The word gene was coined by W. Johannsen in 1909. A gene is a physical and functional unit of heredity. It carries information from one generation to the next. Gene is also defined as a nucleotide sequence that is responsible for the production of a specific protein. When a gene undergoes changes due to mutation, it results in biological variations. These variations are important for evolution. Such variations also arise due to recombination of genes on chromosomes.
The relationship between genes and enzymes was discovered by Beadle and Tatum. They conducted bio-chemical research on the fungus Neurospora and concluded that the major role of genes was to carry information for the production of enzymes. For their work they were awarded Nobel prize in 1958. Their findings are referred to as 'one gene one enzyme hypothesis'. Now, the hypothesis has been modified to 'one gene one polypeptide hypothesis' because the product of gene action is always a polypeptide.
Genome may be defined as the totality of the DNA sequences of an organism including DNAs present in mitochondria and chloroplasts. Each species has a characteristic number of chromosomes in the nuclei of its gametes and somatic cells. The gametic chromosome number constitutes a basic set of chromosomes of the organism. In all organisms it is made up of DNA but in viruses, it is made up of either DNA or RNA.
Sl.No. Name of organism Haploid set of
1. Arabidopsis thaliana 5
2. Garden pea 7
3. Paddy 12
4. Triticum aestivum 21
5. Homo sapiens 23
6. Chimpanzee 24
7. Sugarcane 40
8. Ophioglossum 631
The genome size of an individual is expressed in terms of number of base pairs either in kilobases (1000 bp) or in megabases (one million bp). Arabidopsis thaliana is an annual crucifer weed called 'thale cress'. It has the smallest nuclear genome of 130 mb with five chromosomes (2n = 10). The human genome comprises approximately 3.2 x 109 nucleotides. The human mitochondrial genome contains 37 genes and has 16,569 basepairs.
S. No Name of the organism Size of the genome Approximate
in megabase pairs number of genes
1. Escherichia coli 4.64 4,400
2. Yeast 12.10 5,800
3. Arabidopsis thaliana 130.00 26,000
4. Drosophila 180.00 13,600
5. Homo sapiens 3,300.00 30,000 - 40,000
In human genome, 38.2% of genome is involved in biochemical activities like synthesis of immunological and structural proteins, 23.2% in the maintenance of genome, 21.1% in receiving and giving signals related to cellular activities and remaining 17.5% in the general functions of the cell. The functions of 30,000 to 40,000 human genes are known.