Describe the regulation of lactose operon in bacteria.

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Q. Describe the regulation of lactose operon in bacteria.

Karnataka PUCKarnataka II PUC Board 2026Subjective· 5mImportance★★★★★

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The lac operon is switched off by a repressor bound to the operator when lactose is absent, and switched on when lactose (as allolactose, the inducer) inactivates the repressor so RNA polymerase can transcribe the structural genes z, y and a.

The lac operon of E. coli (worked out by Jacob and Monod) is a classic example of the regulation of gene expression in bacteria by an inducible operon under negative control.

Components of the lac operon:

i gene (regulatory gene): codes for the repressor protein. ("i" stands for inhibitor.)
Promoter (P): site where RNA polymerase binds.
Operator (O): site where the repressor binds; overlaps the promoter region.
Structural genes: z (beta-galactosidase, splits lactose into glucose + galactose), y (permease, increases lactose uptake into the cell), a (transacetylase).

Regulation:

When lactose is ABSENT (operon OFF):
- The i gene continuously produces the active repressor protein.
- The repressor binds to the operator region.
- This blocks RNA polymerase from transcribing the structural genes.
- No enzymes are made — this prevents wastage when there is no substrate.
When lactose is PRESENT (operon ON):
- Lactose acts as the inducer. A small amount enters the cell and is converted to allolactose.
- Allolactose binds to the repressor and changes its shape, inactivating it.
- The inactive repressor can no longer bind the operator. …

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CBSE 2026Set A1 markMCQ
$Q. Which of the following enzymes causes lactose to enter the bacterial cell?$
(a) Beta galactosidase
(b) Permease
(c) Transacetylase
(d) Lactase
›Reveal solutionSolution

In the lac operon, the enzyme permease transports lactose into the E. coli cell.

The lac operon has three structural genes: lacZ (beta-galactosidase, hydrolyses lactose into glucose + galactose), lacY (permease, increases membrane permeability so lactose can enter the cell) and lacA (transacetylas …

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CBSE 2026Set SEM31 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 present and it binds to RNA polymerase
›Reveal solutionSolution
The lac operon is an inducible system. When lactose (the inducer) is present, it binds the repressor protein and inactivates it; the repressor then cannot sit on the operator, so RNA polymerase transcribes the structural genes — the operon is switched ON.

Mechanism of the lac operon:

In the absence of lactose, the repressor (product of the i gene) binds the operator and blocks RNA polymerase → operon OFF.

In the presence of lactose, a little lactose is converted to allolactose (the inducer), which binds to the repressor and changes its shape so it can no longer bind the operator. RNA polymerase is now free to transcribe z, y, a → operon ON. …
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CBSE 2025Set 57/6/11 markMCQ
$Q. Regulation of lac operon by repressor is referred to as : (A) Inducible regulation (B) Repressible regulation (C) Negative regulation (D) Positive regulation$
›Reveal solutionSolution
The regulation of the lac operon by its repressor protein is termed negative regulation because the repressor actively blocks transcription.

The lac operon in E. coli is a classic example of gene regulation, demonstrating how bacteria adapt to their environment by controlling gene expression. This operon is responsible for the metabolism of lactose, a disaccharide sugar. The core idea is that the genes for lactose metabolism are only expressed when lactose is available and glucose (the preferred energy source) is absent.

At the heart of this regulation, particularly concerning the repressor, is a mechanism designed to prevent the wasteful synthesis of enzymes when lactose is not present. The lac operon consists of a regulatory gene (i gene), a promoter (P), an operator (O), and three structural genes (z, y, a). The i gene codes for the lac repressor protein.

Here's how the repressor regulates the lac operon:

Repressor Synthesis: The i gene is constitutively expressed, meaning it is always producing the lac repressor protein, albeit at a low level. This repressor protein is a tetramer.

Repressor's Default Action (Absence of Lactose):

In the absence of lactose, the repressor protein is in its active conformation.

It binds tightly to the operator region (O) of the lac operon.

The operator region is located immediately downstream of the promoter. When the repressor is bound to the operator, it physically blocks the movement of RNA polymerase.

This blockage prevents RNA polymerase from transcribing the structural genes (z, y, a), which code for enzymes like beta-galactosidase, permease, and transacetylase, necessary for lactose metabolism.

Thus, in the absence of lactose, the operon is switched off, and the enzymes are not produced.

Repressor's Action in Presence of Lactose (Induction):

When lactose is present in the medium, a small amount of it enters the bacterial cell.

Lactose is then converted into allolactose, which acts as the actual inducer molecule.

Allolactose binds to the repressor protein. This binding causes a conformational change in the repressor.

Due to this change, the repressor protein can no longer bind to the operator region.

With the operator free, RNA polymerase can now bind to the promoter and move along the DNA, transcribing the structural genes.

This leads to the production of the enzymes required for lactose metabolism, allowing the bacterium to utilize lactose as an energy source.

Important

The repressor protein's primary function is to block transcription. When it is active and bound to the operator, it exerts a negative control over gene expression.

Considering the options: …
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CBSE 2025Set F1 markMCQ
$Q. Which of the following does coding of β-galactosidase in lac operon?$
(a) z-gene
(b) a-gene
(c) y-gene
(d) i-gene
›Reveal solutionSolution
β-galactosidase is coded by the z-gene (lacZ) of the lac operon.

