Let us discuss about the nomenclature and definitions of gene with diagram.
Nomenclature of Gene:
The word gene was introduced by Johannson in 1909 for a single unit of heredity occupying a specific position (locus) in a chromosome.
It was later found that the genes lay in a linear order in the chromosome and were carried along with them to daughter cells in cell division.
Behaviour of chromosomes and genes was found to be parallel. This and some more genetic information led to the following classical definition of a gene. A gene is a unit of inheritance which is carried from a parent by a gamete in a chromosome and controls the expression of a specific character in the young one in cooperation with its allele, other genes and environment.
Modern Definition of the Gene:
The above definition of a gene has become inadequate in view of the new ideas about the gene and its role revealed by intensive research in the last half a century. The following definitions emerged from the outcome of this research.
1. Gene a unique sequence of deoxyribonucleotide pairs:
It has been found that DNA is the hereditary material. It is composed of a linear series of deoxyribonucleotide pairs which form a chromosome. Therefore, a gene is regarded a segment of DNA having a limited number of nucleotide pairs in a unique sequence. Different genes have different sequences of nucleotide pairs. The specific sequence of bases in a gene forms the code that directs the cell to synthesize a particular protein.
2. Gene a code for a single protein (enzyme):
In 1948, Beadle and Tatum proposed their famous one gene-one enzyme (protein) hypothesis, and considered gene as a unit of hereditary material which codes for the formation of a single protein (enzyme).
3. Gene a code for one polypeptide:
Later, it was found that all proteins do not act as enzymes, some proteins have structural role in cells and some function as membrane receptors. Moreover, certain proteins are composed of more than one polypeptide chains. As a result of these findings, the Beadle and Tatum hypothesis has been replaced by one gene-one polypeptide chain principle. It visualizes gene as a unit of hereditary material that codes for the synthesis of a single polypeptide.
4. Gene a unit of function (Cistron):
In 1960s, genes were found to code for rRNA and tRNA besides polypeptides. This discovery made the above concept too narrow to define genes adequately. Regulatory genes or regions, such as the operators or prokaryotic DNA, were also discovered. These genes are never transcribed. Besides these, some viruses have overlapping genes which code for more than one polypeptide.
In addition, certain viruses and higher organisms have genes which code for long polyproteins. The latter are then cleaved by proteolytic enzymes into many separate proteins. This means a part of gene can code for a protein. A part from this, a eukaryote gene may have noncoding introns between coding exons. These researches have led to the concept that a gene is a segment of DNA molecule that codes for a unit of function. The function may be to code for a polypeptide, a polyprotein, ribosomal or transfer RNA, or to regulate the activity of other functional units within the DNA.
5. Gene a segment of nucleic acid:
Some RNA viruses can form genomic RNA and mRNA directly without requiring a DNA intermediate. To cover these viruses, a gene may be defined as a segment of nucleic acid molecule responsible for the synthesis of a specific product. So our concept of a gene has greatly changed from the “factor” of Mendel to the “unique sequence of nucleotide base pairs” of the modern geneticists.
6. Synthetic definition of gene:
According to the modern concept, a gene;
(i) Is a segment of nucleic acid, usually DNA, rarely RNA;
(ii) Has a unique sequence of nucleotide base paris;
(iii) Codes for a specific polypeptide, or one rRNA, or one tRNA, or a polyprotein or has a regulatory role;
(iv) Can undergo crossing over and mutation at times; and
(v) May have continuous or split information.
Even this elaborate definition lacks precision. It does not cover the cases in which one gene influences many traits, or many genes (polygenes) determine one trait. Overlapping genes of certain viruses are also not covered by it.
Benzer in 1955 introduced the concept of a cistron as a unit of function. A cistron is a segment of DNA carrying information for the production of a polypeptide chain. While the concept of a gene changes with new findings, its definition as a unit of heredity is still valid.
Part of a Gene Can Function (Functional Sub-units of Gene):
It was considered earlier that gene is the basic unit of function and parts of gene, if exist, cannot function. But this concept has been outdated now. Based on studies on rll locus of T4 phage, Benzer (1955) concluded that there are three sub-divisions of a gene, viz., recon, muton and cistron.
These are briefly described below:
1. Cistron (Unit of function):
Since the activity of a gene can only be detected by observing its effect, a gene can thus be defined as a unit of function, each unit determining a particular product. It may contain up to 30,000 pairs of nucleotides. This is called cistron (Fig. 8.1-A).
2. Recon (Unit of Recombination):
Earlier, it was thought that crossing over occurs between two genes. Benzer demonstrated that the crossing over or recombination occurs within a functional gene or cistron. In a cistron the re-combinational units maybe more than one. Thus, the smallest unit of a gene (cistron) capable of undergoing recombination is known as recon (Fig. 8.1-B).
3. Muton (Unit of Mutation):
Benzer coined the term muton to denote the smallest unit of cistron that undergoes mutational changes. A recon is further subdivisible into such units of mutation (i.e. mutons) and several mutons in a recon will not be separable due to recombination (Fig. 8.1-C). Thus a gene or cistron can consist of several recons and a recon of several mutons. A recon and a muton may be of the same size, and in such a case recon may not consist of several mutons.