Various external agencies are known to bring about mutational changes in genes. All these are included under the general category of induced mutations. H.J. Muller (1927) may be regarded as pioneer in radiation genetics.
In his studies on Drosophila, he showed that flies subjected to X radiation undergo mutations. Similarly Stadler (1928) demonstrated the mutational effects of X rays in barley and maize.
Various kinds of agents causing mutational changes in organisms are known. These are of the following types, a) Radiation b) Temperature changes and c) Chemicals.
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1. Radiation induced mutations:
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Various types of radiation (excluding visible light) are known to cause mutations in different intensity radiations are of two categories depending upon their capacity of penetration into a substance -Ionizing and non ionizing.
Generally speaking, non ionizing radiations are not a high potentiality to bring about mutations at a rate which is brought about y ionizing radiation.
The Alpha rays, Beta rays and the gamma rays and X rays under the category of ionizing radiations while low penetration rays like rays belong to non ionizing radiation.
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Mayor (1920) experimented on the effects of X radiation in Drosophila that Similar other radiations) to study the effects on germplasm.
He demonstrated result of radiation, there was no disjunction of the X chromosome in g in a XXY individual, which was female phenotypically.
H.J. Muller also worked on the effects of X rays on Drosophila and published a classical paper ‘The artificial transmutation of the gene’.
He also devised a technique to detect induced mutations.
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Stadler studied the effects of X rays on barley plant, while Gager and Blackslee induced mutants in the jimson weed Datura stramonium. It should be noted however that X rays are lethal and the dosage of the rays given to the individual has to be carefully monitored.
On an average it has been found out that 80% of the X ray induced mutations are lethal. Oliver, Tirnofeeff Ressovsky and many others have worked out the relationship between the frequency of mutation and the amount of X radiation absorbed by the organism.
The intensity of the radiation is measured in terms of Roentgen units or r- units. The frequency of mutation seems to be proportional to the amount of r- units absorbed by the plant. The duration of exposure or the distance between the radiation source and the experimental organism, however do not seem to affect the frequency of mutations.
Chlorophyll deficiency has been induced in barley due to X radiation (Stadler). X rays can also induce mutations in the wasp Habrobracon. Among the other types of ionizing radiation Alpha, Beta, and Gamma rays are important.
The First two however do not seem to have a profound mutational role as they cannot penetrate the body cells as deeply as gamma rays.
Mechanism of action of ionizing radiation:
There are two theories which account for the mechanism of mutagenic effect of ionizing radiation. These are the direct hit theory and the indirect hit theory. According to the direct hit theory, the radiation directly strikes the genes bringing about a change in the chemical composition.
The electrons that, strike the atoms of genes bring about a change in the chemical composition, thus altering the gene but in several experiments, it has been seen that, the same dose of radiation does not result in the same frequency of mutation indicating that direct hit theory cannot account for all radiation induced mutations.
According to the indirect hit theory, gene alteration is brought about by changing the molecular environment of the surroundings. Muller is a supporter of this theory.
According to him when the high speed protons of x ray collide with the molecules of the cell, electrons are ejected out of the atoms rendering them positively charged.
The electrons that come out move at high speeds and knock out other electrons that come in their way resulting in an environment which is ionized (positively charged).
The electrons moving freely may attach themselves to other atoms rendering them negatively charged.
Various chemical reactions and shifts take place to bring both the positive and negatively charged atoms to their normal state.
During this process, alteration in the chemical composition takes place, thus mutating the gene. At the molecular level, base pair sequences are altered in the DNA molecule.
Effect of non ionizing radiation:
Ultra violet rays are none ionizing because they have a long wave length and therefore have less energy. They are generally non penetrative. They may bring about many changes in the traits, but are not as effective as X rays.
Altenberg was able to accelerate the rates of mutations by using U.V. rays. Stadler also achieved pollen mutations induced by U.V. rays in maize; the same is the case in Antirrhinum as reported by Noethling and Stubbs.
The U.V. rays are known to cause alterations in the bond characters of purines and pyrimidines. Between the two, the latter are more prone to mutational changes.
2. Chemical mutagens:
A number of chemicals are known to have mutagenic properties. It was C.Auerbach who discovered during Second World War that chemicals could be used to induce mutations.
Thorn and Steinberg (1939) had also reported the chemical induction of mutations. They treated the Fungus Aspergillus with nitrous acid. Since then a long list of mutangenic chemicals have been identified.
Hydrogen peroxide is known to bring about mutations in Neurospora. Urithane and phenol are capable of inducing mutations in higher plants and Drosophila. Formaldehyde may induce mutations during spermatogenesis in Drosophila.
Colchicine is an alkaloid extracted from the bulbs of Colchicum autmnale of the family Liliaceae. This is known to induce polyploidy by suppressing spindle formation.
Colchicine is not known to bring about any point mutation- Mustard gas is another chemical widely employed as a mutagen. Its mutagenic effect has been tested with reference to Drosophila, Neurospora, bacteria, barley, mice etc.
3. Temperature induced, mutations:
Child, Plough have studied the effect of temperature as a mutagenic agent in Drosophila. A range of tempera from 4°C to lethal range was used to find out the possible effect of temperature rate of mutation.
A higher range of might not be an effective mutagen. Any sudden increase or decrease in Temperature might alter the gene copying mechanism thus altering the gene.
The temperature effects largely remain as a somatic temperature is known to accelerate rate of mutation. But it has to be borne in mind that a gradual increase in tne influences.
Vegetable oils as mutagenes:
In one of their experiments concerning oil yielding plants, Swaminathan and Natarajan (1959) noticed that seeds with high oil content are generally resistant to mutagens.
They soaked the cereal grains in oils of ground nut, mustard, and castor and wanted to know whether these oils provide any mutation resistance to the grains.
Much to their surprise, the results were startling. Instead of the oils providing resistance to the cereals, they (oils) themselves had acted as mutagens as noticed by marked reduction in rates of germination.
Chromosome irregularities and many ear head variation mutations in wheat, one of the cereals used in the experiment it was assessed that in wheat (Triticum astivum), the rate of mutation, induced was much higher than that of x rays.