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The following points highlight the five chief agents which bring about mutation in organisms. The agents are: 1. Induction of Mutation by Ionizing Radiations 2. Non-Ionizing Radiations 3. Heat and Temperature 4. Mustard Gas 5. Chemical Mutagens.
Agent # 1. Induction of Mutation by Ionizing Radiations:
X-ray is representative of ionizing radiations- a group including alpha, beta and gamma radiation of radioactive substances in addition of X-rays. We know, an atom is made up of a positively charged atomic nucleus and surrounded by negatively charged electrons.
These charges are so balanced that normal atoms are electrically neutral. When ionizing radiations pass through matter, they loose some electrons. When an atom looses an electron it is no longer neutral but it becomes positively charged and is said to be ionized. Such charged particles are called ions.
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An atom is a part of a molecule. When an atom becomes ionized, the molecules of which it is a part undergoes chemical change. When this charged molecule is a gene or part of a gene and when the modified gene duplicates its new pattern, the result of the change is mutation. This is how X-rays produce gene mutations. X-rays also produce chromosomal mutations i.e., Deletion, Deficiency, Inversion etc.
Among alpha, beta and gamma rays only gamma rays penetrate like X-rays. Besides, Cosmic rays are even more penetrating than X-rays or gamma rays. Muller (1927) treated Drosophila males with X-rays and tested, 1,448 treated X-chromosomes in the progeny of the X-rayed males for sex-linked lethals, semi-lethal and visible mutations. A total of 154 lethals and semi-lethals appeared.
Agent # 2. Non-Ionizing Radiations (Ultra – Violet Rays):
Ultra violet light has very weak penetrating power. It can not penetrate the surface layers of a normal adult fly and reach the reproductive organs, unless it is used in very heavy doses. But heavy doses cause great injury to the body of the fly.
Altenburg found that ultra violet light is very effective in producing mutations in Drosophila. It is specially effective as a mutagenic agent in bacteria and other micro-organisms. In the case of man, ultraviolet penetrates only the most superficial layers of the skin and, therefore, causes no mutation in the germ cells.
Agent # 3. Heat and Temperature:
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Heat increases the mutation rate as found by Muller. However, the amount of effect obtained is far less than with X-rays. Some types of mutation also increase in frequency towards extremes of low temperature. Altenburg and Hanson noted that by exposing Drosophila larvae to high temperature, mutation rate increased to six times the natural rate.
Agent # 4. Mustard Gas:
Auerbach and Robson discovered that mustard gas is a very powerful mutagenic agent. When Drosophila flies are exposed to this gas, their mutation rate goes as high as 10%. It produces all type of mutations e.g., lethals, chromosomal rearrangements, visible etc. A most unusual thing about mustard gas is that its mutagenic effect is often delayed.
Agent # 5. Chemical Mutagens:
In modern time there are large number of chemical substances which can produce mutations are as follows:
1. Reactants:
(i) Nitrous acid
(ii) Formaldehyde.
2. Base analogs:
(i) 2 amino purine, a purine analog.
(ii) 5 bromouracil, a pyrimidine analog. »
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3. Alkylating agents:
(i) Ethyl ethane sulphonate
(ii) Ethylene oxide
(iii) Ethyl-methane sulphonate, EMS
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(iv) Dimethyl sulphate
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(v) Ethyl urethane
4. Acridine dyes:
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(i) Pro-flavin
(ii) Acridine yellow
(iii) Acridine orange
5. Others:
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(i) Methyl cholenthrene
(ii) Peroxides
(iii) Permanganate, Caffeine
(iv) Nitrogen mustards
(v) Sulphur mustards
(vi) Phenols, methyl mercury.
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Charllote, J.M. Robson, Averbach and others gave their valuable investigations in this field.
Chemicals such as nitrous acid hydroxylamine and bromo-uracil produce chemical changes in DNA which results in changes in some proteins.
It is interesting to note that some of the above mutagenic chemicals such as formaldehyde, nitrous acid and peroxides are in-fact products of the cells’ own metabolism. Some mutagens like methyl mercury are industrial wastes.
