Explain how the addition of lactose in the medium regulates the switching on of the lac operon in bacteria.

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Q. Explain how the addition of lactose in the medium regulates the switching on of the lac operon in bacteria.

CBSECBSE Class XII Board 2025Subjective· 3mImportance★★★★★

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Concept understanding — Lac Operon Catabolite Repression

Imagine you are a factory manager. You have two raw materials: a high-grade fuel that your machines run on perfectly, and a low-grade backup fuel that works but is harder to use. As long as the good fuel is available, you would never waste time and energy switching to the backup. But if the good fuel runs out, you immediately switch to the backup to keep production going.

That is exactly what catabolite repression does inside a bacterium like E. coli. It is the cell's way of saying: "Use the best fuel first; don't bother with the second-best until you absolutely have to."

The Two Fuels: Glucose and Lactose

E. coli bacteria love glucose. It is their favourite energy source — easy to break down, gives quick energy. Lactose (milk sugar) is harder to digest; the cell needs to build special enzymes (like β-galactosidase) to break it down. These enzymes are coded by the lac operon.

The cell has a simple rule: If glucose is present, do not waste energy making lactose-digesting enzymes. That is catabolite repression. It is a global regulatory mechanism that ensures glucose is used first, even when lactose is also available.

Note

Catabolite repression is sometimes called the glucose effect. It is not unique to the lac operon — it affects many operons that break down alternative sugars. But the lac operon is the classic textbook example.

How It Works: The Molecular Switch

The key player is a molecule called cAMP (cyclic AMP). Its level inside the cell is inversely related to glucose concentration:

When glucose is high: cAMP levels are low.
When glucose is low: cAMP levels rise.

cAMP binds to a protein called CAP (Catabolite Activator Protein). The cAMP–CAP complex then binds to a specific site near the lac operon's promoter. This binding dramatically increases the rate of transcription — it is like pressing the accelerator pedal.

So here is the logic:

Glucose present (high): Low cAMP → CAP cannot bind → lac operon is barely transcribed, even if lactose is around. The cell ignores lactose.
Glucose absent (low): High cAMP → CAP binds → lac operon is fully activated. Now, if lactose is also present, the operon switches on fully and the cell digests lactose.

Important

Catabolite repression is a positive control mechanism. The CAP–cAMP complex activates transcription. This is different from the lac repressor, which blocks transcription when lactose is absent. The lac operon is controlled by two switches: a negative one (repressor) and a positive one (CAP–cAMP). Both must be in the "on" position for maximum expression.

Why It Matters (Exam Perspective)

The NCERT textbook (Class 12 Biology, Chapter 6) presents catabolite repression as a fine-tuning mechanism. It explains that even when the lac repressor is removed (by lactose binding), transcription is still low unless glucose is absent. The CAP–cAMP complex is the "second key" that unlocks full expression.

Key points to remember for exams: …

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Lactose acts as an inducer that inactivates the lac repressor, allowing RNA polymerase to transcribe the lac operon genes needed for lactose metabolism.

To understand how lactose switches on the lac operon, you first need to picture the operon in its default state. In the absence of lactose, the lac operon is kept switched off by a repressor protein. This repressor is encoded by a separate regulatory gene called lac I, which is located elsewhere on the bacterial chromosome. The repressor protein binds tightly to a specific DNA sequence called the operator, which sits right next to the promoter where RNA polymerase would normally attach. When the repressor is bound to the operator, it physically blocks RNA polymerase from moving forward, so the structural genes — lac Z, lac Y, and lac A — are not transcribed. The bacterium does not waste energy making enzymes it does not need.

Now, what happens when lactose enters the medium? Lactose itself is not the direct trigger. A small amount of lactose that enters the cell is converted into an isomer called allolactose. Allolactose is the true inducer. It binds to the repressor protein and changes the repressor's shape — its three-dimensional structure is altered so that it can no longer attach to the operator. With the repressor dislodged, the operator site is free. RNA polymerase can now bind to the promoter and begin transcribing the three structural genes. This produces the enzymes beta-galactosidase (which breaks lactose into glucose and galactose), permease (which brings more lactose into the cell), and transacetylase (whose precise role is less central but still part of the operon).

Note

Allolactose is not a separate substance added to the medium; it is formed inside the cell from lactose itself. So the presence of lactose in the medium ultimately leads to allolactose production. …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

CBSE 2026Set ANNUAL1 markMCQ
$Q. In E.coli, the lac operon gets switched on when :$
(a) lactose is present and it binds to the repressor.
(b) repressor binds to operator.
(c) RNA polymerase binds to the operator.
(d) lactose is absent.
›Reveal solutionSolution
The lac operon is switched ON when lactose (the inducer) binds and inactivates the repressor, so RNA polymerase can transcribe the structural genes; option (a) is correct.

In the CBSE/NCERT Molecular Basis of Inheritance chapter, the lac operon of E. coli is regulated by negative control:

No lactose: the repressor protein (from the i gene) binds the operator, blocking RNA polymerase — operon OFF. …
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CBSE 2020Set ANNUAL1 mark
$Q. Name the inducer which regulates the switching on and off of the lac operon.$
›Reveal solutionSolution

Lactose is the inducer that switches the lac operon on and off. In its presence the operon is transcribed; in its absence the repressor keeps it off.

Concept. The lac operon of E. coli (worked out by Jacob and Monod) is a classic example of a negatively regulated inducible operon. It has a regulator gene (i), a promoter, an operator, and three structural genes (z, y, a) coding for β-galactosidase, permease and transacetylase.

…

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CBSE 2019Set ANNUAL1 markMCQ
$Q. Lac operon represents-$
(a) Inducible gene system
(b) Repressible gene system
(c) Housekeeping gene system
(d) All of these
›Reveal solutionSolution

The lac operon is an inducible gene system.

In the lac operon of E. coli, lactose acts as an inducer: it binds the repressor, inactivating it, so RNA polymerase can transcribe the structural genes (z, y, a) for lactose metabolism. In the absence of lactose the genes are switched off. Becau …

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CBSE 2015Set ANNUAL1 mark
$Q. Identify the diagram and write how it acts?$
›Reveal solutionSolution

The grainy vertical banding image is a DNA fingerprint (autoradiograph); it works because repetitive DNA sequences called VNTRs (minisatellites) are highly variable in repeat number between individuals, so the pattern of bands each person shows is essentially unique and can be used to identify them.

Identifying the diagram

A vertical smear of dark parallel bands produced on X-ray film is the classic appearance of a DNA fingerprint, generated by autoradiography as the final step of the DNA fingerprinting/profiling technique.

How it acts (the principle)

DNA fingerprinting exploits repetitive DNA sequences (satellite DNA), particularly a class called VNTRs (Variable Number of Tandem Repeats, or minisatellites). These repeat sequences show a very high degree of polymorphism — i.e. the number of repeats at a given locus differs from person to person (and even between the two chromosome copies of the same person), because such regions are not being selected against and mutate/change repeat-number relatively freely. In the technique, an individual's DNA is cut with restriction enzymes, the fragments are separated by electrophoresis, transferred to a membrane, and hybridised with a labelled VNTR probe; the labelled fragments are then visualised by autoradiography as a series of bands. Because the exact sizes/positions of t …

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Q.Explain how the addition of lactose in the medium regulates the switching on of the lac operon in bacteria.

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