The lac operon of E. coli has three structural genes:

z (lacZ) → β-galactosidase, which hydrolyses lactose into glucose and galactose,

y (lacY) → permease, for lactose uptake,

a (lacA) → transacetylase. …
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CBSE 2024Set E1 markMCQ
$Q. What is the role of y gene in lac operon?$
(a) Codes for β-galactosidase
(b) Codes for permease
(c) Codes for repressor gene
(d) Codes for transacetylase
›Reveal solutionSolution

In the lac operon, the y gene codes for permease.

The lac operon has three structural genes: z, y and a. The z gene codes for beta-galactosidase (which hydrolyses lactose into glucose and galactose); the y gene codes for permease (which increases permeability of the cell to lactose/beta-galactosides); and the a gene codes for transacetylase. T …

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CBSE 2024Set D1 mark
$Q. Match the following and write the correct pair. Column 'A':$
(i) Structural genes
(ii) T-Lymphocytes
(iii) Competition
(iv) PBR322
(v) Corona radiata. Column 'B':
(a) bone marrow
(b) vectors
(c) ovum
(d) detrimental interaction
(e) z, y and a
(f) RBCs. Find the correct match for item
(i) 'Structural genes' from Column 'B'.
›Reveal solutionSolution

In the matching exercise, 'Structural genes' pairs with option (e) 'z, y and a' — the three structural genes of the lac operon.

The lac operon of E. coli, the classic model of gene regulation studied in the Molecular Basis of Inheritance chapter, consists of a regulatory gene (i) and three structural genes: z, y, and a.

The z gene codes for β-galactosidase (which hydrolyses lactose into glucose and galactose), the y gene codes for permease (which increases the permeability of the cell to β-galactosides), and the a gene codes for transacetylase.

…

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CBSE 2024Set ANNUAL1 markMCQ
$Q. Which one of the following is not a part of lac operon?$
(a) Promoter
(b) Regulator
(c) Inducer
(d) Operator
›Reveal solutionSolution

Correct answer: (c) Inducer.

The lac operon's structural parts are the promoter, operator and regulator gene; the inducer is a small molecule, not a structural component.

The lac operon of E. coli is built of the regulator gene (i, coding for the repressor), the promoter (RNA-polymerase binding site), the operator (repressor binding site) and the structural genes (z, y, a). The inducer — allolactose (or its analogue, lactose its …

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CBSE 2023Set BOTANY1 markMCQ
$Q. Fill in the blank selecting the appropriate term given under the bit: Repressor protein is produced by _____.$
(a) regulator gene
(b) operator gene
(c) structural gene
(d) terminator gene
›Reveal solutionSolution

The repressor protein of an operon (e.g., the lac operon) is coded for by the regulator gene, which lies outside the operon proper but controls it.

Jacob and Monod's operon model of gene regulation in E. coli (lac operon) consists of a regulatory gene and structural genes under the control of a common promoter and operator. The regulator gene (i) is transcribed constitutively at a low, constant rate and translated to give the repressor protein. This repressor protein binds to the operator gene (o) region in the absence of the inducer (lactose/allolactose), physically blocking RNA polymerase from transcribing the downstream structural genes (z, y, a), which code for the enzymes β-galactosidase, permease and transacetylase involved in lactos …

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CBSE 2022Set ANNUAL1 markMCQ
$Q. Which of the following gene produced repressor protein?$
(a) Regulator gene
(b) Operator gene
(c) Structural gene
(d) Promoter gene
›Reveal solutionSolution

The regulator gene (i gene) of the lac operon is transcribed and translated into the lac repressor protein.

The lac operon of E. coli consists of a regulator gene (i), a promoter (p), an operator (o), and three structural genes (z, y, a). The i gene is constitutively expressed at a low, constant level, and its mRNA is translated into the lac repressor protein. In the absence of lactose, this repressor binds to the operator region and physically blocks RNA polymerase from transcribing the structural genes.

…

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CBSE 2021Set D1 markMCQ
$Q. Operon model was proposed by$
(a) Watson and Crick
(b) Nirenberg
(c) Jacob and Monad
(d) None of them
›Reveal solutionSolution

The operon concept was given by Jacob and Monod.

In 1961, the French scientists Francois Jacob and Jacques Monod, working on the lactose (lac) operon of Escherichia coli, put forward the operon model to explain how a group of structural genes is switched on and off together under the control of a common promoter and operator. …

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CBSE 2021Set D1 markMCQ
$Q. What does operon model represent ?$
(a) Gene synthesis
(b) Gene expression
(c) Gene regulation
(d) Gene function
›Reveal solutionSolution

The operon model represents gene regulation.

The operon (e.g. the lac operon of E. coli) is a unit of coordinated gene control consisting of a promoter, an operator and a cluster of structural genes, together with a regulatory gene coding for a repressor. By controlling whether the repressor binds the operator, the cell switches transcription of the structural genes on or off in response to …

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CBSE 2020Set ANNUAL1 markMCQ
$Q.______ is known as the regulatory gene in Lac-Operon.$
(a) i gene
(b) z gene
(c) y gene
(d) a gene
›Reveal solutionSolution
The i (regulatory) gene of the lac operon codes for the repressor protein that controls whether the operon's structural genes are transcribed.

The lac operon of E. coli consists of a regulatory gene and three structural genes, together controlling lactose metabolism:

i gene (regulatory gene): located just upstream of the promoter, it is expressed constitutively (continuously, at a low level) and codes for the lac repressor protein.

z, y, a genes (structural genes): code for β-galactosidase, permease, and transacetylase respectively, the enzymes needed for lactose uptake and catabolism. …
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Q.Describe the regulation of lactose operon in bacteria.

Concept understanding — Lac Operon

Q.Describe the regulation of lactose operon in bacteria.

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