29. A chemist synthesized three new chemical compounds, MI, M2 and M3. The compounds were tested for their mutagenic potential and were found to be highly mutagenic. Tests were made to characterize the nature of mutations by allowing the reversion with other mutagens. The following results were obtained:

Mutations produced by	Reversed by
	2-amino purine	Nitrous Acid	Hydroxyl amine	Acridine orange
M1	NO	NO	NO	NO
M2	YES	YES	NO	NO
M3	NO	NO	NO	YES

C4

NO

NO

NO

YES

Which one of the following conclusions drawn regarding the nature of mutations by the compounds is correct?
(1) M1- transversion, M2 – insertion, M3 –deletion
(2) M1- transition, M2- transversion, M3 – insertion
(3) M1 – insertion, M2 – transition, M3 – transversion
(4) M1 – transversion, M2 – transition, M3 – insertion

The correct answer is option (4) M1 – transversion, M2 – transition, M3 – insertion.

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Question breakdown

In this problem, three new mutagenic compounds (M1, M2, M3) are tested.
Reversion of the mutations they produce is checked using four standard mutagens:

• 2‑aminopurine (2‑AP) – a base analogue that mainly induces transition base substitutions.

• Nitrous acid – deaminates bases and also mostly causes transition mutations.

• Hydroxylamine – modifies cytosine, again giving mainly transition mutations.

• Acridine orange – an intercalating agent that causes frameshift mutations, usually insertions or deletions of base pairs.

The key logic:

• If a mutation is reversed by transition‑type mutagens (2‑AP, nitrous acid, hydroxylamine), the original mutation is most likely a base substitution, often a transition.

• If a mutation is reversed specifically by a frameshift mutagen like acridine orange, the original mutation is most likely an insertion or deletion (frameshift).

From the table in the question:

• M1: not reversed by any of the four mutagens.

• M2: reversed by 2‑AP and nitrous acid, but not by hydroxylamine or acridine orange.

• M3: reversed only by acridine orange.

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Identifying M2 and M3

Nature of M2 mutation

M2 is reverted by 2‑AP and nitrous acid, both classic transition‑inducing agents.
This strongly indicates that the M2‑induced mutation is a transition (purine↔purine or pyrimidine↔pyrimidine substitution).

So:
M2 = transition mutation.

Nature of M3 mutation

M3 is reverted only by acridine orange, an intercalating dye that typically corrects or induces frameshift mutations through base insertions or deletions.
Reversion by acridine orange means the original mutation must also be a frameshift, and in single‑step reasoning such questions usually classify this under insertion (rather than deletion) when one specific option must be chosen.

So:
M3 = insertion (frameshift) mutation.

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Interpreting M1

M1 is not reversed by any of the mutagens given:

• Not reversed by transition mutagens (2‑AP, nitrous acid, hydroxylamine), so it is unlikely to be a simple transition.

• Not reversed by acridine orange, so it is unlikely to be a straightforward insertion or deletion frameshift that acridine could correct.

Among the options, the only remaining base‑substitution type is a transversion (purine↔pyrimidine change).
Transversions are less efficiently or less specifically reversed by these standard transition mutagens, so lack of reversion is consistent with a transversion mutation.

So:
M1 = transversion mutation.

Putting all three together gives:
M1 – transversion, M2 – transition, M3 – insertion, which matches option (4).

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Option‑wise explanation

Option (1): M1 – transversion, M2 – insertion, M3 – deletion

• M2 is clearly reverted by transition‑inducing mutagens (2‑AP and nitrous acid), not by acridine orange, so calling M2 an insertion contradicts its reversion pattern.

• M3 is reverted by acridine orange, which supports a frameshift, but the option specifies deletion while the key in such questions conventionally groups acridine‑correctable frameshifts as “insertion” in a single‑choice framework.

Therefore, option (1) is inconsistent for M2.

Option (2): M1 – transition, M2 – transversion, M3 – insertion

• If M1 were a transition, it should sometimes be corrected by transition mutagens like 2‑AP or nitrous acid, yet the table shows no reversion at all.

• For M2, the strong reversion by 2‑AP and nitrous acid fits transition, not transversion, so M2 is mis‑classified here.

Thus, option (2) is rejected for both M1 and M2.

Option (3): M1 – insertion, M2 – transition, M3 – transversion

• M2 as transition is acceptable, because 2‑AP and nitrous acid revert it, but the other two assignments fail.

• M1 as insertion should have at least some reversion in the presence of acridine orange, which is not observed.

• M3 as transversion conflicts with its clear reversion by acridine orange, a frameshift mutagen, not a base‑substitution mutagen.

Hence, option (3) cannot be correct.

Option (4): M1 – transversion, M2 – transition, M3 – insertion

• M2: Reverted by transition mutagens ⇒ correctly labeled as transition.

• M3: Reverted solely by acridine orange ⇒ correctly labeled as insertion (frameshift).

• M1: Not reverted by any of these agents ⇒ best explained as a transversion that is not efficiently corrected by the tested mutagens.

Therefore, option (4) correctly matches the experimental data and is the right choice.