Control of gene expression in prokaryotes take place at the level of: (a) DNA-replication (b) Transcription (c

NCERT Exemplar · Q16

Q. Control of gene expression in prokaryotes take place at the level of:

(a) DNA-replication

(b) Transcription

(d) None of the above

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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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In prokaryotes, the primary and most efficient level of control over gene expression occurs during transcription, determining whether a gene is copied into mRNA.

Prokaryotic organisms, such as bacteria, live in constantly changing environments and must adapt rapidly to survive. This necessitates a highly efficient and responsive system for controlling which genes are expressed and when. Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product, typically a protein. Regulating this process allows prokaryotes to conserve energy and resources by only producing proteins that are needed at a particular time.

Let's consider the various stages where gene expression could theoretically be controlled:

DNA Replication: This process involves making copies of the entire genome before cell division. While fundamental to life, DNA replication itself is not the primary point of control for the expression of individual genes. It's about duplicating the genetic material, not about regulating whether a specific gene within that material is turned "on" or "off" to produce a protein.
Transcription: This is the synthesis of messenger RNA (mRNA) from a DNA template. In prokaryotes, controlling transcription is the most crucial and common level of gene expression regulation. If a gene is not transcribed, no mRNA is produced, and consequently, no protein can be made from that gene. This allows the cell to stop the process at the earliest possible stage, saving significant energy and resources. The classic example of this is the lac operon in E. coli.
- When lactose is absent, a repressor protein binds to the operator region of the lac operon, physically blocking RNA polymerase from initiating transcription of the genes required for lactose metabolism.
- When lactose is present, it acts as an inducer, binding to the repressor protein and altering its shape, preventing it from binding to the operator. This allows RNA polymerase to transcribe the structural genes, leading to the production of enzymes that metabolize lactose. This entire mechanism operates at the level of transcription initiation.

Important

The lac operon is a prime example of transcriptional control in prokaryotes, where the presence or absence of a substrate (lactose) directly regulates whether genes are transcribed. …

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

KCET 2025Set C-41 markMCQ

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›Reveal solution

Recall the three structural genes of the lac operon and what each product does; the permeability function is the $y$ gene product, permease.

Step 1 — The lac operon's three structural genes.

In E. coli, the lactose (lac) operon has a regulatory gene $i$ plus three structural genes:

Gene	Product	Function
$z$	β-galactosidase	Hydrolyses lactose → galactose + glucose
$y$	Permease	Increases permeability of the cell to β-galactosides (lactose) — it is the membrane transporter
$a$	Transacetylase	Transfers an acetyl group to β-galactosides

Step 2 — Read the stem carefully.

The stem asks for the enzyme that increases the permeability of the bacterial cell to lactose — i.e. the one that lets lactose in, not the one that breaks it down.

Step 3 — Eliminate. …

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Q.Control of gene expression in prokaryotes take place at the level of:

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