Mutagenic Agents

Any agent that raises the natural rate of mutation above its spontaneous background level is called a mutagen, and the process by which mutagens produce mutants is called mutagenesis. Mutagens fall into two broad classes: physical mutagens and chemical mutagens. H.J. Muller was the first to demonstrate, in 1927, that a physical agent, X-rays, could deliberately induce mutations, using Drosophila as his experimental subject, and L.J. Stadler subsequently extended the same approach to plants using both X-rays and gamma rays. Among physical mutagens, a rise in temperature increases the mutation rate because added heat breaks the hydrogen bonds holding the two DNA strands together, which interferes with accurate replication and transcription. Radiation is split into ionising radiation (X-rays, gamma rays, alpha rays, beta rays and cosmic rays), which carries enough energy to strip electrons from atoms and can break chromosomes and chromatids outright, and non-ionising radiation (ultraviolet rays), which carries less energy and correspondingly less penetrating power, so it is mainly used on single-celled organisms, spores and pollen grains whose nuclei sit close to the cell surface. Sharbati Sonora, a gamma-ray-induced mutant wheat variety developed from the Mexican Sonora 64 variety by Dr M.S. Swaminathan, India's 'Father of the Green Revolution', and Castor Aruna, a thermal-neutron-induced mutant castor variety that matures in just 120 days instead of the usual 270, are two well-known Indian examples of physical mutagenesis applied in plant breeding. Chemical mutagens include mustard gas (used as a chemical weapon in the First World War), nitrous acid (which alters the nitrogenous bases of DNA and disrupts replication and transcription), and the alkylating agents ethyl and methyl methane sulphonate (EMS and MMS); C. Auerbach first r …