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INDUCED MUTATIONS

The mutations in the genetic material can be induced artificially by some physical or chemical mutagens that increase the rate of mutation.

1.Physical Mutagens. They can be ionizing or non ionizing radiations.

(i)                 Ionizing Radiations: They are short-wavelength and highly energetic radiations that can cause ionization of various cellular molecules, e.g., X-rays, γ-rays, β-rays, neutrons, etc. Muller (1927) was the first person who demonstrated the mutagenic effect of X-rays in the living system (in Drosophila or the fruit fly). X-rays induce mutations by:

1. Ionization of bases due to loss of electrons.

2. Breakage of DNA strands that may cause deletions or rearrangement of bases after reunion.

3. Formation of free radicals and H2O2, that in turn may induce breaks and other changes in DNA, e.g., the warer molecules in the cells may lead to free radical formation as:

H2O          Ho     +     OHo

Ho        +     Ho    →     H2

OHo     +    OHo  →     H2O2

(ii) Non-ionizing Radiations: They are long-wavelength, lesser energetic radiations and, therefore, are unable to cause ionization of cellular molecules, e.g., UV (190-390 nm). Altenberg (1930) was the first person who demonstrated the mutagenic effect of UV rays in the living system (in Drosophila eggs). UV radiation usually acts on pyrimidine bases in two ways:

Most frequently intrastrand pyrimidine dimmer formation (Fig. 10.9):

 

(a) TT (about 50% frequency)

(b) TC (about 40% frequency)

(c) CC (about 10% frequency)

(d) UU (where RNA is the genetic material, e.g., some viruses)

 

These pyrimidine dimmers may interrupt the DNA replication to cause lethal effects or may lead to wrong base incorporations or deletions of bases.

2. Formation on Hydoxyl pyrimidines, e.g., hydroxyl cytosine (HC), hydroxyl thymine (HT):

Fig. 10.9 Thymine dimer formation by exposure to UV radiation.

Fig. 10.9 Thymine dimer formation by exposure to UV radiation.

The hydroxyl pyrimidines base pair wrongly to cause transitions.

3. UV also induces free radicals and peroxides formation to cause mutations.
2. Chemical Mutagens: There are many types of chemicals that are mutagenic.

(i) Base Analogues: They are almost similar in structure to the normal bases found in DNA and may get incorporated in DNA at their places. Moreover, their mutagenic effect is due to the fact they form tautomers more readily than the normal bases, therefore, during DNA replication they readily pair with wrong bases and cause base substitutions. Some of the important base analogues are:

(a)   5-Bromouracil: It is analogue of the base thymine (keto base) and after getting incorporated into DNA, it more frequently forms the enol tautomer (that base pairs wrongly with guanine) due to higher electronegativity of -Br than the -CH3 group. Thus, it causes transitions.

2-Amino purine: It is analogue of the base adenine (amino base) and alter getting incorporated into DNA, it more frequently forms the imino tautomer that base pairs wrongly with cytosine. Thus, it also causes transitions.

(ii) Alkylating Agents: Examples of some alkylating agents are methyl methane sulphonate (MMS), ethyl methane sulphonate (EMS), etc. They cause mutations by affecting the DNA in different ways:

1.EM, most frequently after adding an ethyl group in guanine base, forms the 7-ethyl guanine (7-EG) that due to wrong base pairing causes transition.

2. Since, 7-ethyl guanine is weakly attached to the deoxy-ribose sugar in DNA, I may get detached (depurination) from the sugar. This gap in DNA may interfere with the DNA replication or any of the four bases (T, C, A or G) may get incorporated in front this vacant site to cause transitions, transversion or no change at all, depending upon the incorporated base.
3. Alkylating agents may also cause mutations by inducing alkylation of PO43- group in the DNA backbone to cause breakage of DNA strand.
(iii) Dyes: Dyes, being larger in size, get intercalated either in front of or in between the bases in
DNA to cause additions or delitions that lead to frame shift mutations, e.g., proflavin, acridine orange, etc. (Fig. 10.10 and 10.11).
Fig.10.11

Fig. 10.11 Chemical structures of mutagenic dyes prot1avin (left) and acridine orange (right).