In this article we will discuss about the concept and units of genes.
Concept of Gene:
Although the role of hereditary units (factors) in transfer of genetic characters over several generations in organisms was advocated by Gregor John Mendel, yet the mystery of the ‘hereditary units’ was un-ravelled during early 1900s. In 1909, W. Johanson coined the term ‘gene’ that acts as hereditary units. However, early work done by several workers proposes various hypotheses to explain the exact nature of genes.
In 1906, W. Bateson and R.C. Punnet reported the first case of linkage in sweet pea and proposed the presence or absence theory. According to them the dominant character has a determiner, and the recessive character lacks determiner. In 1926, T.H. Morgan discarded all the previous existing theories and put forth the particulate gene theory.
He thought that genes are arranged in a linear order on the chromosome and look like beads on a string. In 1928, Belling proposed that the chromosome that appeared as granules would be the gene. This theory of gene was well accepted by the cytologists.
In 1933, Morgan was awarded Nobel Prize for advocating the theory of genes. After the discovery of DNA as carrier of genetic information’s, the Morgan’s theory was discarded. Therefore, it is necessary to understand both, the classical and modern concepts of gene.
According to the classical concepts a gene is a unit of:
(i) Physiological functions,
(ii) Transmission or segregation of characters, and
(iii) Mutation.
In 1969, Shapiro and co-workers published the first picture of isolated genes. They purified the lac operon of DNA and took photographs through electron microscope.
Avery, McCleod and Mc Carty (1944) gave the first experimental proof for the role of DNA as genetic material. Therefore, the presence of genes was supposed on DNA. However, in some viruses like TMV, retroviruses, revoviruses, etc. the chemical nature of genes is RNA but not DNA.
In 1908, the British physician Sir E.R. Garrod first proposed one-gene-one product hypothesis. In 1941, G.W. Beadle and E.L. Tatum working at St. Standford university clearly demonstrated one-gene-one enzyme hypothesis based on experiments on Neurospora crassa. They made it clear that genes are the functional units and transmitted to progenies over generations; also they undergo mutations.
They treated N. crassa with X-rays and selected for X-ray induced mutations that would have been lethal. Their selection would have been possible when N. crassa was allowed to grow on nutrient medium containing vitamin B6.
This explains that X-rays mutated vitamin B6. synthesizing genes. They concluded that a gene codes for the synthesis of one enzyme. In 19.58, Beadle and Tatum with Lederberg received a Nobel Prize for their contribution to physiological genetics.
Units of a Gene:
After much extensive work done by the molecular biologists the nature of gene became clear. A gene can be defined as a polynucleotide chain that consists of segments each controlling a particular trait. Now, genes are considered as a unit of function (cistron), a unit of mutation (mutant) and a unit of recombination (recon).
1. Cistron:
One-gene-one enzyme hypothesis of Beadle and Tatum was redefined by several workers in coming years. A single mRNA is transcribed by a single gene. Therefore, one-gene-one raRNA 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 has been given as one-gene-one protein hypothesis. The proteins are the polypeptide chain of amino acids translated by mRNA. Therefore, it has been correctly used as one-gene-one polypeptide hypothesis.
Moreover, genes are present within the chromosome and their cis-trans effect govern the function. Therefore, S. Benzer termed the functional gene as cistron (Fig. 6.1 A). Crossing over within the functional genes or cistron is possible.
The cis and Trans arrangement of alleles may be written as below:
2. Recon:
Earlier, it was thought that crossing over occurs between two genes. In 1962, Benzer demonstrated that the crossing over or recombination occurs within a functional gene or cistron. In a cistron the recombinational units may be more than one. Thus, the smallest unit capable of undergoing recombination is known as recon (Fig. 6.1B).
Benzer (1955) found that the cultures of T4 bacteriophage formed plaques on agar plates of Escherichia coli. Normally T4 formed small plaques of smooth edges, whereas the mutant T4 phage formed the larger plaques of rough edges (Fig. 6.2).
The DNA molecule of T4 phage consists of several genes one of which is called rll region. Formation of rough edged plaques was governed by two adjacent genes (cistrons rllA and rllB) in mutant bacteriophage (Fig. 6.3 A).
Both the regions function independently and consist of 2,500 and 1,500 nucleotides, respectively. In rllA gene over 500 mutational sites are present where crossing over may occur. Through crossing over exchange of two segments of DNA occurs. If crossing over takes place within the gene by mating two rll mutant of T4 phage, a normal wild type phage can be produced.
Similar was the result of Benzer. Thus, the work of Benzer lends support that crossing over within a gene occurs (Fig. 6.3B) which explains that the recombinational unit (recon) is much smaller than the functional unit i.e. cistron.
3. Muton:
Benzer (1962) coined the term muton to denote the smallest unit of chromosome that under goes mutational changes. Hence, muton may be defined as ‘the smallest unit of DNA which may be changed is the nucleotide’. Thus, changes at nucleotide level are possible (Fig. 6.1C). The smallest unit of muton is the nucleotide.
Therefore, cistron is the largest unit in size followed by recon and muton. This can be explained that a gene consists of several cistron, a cistron contains many recon, and a recon a number of mutons. However, if the size of a recon is equal to muton, there would be no possibility in recon for consisting of several mutons.