Agents Modifying Purines and Pyrimidines OR Agents Labilize the Bases

Agents Modifying Purines and Pyrimidines OR Agents Labilize the Bases

	Agents Modifying Purines and Pyrimidines OR Agents Labilize the Bases - It include nitrous oxide, hydroxylamine and alkylating agents. (i) Nitrous oxide (HNO2) reacts with bases containing amino groups. It can change the structure of such bases by deamination (removal of the amino group). When purines or pyrimidines containing the amino group are treated with nitrous oxide, the amino group (-NH2) is replaced by the hydroxyl group (-OH). The order of frequency of deamination is adenine, cytosine and guanine.
Deamination of adenine results in the formation of hypoxanthine. The pairing behaviour of hypoxanthine is like that of guanine. Therefore hypoxanthine pairs with cytosine rather than with thymine. Thus A-T pairing is replaced by G-C pairing
The deamination of cytosine (at the 6-position) results in the formation of uracil. The hydrogen bonding properties of uracil are similar to those of thymine. Therefore, instead of C-G pairing there is U -A pairing. Guanine is deaminated to xanthine. There is no change in pairing behaviour in this case, because xanthine behaves like guanine and pairs with cytosine. Instead of G-C pairing there is X-C pairing. Thus deamination of guanine is not mutagenic.
The bases formed after deamination of adenine and cytosine have a different pairing behaviour. As a result changes in DNA take place in 50% of the progeny. Deamination of guanine, however, does not result in a heritable mutation, since there is no change in the pairing behaviour of the deaminated base (xanthine).
	(ii) Hydroxylamine (NH2OH) is very specific in its action. It reacts mainly with cytosine and guanine residues and brings about transitions and mispairing. It derminates cytosine to a base which pairs with adenine instead of guanine. Thus C-G pairing is changed to A-T pairing. Alkylating agents are the most widely used mutagenic reagents. They include: dimethyl sulphate (OMS) ethyl methane sulphonate (EMS) –CH3CH2SO3CH3 ethyl ethane sulphonate (EES) -CH3CH2SO3CH2CH3

The main chemical reaction of these agents is alkylation at the N-7 position of guanine residues or at the N-3 position of adenine residues.

Alkylation increases the probability of ionization and introduces pairing error. The base-sugar linkage undergoes hydrolysis and releases the base from the DNA molecule. This creates a gap in one chain.

EMS specifically removes guanine from the chain. During replication the chain without gaps will give rise to normal DNA. In the chain with gaps, however, any base (A, T, G or C) maybe inserted across the gap.

This may be a correct base or an incorrect one. In the next replication the gap is filled by a base which is complementary to the inserted base.

Where the correct base is inserted the DNA is normal. Insertion of an incorrect base may result in a transversion (purine replaced by a pyrimidine and vice versa) or a transition (purine replaced by a purine and a pyrimidine by a pyrimidine)

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Suppressor Genes Frameshift Suppressions Molecular Basis of Mutations Spontaneous Mutations Tautomerism Induced Mutations Chemical Mutagens Base Analogues Agents Modifying Purines and Pyrimidines OR Agents Labilize the Bases Agents Producing Distortions in DNA Radiations Types of Mutations

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