Authors: Satish Kumar, Vikas Gupta and Chandra Nath Mishra
ICAR-Indian Institute of Wheat and Barley Research, Karnal-132001 Haryana
Mutation is a sudden heritable change in a characteristic of an organism. Mutations produced by changes in the base sequences of genes are known as gene or point mutations. The term mutations was introduced by Hugo de Vries in 1900.
Mutations occur in natural populations (without any treatment by man) at a low rate. These are known as spontaneous mutations. The frequency of natural mutations is generally one in ten lacs.
Mutations may be artificially induced by a treatment with certain physical or chemical agents. Such mutations are known as induced mutations, and the agents used for producing them are termed as mutagen. The utilization of induced mutations for crop improvement is known as mutation breeding. Induced mutations have a great advantage over the spontaneous ones, they occur at a relatively higher frequency so that it is practical to work with them.
Characteristics of mutations
1. Mutations are generally recessive, but dominant mutations also occur.
2. Mutations are generally harmful to the organism, but a small proportion (0.1 percent) of them is beneficial.
3. Mutations are random i.e., they may occur in any gene. However some genes show higher mutations rate than others.
4. Mutations are recurrent, that is the same mutations may occur again and again.
5. Induced mutations commonly show pleiotropy, often due to mutations in closely linked genes.
Agents used for induction of mutations are known as mutagens. The mutagens are classified into two groups, physical and chemical mutagens.
The mutations inducing radiationâ€™s are of two kinds.
i. Ionizing radiation
ii. Non ionizing radiation.
Alpha, Beta and gamma rays of radio active substances, Neutrons and X rays are examples of ionizing radiation. When ionizing radiations passes through matter, atoms, absorb energy from them and lose electrons. When an atom becomes ionized, molecule of which it is a part undergoes chemical change. If the molecule is a gene and if this changed gene duplicate its new pattern, the result of the change is a mutation.
When compounds absorb energy from non-ionizing radiations, their electrons are raised to higher energy levels (excitation). It results in increased reactivity of the affected molecules leading to mutations.
The only one non-ionizing radiation capable of inducing mutations is ultra violet light. U-V radiation can be obtained from a mercury vapour lamp. U V rays have much longer wave lengths (about 2500 Angstroms)
1. Alkylating agents â€" eg., EMS (Ethyl Methane Sulphonate) MMS (Methyl Methane Sulphonate)
2. Acridine dyes eg., Ethidium Bromide, acriflavine proflavine
3. Base analogue â€" eg. 5 Bromouracil, 5 â€" Chlorouracil
4. Others - eg., Nitrous acid, hydoxyl amine , sodium azide.
The gamma garden of the Indian Agricultural Research Institute, New Delhi is a three-acre plot. In the centre of this field, there is a large source of radioactive cobalt (CO 60) and plants in pots are kept at varying distances from the source, irradiated and studied. It is used for irradiating whole plants during different stages and for varying durations.
Gamma rays are of shorter wavelength than X-rays and hence are penetrating. Gamma rays are commonly measured in terms of Roentgen units (r).
Treating a biological material with a mutagen in order to induce mutations is known as mutagenesis. Exposure of a biological material to radiation (x-rays, gamma rays etc.,) is known as irradiation.
Part of the plant to the treated
Seeds, pollen grains, or vegetative propagules (buds and cuttings) may be used for mutagenesis. Chemical mutagens are best used with seeds.
Dose of the mutagen
Mutagen treatments reduce germination, growth rate, vigour and fertility (pollen as well as ovule). An optimum dose in the on which produces the maximum frequency of mutations and causes the minimum killing. LD 50 in that dose of a mutagen which would kill 50 percent of the treated individuals. LD 50 value varies with the crop species and with the mutagen used. A preliminary experiment is generally conducted to determine the suitable mutagen dose. Dose of the mutagen may be varied by varying the intensity or the treatment time. Intensity in the case of chemical mutagens may be varied by changing the concentration of mutagens.
The selected plant part is exposed to the desired mutagen dose. The case of chemical mutagens, seeds are usually presoaked for a few hours, to initiate metabolic activities, exposed to the desired mutagen and then washed in running tap water to remove the mutagen present in them. The treated seeds are immediately planted in the field to raise the M1 generation. M2, M3, M4 etc are the subsequent generations derived from M1, M2, M3 etc., plants through selfing.
About Author / Additional Info:
I am working as a Scientist in ICAR- Indian Institute of Wheat and Barley Karnal, Haryana under ICAR, New Delhi.