Any substance or agent which is responsible for mutation is called mutagen.
Now we will discuss about types of mutagens:
Radiations
The radiations which are important in mutagenesis are of two categories : one type is ionizing radiations such as X-rays and gamma rays; alpha and beta rays; electrons, neutrons, protons and other fast moving particles.
The second type is non-ionizing radiations such as ultraviolet and visible light. Both types of radiations induce mutations by following methods :
Ionizing radiations
When ionizing radiations pass through matter, they transfer their energy to the outer most electrons of atom and as a result of this gain in energy the electrons become excited and leave the atom as a result of which the atom becomes positively charged.These excited electron also knock out electrons from other atoms.when these excited electrons come back to the ground position they release energy and this released energy causes mutation.Further, ionizing radiations cause breaks in polysugar phosphate backbone of DNA and, thus, causing chromosomal mutations such as break, deletion, addition, inversion and translocation.
In 1927 H.J Muller demonstrated that x rays could induce mutations in Drosophila.L.J.Stadler who used x rays to induce mutations in barley.With x rays it became possible to obtain large numbers of mutant genes in many organisms.These mutants are essential for investigation of the transmission of genetic material.
Non-ionizing radiations
Mutations can be induced by ultraviolet light (UV) which is a non-ionizing radiation. The most effective wave length of ultraviolet for inducing mutations is about 2,600 A°. This is a wave length that is best absorbed by DNA and a wave length at which proteins absorb little energy. When a substance absorbs sufficient energy from the ultraviolet light, some of their electrons are raised to higher energy levels, a state called excitation. The excited molecule becomes reactive and mutated and is called photoproducts.
UV rays also effect DNA by the formation of Bonds between two adjacent pyrimidines,for example between cytosine and thymine.The hydrated form of a pyrimidnie can also be formed, for example, cytosine can be hydrated.In both cases, following dimerization or hydration, the cytosine can be converted to Uracil.
Temperature as mutagen
The rate of all chemical reactions are influenced by temperature. It is not surprising that temperature can be mutagenic. It is reported that the rate of mutation is increased due to increase in temperature. For example, an increase of 10°C temperature increases the mutation rate two or three fold. Temperature probably affects both thermal stability of DNA and the rate of reaction of other substances with DNA.
Chemical mutagens
There are many chemical substances which can induce mutations, for example hydrogen peroxide.During world war two investigations in England concerned with poison gases like mustard gas and related compounds as the nitrogen and sulphur mustards, mustard oil revealed that they were mutagenic in Drosophila and capable of inducing both gene and chromosomal mutations.. Since then many chemical compounds which were considered to be non-toxic have been found to be mutagenic in some situations.
Chemical mutagen can affect the chromosomal DNA by following two way
(1) Direct gene change
Certain chemical mutagens affect DNA directly. They affect the DNA when DNA is not replicating. For example, nitrous acid converts adenine into hypoxanthine and cytosine to uracil by deamination. Like the nitrous acid, nitrogen mustard, formaldehyde, epoxides, dimethyl and diethyl sulphonate, methyl and ethyl methanesulphonate (MMS and EMS) and nitrosoguanadine (NG) also have direct mutagenic effect on the DNA molecule.
(2) Copy error
Certain chemical compounds, called base analogues (e.g., 5-bromouracil, 2-aminopurine, etc.) closely resemble with certain DNA bases and are, therefore, act as mutagens. During DNA replication, they are incorporated into the DNA in place of the normal DNA bases. Some other base analogues are urethane triazine, caffiene (in coffee, tea and soft drinks), phenol.
Now we will discuss about types of mutagens:
Radiations
The radiations which are important in mutagenesis are of two categories : one type is ionizing radiations such as X-rays and gamma rays; alpha and beta rays; electrons, neutrons, protons and other fast moving particles.
The second type is non-ionizing radiations such as ultraviolet and visible light. Both types of radiations induce mutations by following methods :
Ionizing radiations
When ionizing radiations pass through matter, they transfer their energy to the outer most electrons of atom and as a result of this gain in energy the electrons become excited and leave the atom as a result of which the atom becomes positively charged.These excited electron also knock out electrons from other atoms.when these excited electrons come back to the ground position they release energy and this released energy causes mutation.Further, ionizing radiations cause breaks in polysugar phosphate backbone of DNA and, thus, causing chromosomal mutations such as break, deletion, addition, inversion and translocation.
In 1927 H.J Muller demonstrated that x rays could induce mutations in Drosophila.L.J.Stadler who used x rays to induce mutations in barley.With x rays it became possible to obtain large numbers of mutant genes in many organisms.These mutants are essential for investigation of the transmission of genetic material.
Non-ionizing radiations
Mutations can be induced by ultraviolet light (UV) which is a non-ionizing radiation. The most effective wave length of ultraviolet for inducing mutations is about 2,600 A°. This is a wave length that is best absorbed by DNA and a wave length at which proteins absorb little energy. When a substance absorbs sufficient energy from the ultraviolet light, some of their electrons are raised to higher energy levels, a state called excitation. The excited molecule becomes reactive and mutated and is called photoproducts.
UV rays also effect DNA by the formation of Bonds between two adjacent pyrimidines,for example between cytosine and thymine.The hydrated form of a pyrimidnie can also be formed, for example, cytosine can be hydrated.In both cases, following dimerization or hydration, the cytosine can be converted to Uracil.
Temperature as mutagen
The rate of all chemical reactions are influenced by temperature. It is not surprising that temperature can be mutagenic. It is reported that the rate of mutation is increased due to increase in temperature. For example, an increase of 10°C temperature increases the mutation rate two or three fold. Temperature probably affects both thermal stability of DNA and the rate of reaction of other substances with DNA.
Chemical mutagens
There are many chemical substances which can induce mutations, for example hydrogen peroxide.During world war two investigations in England concerned with poison gases like mustard gas and related compounds as the nitrogen and sulphur mustards, mustard oil revealed that they were mutagenic in Drosophila and capable of inducing both gene and chromosomal mutations.. Since then many chemical compounds which were considered to be non-toxic have been found to be mutagenic in some situations.
Chemical mutagen can affect the chromosomal DNA by following two way
(1) Direct gene change
Certain chemical mutagens affect DNA directly. They affect the DNA when DNA is not replicating. For example, nitrous acid converts adenine into hypoxanthine and cytosine to uracil by deamination. Like the nitrous acid, nitrogen mustard, formaldehyde, epoxides, dimethyl and diethyl sulphonate, methyl and ethyl methanesulphonate (MMS and EMS) and nitrosoguanadine (NG) also have direct mutagenic effect on the DNA molecule.
(2) Copy error
Certain chemical compounds, called base analogues (e.g., 5-bromouracil, 2-aminopurine, etc.) closely resemble with certain DNA bases and are, therefore, act as mutagens. During DNA replication, they are incorporated into the DNA in place of the normal DNA bases. Some other base analogues are urethane triazine, caffiene (in coffee, tea and soft drinks), phenol.
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