Q.77 A culture of lac⁺ Escherichia coli is grown in a medium lacking lactose or any
other β–galactoside. The response of the lac operon upon induction by lactose
can be monitored by measuring the levels of lac mRNA, β–galactosidase enzyme
and permease enzyme. Which one of the following profiles correctly captures the
on–off response to lactose?
(A) P
(B) Q
(C) R
(D) S

The correct profile for the on–off response of the lac operon to lactose is Option (A): P.​

Concept behind the question

When lactose is added to a culture of E. coli growing without lactose, lactose (actually allolactose) inactivates the lac repressor, allowing transcription of the lac operon. The structural genes are transcribed as one polycistronic mRNA, which is then translated to produce β-galactosidase (lacZ product) and permease (lacY product).​

Key kinetic facts needed to read the graphs:

• lac mRNA appears rapidly after induction and also decays rapidly once transcription stops, so its level falls quickly when lactose is removed.​

• Enzymes (β-galactosidase and permease) accumulate more slowly and are more stable than mRNA, so their activities decline more gradually after lactose removal.​

A correct “on–off” profile must therefore show:

• Fast rise and fast fall of lac mRNA.

• Slower rise and slower fall of β-galactosidase.

• Similar enzyme behavior for permease, possibly with small differences but not faster than mRNA.

Why profile P is correct

In profile P (the correct graph):

• When lactose is added, lac mRNA (dashed line) rises first and reaches a peak quickly, reflecting rapid transcription initiation after repressor inactivation.​

• β-galactosidase (BG, dotted line) and permease (PE, solid line) rise more slowly because translation and protein accumulation take time.​

• When lactose is removed, transcription stops; lac mRNA declines sharply to near zero within a short time, consistent with its short half‑life.​

• The enzyme levels fall more slowly, remaining for some time after lactose removal, representing protein stability.​

Thus, profile P correctly captures an on–off response where transcription responds quickly to lactose availability, while enzyme activities lag both in onset and in decay.

Why profile Q is incorrect

In profile Q:

• After lactose is added, permease rises earliest and fastest, while lac mRNA appears to rise later, which contradicts the central mechanism because permease cannot increase before the mRNA encoding it is made.​

• The order of appearance should always be mRNA first, proteins second; here the timing is reversed.​

• After lactose removal, enzyme decay is again slower than mRNA, which is reasonable, but the early rise of permease ahead of mRNA makes the overall profile mechanistically impossible.

Therefore, Q does not represent the correct lac operon on–off response to lactose.

Why profile R is incorrect

In profile R:

• lac mRNA shows a delayed, broad peak, whereas both enzyme curves (β-galactosidase and permease) rise very early and reach high levels before mRNA peaks, again violating the requirement that translation depends on prior mRNA synthesis.​

• After lactose removal, permease and β-galactosidase drop almost as fast as mRNA, implying unrealistically short protein half‑lives.​

• Experimental data for lac operon indicate that proteins are significantly more stable than their mRNAs; hence their activity should decay much more slowly.​

Because both the timing and decay kinetics are inconsistent with known lac operon behavior, R is not correct.

Why profile S is incorrect

In profile S:

• After lactose addition, β-galactosidase rises significantly earlier and faster than lac mRNA, which is not feasible because enzyme synthesis requires existing mRNA.​

• lac mRNA is portrayed as the slowest component to rise and fall, suggesting it is more stable than the proteins. This contradicts measured half‑lives, where lac mRNA is short‑lived and degraded rapidly, while the enzymes persist.​

• Although the enzymes do decline somewhat slowly after lactose removal, the relative shapes and timing still misrepresent the fundamental transcription‑translation sequence of the operon.​

Therefore, S also fails to depict the true on–off kinetics of the lac operon.

In summary, only profile P correctly reflects that lac operon induction by lactose causes a rapid, transient rise in lac mRNA, followed by slower, more sustained increases in β-galactosidase and permease, with mRNA disappearing quickly once lactose is removed while enzymes decay gradually.​