Any extracellular physical or chemical factor which can cause mutations or increase the frequency of mutations in organism is termed as mutagen. In other words, the specific factors and chemicals of the environment that induce mutations are termed as mutagens.
Mutations can be artificially produced by certain agents termed as mutagens on mutagenic agents. Muller (1927) was the first to produce induced mutations in Drosophila by exposing them to X-rays.
Following are the two major classes of mutagens:
- Physical mutagens mainly radiations and
- Chemical mutagens
Physical mutagens are of two types; high energy radiations and temperature.
Radiations are the most important physical mutagens. H.J. Muller who used X-rays to increase the rate of nutrition in Drosophila opened an entirely new field in inducing mutations. The spectrum of wavelengths that are shorter (i.e., of higher energy) than the visible light can be subdivided into following two groups:
- Ionizing radiations such as X-rays, gamma rays and cosmic rays and
- Non ionizing radiations such as ultraviolet light.
The main source of spontaneous mutations are the natural radiations coming from the cosmic rays of the sun. They occur in small amounts in the environment and are termed as background radiations.
The following are biological effects of radiations:
Effects of ionizing radiations:
The ionizing radiations include X-rays, gamma rays, alpha rays and beta rays. Since alpha rays and beta rays do not penetrate the human skin, these do not affect the internal body cells. Ionizing radiations cause breaks in the chromosome. These cells then show abnormal cell divisions. If these include gametes, they may be abnormal and even die prematurely. Different types of cancers are the result of radiations. The frequency of induced mutation is directly proportional to the dose of radiations.
Effects of non ionizing radiations:
The non ionizing radiations have longer wavelengths but carry lower energy. This energy is insufficient to induce ionization. Therefore, non ionizing radiations such as ultraviolet light do not penetrate beyond the human skin.
Ultraviolet rays are easily absorbed by purines and pyrimidines. The changed bases are termed as photoproducts. Ultraviolet rays cause two changes in pyrimidine to produce pyrimidine hydrates and pyrimidine dimmers. Thymine ( pyrimidine) dimers is a major effect of ultraviolet rays that disturbs DNA double helix and thus DNA replication.
Increase in temperature increases the rate of mutations with Q10=5. It is further enhanced at higher temperature. Rise in temperature breaks the hydrogen bonding between the two strands of DNA and hence denatures the latter. It disturbs the synthetic process connected with replication and transcription. In rice low temperature increases the rate of mutations.
Large number of chemical mutagens are now known. These are more injurious to man than radiations. The first chemical mutagen discovered was mustard gas by C. Aurebach et al; during world war II.
Chemical mutagens are placed into two groups:
- those which are mutagenic to both replicating and non replicating DNA such as alkylating agents and nitrous acid and
- those which are the mutagenic only to replicating DNA, such as acridine dyes and base analogues.
The following are the effects of the chemical mutagens:
Alkylating and hydroxylating agents:
These include nitrogen and sulphur mustards, methyl and ethylsulphonate ethylnitroso urea (MMS and EMS), nitrosoguanidine (NTG), ethylethane sulphonates (EES), etc. These chemicals are mutagenic to both replicating and non replicating DNA. The major effect includes the transfer of methyl or ethyl groups to the bases in such a way that their base pairing potential changes producing transitions .
Nitrous acid is mutagenic to both replicating and non replicating DNA. It is a deaminating agent which changes cytosine to uracil, guanine to xanthine, and adenine to hypoxanthine. It acts directly by oxidative deamination on A (adenine), G (guanine) and C (cytosine) bases which contain amino groups. A is deaminated to hypoxanthine which is complementary to cytosine. G converts to xanthine that, though pairs which C has only two hydrogen bonds between them. C (cytosine) converts to U (uracil) which pairs with A (adenine). In this way nitrous acid induces AT GC (adenine-thymine guanine-cytosine) transitions.
They are tar derived heteroaromatic flat molecules from which a number of dyes and pharmaceuticals are prepared. Acridines and proflavins are powerful mutagens. Acridines such as acriflavine, proflavine, euflavine, acridine orange can intercalate between DNA bases and interfere with DNA replication, producing insertion or deletion of single bases. This induces frameshift mutations or gibberish mutation. The frame of nucleotide sequence of DNA will be thus disturbed and read differently.
Frameshift mutations are those mutations in which the reading of the frame of base shifts laterally either in the forward direction due to insertion (addition) of one or more nucleotides or in the backward direction due to deletion of one or more nucleotides.
These have structures similar to the normal bases and incorporate into DNA only during DNA replication. Base analogues cause mispairing and eventually give rise to mutations. The base analogues may either be natural or artificial. Natural base analogues include 5 methyl cytosine (in plants such as wheat and grasses), 5 hydroxy methyl cytosine (in Escherichia coli), 6 methyl purine (in some bacteria), etc. The artificial base analogues are 5-bromouracil, 2-aminopurine, 5- bromodeoxyuridine,etc.
The most commonly used base analogues are 5 bromouracil (5 BU) and 2-aminopurine (AP). 5 bromouracil is a structural analogue of thymine. It incorporates into the newly replicated DNA in place of thymine (T). 2 amino purine is an artificial base analogue of adenine. It acts as a substitute of adenine (A) and can also pair with cytosine (C).