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In this article we will discuss about the definition of cistron.
The earlier geneticists considered a gene to be the smallest unit (a bead or a chromomere) on a chromosome which could be distinguished as a functional unit, or as a unit of recombination, or as a unit of mutation.
It was also believed that crossing over occurred only between genes, not within a gene itself. This concept emerged from results of genetic experiments which indicated that crossing over occurred between genes which were spatially distant from each other.
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Now we know that all crossing over may occur by breakage and reunion of molecules in DNA. When genes are closely spaced, as in the case of multiple alleles, intragenic recombination occurs, with rare frequency, so that a very large testcross progeny is required for its detection.
The occurrence of mutant alleles has given us insight into the functional composition of the gene. Such alleles are separated by small distances within a gene and are functionally related. The gene is therefore a unit of function called cistron. A genetic test has been devised to define a cistron. This test is applicable to both higher organisms and phages.
Let us consider a hypothetical gene α having two mutant alleles α1 and α2. When α1 and α 2 are present on different members of a pair of homologous chromosomes (+ α1 on one chromosome, α 2+ on the other), the mutant phenotype is produced. The alleles are said to be in trans arrangement (in opposite positions on the two chromosomes) and they are non-complementing because they produce a visible mutant phenotype.
When both alleles are present on the same chromosome (α1 α2/++) it is called cis arrangement (Fig. 22.1) and produces the wild phenotype. Now consider a third allele α3 present in trans arrangement with α 1 on the paired chromosomes, and it produces the wild-type phenotype. In this case the alleles a1 and a3 are said to complement each other.
Thus when two mutations present in the trans position produce a mutant phenotype, they are said to be members of the same functional unit called cistron. But if two mutations present in the trans position complement each other, they are said to belong to different cistrons.
The concept of the cistron is based on the cis-trans position effect. From his studies on intragenic recombination in Drosophila, Lewis in 1951 devised the cis-trans test for complementarity between alleles. In essence it consists in comparing phenotypes produced by two mutations when the two mutations are present in cis and trans configurations.
In terms of complementation, the word cistron can be used in place of gene. The gene as a functional unit is a sequence of nucleotides in the DNA molecule that codes for one polypeptide chain.
Genetic complementation is applicable to haploid organisms like Neurospora. In the case of higher organisms, complementation has been extensively studied in Drosophila; in some other cases it is difficult to understand and does not relate with the functional product of the gene, that is protein.
An abnormal eye condition in Drosophila called lozenge was used by Green and Green (1949) for mapping alleles of this locus. The sequence of the mutant sites was determined from the frequency of intragenic recombination between the alleles. They performed testcrosses of females heterozygous for the different lozenge alleles and obtained a linear map of the single lozenge gene.
After that other investigators constructed linear maps of the gene in Aspergillus, Neurospora and in bacteria. By far the most refined analysis of intragenic recombination, in relation to nucleotides in the DNA molecule was done in bacteriophages by Seymour Benzer in 1955. He mapped thousands of independently arising rII mutants in T4. Such high resolution studies of intragenic recombination are called fine structure mapping.
