Q.Explain the role of the lac repressor protein in the lac operon when lactose is ABSENT from the growth medium. How does this keep the operon switched off?

Showing the 12 most recent of 15 on this concept.

1. 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 (transacetylase). The enzyme responsible for bringing lactose into the bacterial cell is the permease encoded by lacY.

   ✓Final answer

   (B) Permease.

2. 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.

   So the operon is switched on when lactose is present and binds to the repressor — option (a). Option (b) describes the OFF state; (c) and (d) are incorrect (RNA polymerase binds the promoter, and lactose acts on the repressor, not RNA polymerase).

   A classic NCERT/CBSE Class 12 Biology gene-regulation board question.

   ✓Final answer

   (a) Lactose is present and it binds to the repressor.

3. 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:

   • (A) Inducible regulation: This describes the overall nature of the lac operon, where the presence of a substrate (lactose) induces or switches on the transcription of genes. While true for the operon as a whole, it doesn't specifically describe the repressor's action.
   • (B) Repressible regulation: This type of regulation is typically seen in anabolic pathways (e.g., tryptophan operon), where the end product of a pathway acts as a corepressor to activate a repressor, thereby switching off gene expression. This is the opposite of the lac operon's mechanism.
   • (C) Negative regulation: This term precisely describes the repressor's role. The repressor protein negatively controls gene expression by binding to the operator and preventing transcription. Its removal (by the inducer) allows transcription to proceed.
   • (D) Positive regulation: This involves an activator protein that binds to DNA and promotes transcription. While the lac operon also exhibits positive regulation through Catabolite Activator Protein (CAP) in the absence of glucose, the question specifically asks about the regulation by the repressor.

   The NCERT textbook clearly states that the regulation of the lac operon by its repressor is an example of negative regulation. The repressor acts as a negative regulator by preventing transcription.

   ✓Final answer

   The regulation of the lac operon by the repressor is referred to as (C) Negative regulation because the repressor protein actively blocks the transcription of the structural genes.

4. 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.

   The i-gene is the regulatory gene coding the repressor. Hence β-galactosidase is coded by the z-gene.

   ✓Final answer

   (A) z-gene.

5. 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. The i gene (a separate regulatory gene) codes for the repressor. So the role of the y gene is to code for permease.

   ✓Final answer

   (b) Codes for permease.

6. 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.

   These three structural genes are transcribed together as a single polycistronic mRNA under the control of a common promoter and operator, and are switched on only in the presence of the inducer (lactose/allolactose).

   Matching against Column B: (a) bone marrow pairs with T-lymphocytes' origin, (b) vectors pairs with PBR322, (c) ovum pairs with corona radiata, (d) detrimental interaction pairs with competition, (f) RBCs is a distractor. So 'Structural genes' correctly pairs with (e) z, y and a.

   ✓Final answer

   (e) z, y and a.

7. 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 itself) — is a small molecule that binds the repressor and inactivates it, switching the operon on; it is not part of the operon's DNA structure, only a regulatory trigger.

   ✓Final answer

   (c) Inducer.

8. 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 lactose metabolism. When the inducer is present, it binds the repressor, changing its shape so it can no longer bind the operator - transcription then proceeds ('switching on' the operon). The terminator is a separate sequence marking the end of transcription, not a gene that produces the repressor.

   ✓Final answer

   (a) regulator gene.

9. 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.

   Why the other options are wrong: the operator gene is a DNA regulatory region that the repressor binds to — it does not itself code for a protein; the structural genes (z, y, a) code for the enzymes β-galactosidase, permease and transacetylase used in lactose metabolism, not for the repressor; the promoter is simply the RNA-polymerase-binding site for transcription initiation, and also does not code for any protein.

   ✓Final answer

   (a) Regulator gene — it produces the repressor protein of the lac operon.

10. 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. Watson and Crick gave the DNA double-helix; Nirenberg decoded the genetic code; neither proposed the operon.

    ✓Final answer

    (C) Jacob and Monod.

11. 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 the substrate. Thus the operon model is a model of gene regulation (control of gene expression), not merely of gene function or synthesis.

    ✓Final answer

    (C) Gene regulation.

12. 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.

    In the absence of lactose, the repressor protein produced by the i gene binds tightly to the operator region, physically blocking RNA polymerase from transcribing the structural genes, keeping the operon switched off. When lactose is present, a metabolite of lactose (allolactose) acts as an inducer, binding to the repressor and changing its shape so it can no longer bind the operator; this allows RNA polymerase to transcribe the structural genes, switching the operon on — the basis of the lac operon's inducible regulation.

    ✓Final answer

    (a) i gene