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Chemical
Mutagens - Chemical mutagens can be divided into two classes those mutagens which can cause mutations to both replicating and non replicating DNA, are called as class I mutagens, those chemicals, which affect replicating nucleic acids are called as class II mutagens.
The molecules of class II mutagens look like nucleic acid; hence they are incorporated into the replicating DNA molecule. The most prominent mutagenic members of this class are base analogs and dyes.
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Base analogs Base analogs are mutagenic and have a structure sufficiently similar to the normal bases so that they are metabolized and incorporated into DNA during replication, but sufficiently different such that they increase the frequency of mis-pairing and thus cause mutation.
The two most commonly used base analogs are 5 ' -bromouracil and 2 ' -aminopurine. 5 ' -bromouracil (5-BU) is a thymine analog. CH3 in 5' position of thymine and bromine at 5 ' -position in 5- Bu have similar effect. But probability of tautomeric shifts increase because of bromine molecule.
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After a tautomeric shift to its enol form, 5' -bromouracil pairs with guanine and thus causes A. T to G. C transitions. 2-aminopurine can pair in its normal tautomeric form with two bases, with thymine by two hydrogen bonds and with cytosine by one hydrogen bond. The normal base that pairs with base analogues, 2-aminopurjne and 2, 6-diaminopurine is not known.
Base analogs cause transitions in the bidirectional mode. Hence mutations induced by these substances can be reversed by using the same compounds, e.g. 5' -bromouracil, 5chlorouracil, 5-iodouracil, 2-aminopurine, 2, 6-diaminopurine, etc.
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Acridine dyes Acridine dyes bind directly to the DNA by using their positive charge. Positively charged acridines intercalate or sandwich themselves between the stacked base pairs in DNA. Acridine dyes separate two bases by 6.8 A, thus a base will be missed.
Acridine dyes can insert or delete only one base pair in DNA, and thus result in frameshift mutations. The examples of this class of mutagens are nitrous acid (HNO2), hydroxylamine, hydrazine, H2O2 etc.
Nitrous acid Nitrous acid is a very potent mutagen that acts directly on either replicating or non replicating DNA by oxidation or deamination of the bases that contain amino groups (adenine, guanine and cytosine). Conversion of the amino groups to\keto groups changes the hydrogen bonding potential of the bases.
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Adenine is deaminated to hypoxanthine, which then pairs preferentially with cytosine in the place of thymine. Cytosine is deaminated to uracil, which now pairs with adenine in the place of guanine. Deamination of guanine has zero effect, as deaminated guanine also pairs with cytosine.
Since the deamination of adenine leads to AT. GC transition and the deamination of cytosine results in CC. AT transitions, nitrous acid induces transitions in both directions. Nitrous acid also causes interstrand cross-linking of DNA. The DNA strands fail to separate and there is no DNA duplication, which is lethal or deleterious.
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Hydroxylamine: Hydroxylamine (NH2OH) reacts with pyrimidine bases. Its effect is strong on cytosine while on uracil the effect is less. Hydroxylamine breaks and removes pyrimidine ring of uracil thus producing phosphoribosyl urea and 5'isoxasolone with cytosine, hydroxylamine finally produces hydroxyl amino (-NOH) derivative, which might be responsible for base pair change (GC-AT pair). Mutations induced by hydroxylamine cannot be reversed with hydroxylamine.
Alkylating agent Alkylating agents carry one, two or more alkyl groups in reactive form, which are capable of being transferred to other molecules where electron density is high. One major mechanism ,of mutagenesis by alkylating agents involves the transfer of methyl or ethyl group to the bases such that their base-pairing potentials are altered and transitions result.
Dimethyl sulphate upon reaction with guanine in DNA produces 7 methyl guanine, which may pair with thymine. The result is transition of GC to AT. Alkylating agents may cause cross-linking of DNA and thus cause lethal effect by inhibiting DNA duplication because of cross linking.
Alkylating agents may readily react with PO 4 group and may break the sugarphosphate backbone of DNA. This may induce large alterations. Difunctional alkylating agents cause cross linking of DNA, induce chromosome breaks and chromosomal aberrations. Alkylating agents as a class exhibit less specificity in their mutagenic effect than base analogs or dyes.
They induce all types of mutations, (transitions, transversions, frameshifts and even chromosome aberrations) with various relative frequencies depending on the specific alkylating agent.
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