A cistron is an alternative term for "
gene
In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...
". The word cistron is used to emphasize that genes exhibit a specific behavior in a
cis-trans test; distinct positions (or
loci) within a
genome
In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding gen ...
are cistronic.
History
The words ''cistron'' and ''gene'' were coined before the advancing state of
biology
Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of cells that process hereditary ...
made it clear that the concepts they refer to are practically equivalent. The same historical naming practices are responsible for many of the
synonym
A synonym is a word, morpheme, or phrase that means exactly or nearly the same as another word, morpheme, or phrase in a given language. For example, in the English language, the words ''begin'', ''start'', ''commence'', and ''initiate'' are a ...
s in the life sciences.
The term cistron was coined by
Seymour Benzer
Seymour Benzer (October 15, 1921 – November 30, 2007) was an American physicist, molecular biologist and behavioral geneticist. His career began during the molecular biology revolution of the 1950s, and he eventually rose to prominence in the ...
in an article entitled ''The elementary units of heredity''.
[ also reprinted in ] 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
In genetics, complementation occurs when two strains of an organism with different homozygous recessive mutations that produce the same mutant phenotype (for example, a change in wing structure in flies) have offspring that express the wild-typ ...
.
Definition
For example, suppose a
mutation
In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, mitos ...
at a chromosome position
is responsible for a change in
recessive trait
In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome. The first variant is termed dominant and ...
in a
diploid
Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respectiv ...
organism (where chromosomes come in pairs). We say that the mutation is recessive because the organism will exhibit the
wild type
The wild type (WT) is the phenotype of the typical form of a species as it occurs in nature. Originally, the wild type was conceptualized as a product of the standard "normal" allele at a locus, in contrast to that produced by a non-standard, " ...
phenotype
In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological prope ...
(ordinary trait) unless both chromosomes of a pair have the mutation (
homozygous
Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism.
M ...
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 experiments
T4 rII system The T4 ''r''II system is an experimental system developed in the 1950s by Seymour Benzer for studying the substructure of the gene. The experimental system is based on genetic crosses of different mutant strains of bacteriophage T4, a virus tha ...
).
For example, an
operon
In genetics, an operon is a functioning unit of DNA containing a cluster of genes under the control of a single promoter. The genes are transcribed together into an mRNA strand and either translated together in the cytoplasm, or undergo splic ...
is a stretch of DNA that is
transcribed to create a contiguous segment of
RNA
Ribonucleic acid (RNA) is a polymeric molecule essential in various biological roles in coding, decoding, regulation and expression of genes. RNA and deoxyribonucleic acid ( DNA) are nucleic acids. Along with lipids, proteins, and carbohyd ...
, but contains more than one cistron / gene. The operon is said to be polycistronic, whereas ordinary genes are said to be monocistronic.
References
Genes
{{genetics-stub