of selection is the question that governs whether cistrons are the same thing as genes. The wordcistron is used to emphasize that molecular genes exhibit a specific behavior in acomplementation test (cis-trans test); distinct positions (orloci) within agenome arecistronic.
The wordscistron andgene were coined before the advancing state ofbiology made it clear to many people that the concepts they refer to, at least in some senses of the wordgene, are either equivalent or nearly so. The same historical naming practices are responsible for many of thesynonyms in the life sciences.
The termcistron was coined bySeymour Benzer in an article entitledThe 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 acomplementation test.
Richard Dawkins in his influential bookThe Selfish Gene arguesagainst the cistron being theunit of selection and against it being the bestdefinition of a gene. (He also argues againstgroup 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 callgene clusters, as the unit of selection. He also defines replicators, more general than cistrons and genes, in thisgene-centered view of evolution.
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 amutation at a chromosome position is responsible for a change inrecessive trait in adiploid organism (where chromosomes come in pairs). We say that the mutation is recessive because the organism will exhibit thewild typephenotype (ordinary trait) unless both chromosomes of a pair have the mutation (homozygous mutation). Similarly, suppose a mutation at another position,, is responsible for the same recessive trait. The positions and are said to be within the same cistron when an organism that has the mutation at on one chromosome and has the mutation at position 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 experimentsT4 rII system).
For example, anoperon is a stretch of DNA that istranscribed to create a contiguous segment ofRNA, but contains more than one cistron / gene. The operon is said to be polycistronic, whereas ordinary genes are said to be monocistronic.
