Cistron, Chromosome, and Gene
In the earlier period it was said that a gene is the smallest unit which is present on the chromosome, and it is also considered as the structural and functional unit of a genome, and it is said that gene is an important molecule in causing mutations and which is also important in recombination.
It has also been believed that crossing over happens in between the genes and it does not occur between the same genes. These concepts were found in the experiments done in early stages.
But on later researches it has been clear that crossing over occurs by the breakage and reunion of the DNA molecules, when the genes are closely spaced such as in case f multiple alleles it results in rare frequency of intragenic recombination, so that a very large test cross progeny is required for its detection.
Occurring of mutant alleles gave an insight into the composition of the gene. Such alleles in a gene are separated by a small distance within themselves and are related functions.
Therefore, a unit of gene is a cistron. Generally, a genetic test is used to define the cistrons.
What is Cistron?
The term cistron was first coined even before the advances that are being made in biology, by the scientist named Seymour Benzer in the article the elementary unit if hereditary cistron is shortly defined as the alternative term of gene.
The term cistron is often used to emphasize the genes that exhibit a specific behavior in cis-trans set, with distinct loci within their genome as cistron.
Cistron is generally a segment of DNA, which is equivalent to a gene. It is also considered as the smallest unit of the genetic material that codes for single polypeptide and their functions by acting as a transmitter for transmitting genetic information.
Hence it is said that gene consists of more than a one cistron.
Mutation and Cistron
To understand the function of cistron, considering an example of mutation, Mutation that is taking part due to the position of the chromosomes. Which is responsible for the changes caused in a recessive trait in the diploid organism.
We say, Mutation is recessive until both chromosomes of a pair have mutation, in a particular trait, will not express any disturbances or defects.
On the other hand, if mutation takes place in any other position B which is responsible for the same recessive trait; then it is considered that both the positions A and B are in the same cistron.
We can take a hypothetical gene containing two alleles which is mutant in condition and they are assumed as α1 and α2.
When these two alleles are present in different numbers in a pair of homologous chromosomes, the mutant phenotype is produced.
The alleles that are present is in opposite positions on the two chromosomes which are said be trans arrangement and they are also non-complementing because they produce a mutant phenotype that is visible.
If both the alleles are present on the same chromosome, then it is said to be as Cis- arrangement which produces a wild phenotype.
Let us consider a third allele termed as α3 which is present on s trans arrangement with the α1 allele in the paired chromosome, it also results in the formation of a wild phenotype.
In such cases it is said that the alleles α1 and α3 are considered to be complementary to each other.
Hence it is said that when 2 mutations are present in the trans position, it produces a mutant phenotype and it is also said to be member of the same functional unit, which is said to be the cistron.
But in other case, where the two mutations are present in the trans position complementing each other then it is said that they belong to different cistrons.
From the concept of the cistron it is said that cistron is dependent on the position of cis- trans effect.
The scientist, Lewis from his work in 1951 devised the effect of Cis- trans test for the complementarity between two alleles.
During essence, it consists of comparing the phenotypes which are produced between two mutations, when they are present in the Cis-trans configuration.
When considering the terms of complementation, Cistron is used instead in the place of gene. The gene is a functional unit in a sequence of nucleotides in the molecule of DNA which codes for one of the polypeptide chains.
This genetic complementation is also applicable for haploid organisms like that of Neurospora. Where as in higher organisms, the complementation can be extensively studied in insects like Drosophila.
But in some cases, it is difficult to identify and understand that the functional product of gene is a protein which is unrelated at times.
An abnormal eye condition in Drosophila is called as lozenge which was used by the researcher Green and Green in the year 1949 for mapping the alleles on the locus.
This sequence of mutant sites was determined by the frequency pf intragenic recombination that is occurring because of the rare frequency, which is required for detecting the large test cross progeny.
They performed testcrosses of the heterozygous females for the different lozenge alleles and they obtained a single lozenge gene on a linear map.
These intergenic recombination of the alleles are said to be as fine structure mapping,