61. Base-pair substitutions caused by the chemical mutagen ethyl methane sulfonate are a result
of
(A) hydroxylation
(B) alkylation
(C) deamination
(D) intercalation

Ethyl methanesulfonate (EMS) induces base-pair substitutions through alkylation of DNA bases. The correct answer to the query is (B) alkylation.​

Correct Answer

EMS primarily causes G:C to A:T transition mutations by alkylating guanine at the O6 position, forming O6-ethylguanine. This modified base pairs with thymine instead of cytosine during replication, leading to a permanent substitution after further rounds of DNA synthesis. EMS acts as a monofunctional alkylating agent with mixed SN1/SN2 reactivity, targeting nucleophilic sites like N7 and O6 of guanine.​

Option Explanations

• (A) Hydroxylation: Involves adding a hydroxyl group, as seen with hydroxylamine, which modifies cytosine at position 4 to pair with adenine, causing C:G to T:A transitions. EMS does not hydroxylate bases.​

• (B) Alkylation: EMS transfers an ethyl group to DNA bases, especially O6-guanine, mispairing with thymine and resulting in transitions. This matches EMS’s primary mutagenic action.​

• (C) Deamination: Removes amino groups from bases like cytosine (to uracil) or adenine, often via nitrous acid, leading to G:C to A:T transitions. EMS mechanism is alkylation, not deamination.​

• (D) Intercalation: Planar molecules insert between base pairs, causing frameshifts via addition/deletion of bases during replication. EMS is not an intercalator; it chemically modifies bases.​