Q.In the medium where E. coli was growing, lactose was added, which induced the lac operon. Then, why does lac operon shut down some time after addition of lactose in the medium?

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.

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.

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: …