units of gene / How it was discovered

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Gene: A gene can be defined as a polynucleotide chain that consists of segments each controlling the particular trait. 

Fig:- science lab

Units of gene: there are three units of gene

(1    (1)Cistron (unit of function)

One-gene-one enzyme hypothesis of Beadle and Tatum. A single mRNA is transcribed by a single gene. Therefore, one-gene-one mRNA hypothesis was put forth. Exceptionally, a single mRNA is also transcribed by more than one gene and it is said to be polycistronic. Therefore, the concept of one gene one protein hypothesis. The protein are the polypeptide chain of amino acid translated by mRNA. Therefore, it has been correctly used as one-gene-one polypeptide hypothesis.

Moreover,  genes are present within the chromosomes and their cis-trans effect governs the chromosome function.  Therefore, S.Benzer term the functional gene as cistrone (shown in figure A). Crossing over within the functional gene or cistrone are possible. The cis-trans arrangement of the alleles may be written as below:

 

++/a b                                     a+/+b                                           / = it shows divide sign

Cis(wild)                                trans(mutant)

fig. The genes as unit of functions i.e. custron (A), recombination i.e. recon (b) and mutation i.e. muton (c)

(2)    Recon (unit of recombination)

Earlier, it was thought that crossing over occurs between two genes. In 1962, S.Benzer demonstrated that crossing over or recombination occurs within a functional gene or cistron. In a cistron recombinational unit may be more than one. Thus, the smallest unit capable of undergoing recombination is known as recon (fig.B). 

S.Benzer (1955) found that the culture of T4 bacteriophage formed plaques on agar plates of Eschcherchia coli. Normal T4 formed small plaques of smooth edges, where the mutant T4 phage formed the large plaques of rough edges. The DNA molecule of T4 phage consists of several genes one of which is called r2 region. Formation of rough edged plaques was govern by two adjacent genes ( cistrons r2A and  r2B ) in mutant bacteriophage. Both the region function independently and consists of 2,500 and 1,500 nucleotide, respectively. In r2A gene over 500 mutant sites are present where crossing over may occur. Though crossing over exchange of two segments of DNA occurs. If crossing over occur within the gene, by matting two r2 mutant of T4 phage a normal wild type will be produced. Thus, the work of benzer lends supports the crossing over within the gene occurs, which explains that the recombinational unit (recon) is much less smaller than the functional unit i.e. cistron. 

 (3) Muton (unit of mutation)

S.Benzer (1962) coined the term mutan to donate the smallest unit of chromosome that undergoes mutational changes. Hence, mutan may be defined as ‘the smallest unit of DNA which may be changed in the nucleotide. Thus, changes at the nucleotide level are possible (fig.C). The smallest unit of mutan is nucleotide. Therefore, cistron is the largest unit in size followed by recon and mutan. This can be explained that the gene consists of several cistron contains many recon, and a recon a number of mutans. However, if the size of a recon is equal to mutan, there would be no possibility in recon for consisting of several mutans

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