Lac Operon: Role of Inducer, Repressor and the i Gene
Biology · Molecular Basis of Inheritance · NEET
In the lac operon, the i gene is always ON (constitutive) and makes a repressor protein. This repressor sits on the operator and blocks RNA polymerase, so the operon stays OFF. When lactose (or its isomer allolactose) is present, this inducer binds the repressor and switches it OFF, freeing the operator so the genes turn ON. Memory hook: "i makes the OFF-switch, lactose kills the OFF-switch."
The i gene constitutively makes the repressor. Without lactose the repressor binds the operator (O) and blocks RNA polymerase (operon OFF). Lactose/allolactose (the inducer) binds and inactivates the repressor, freeing the operator so z, y, a are transcribed (operon ON).
Your doubts, answered
Does the 'i' in i gene stand for inducer?
No. This is the single most common NEET trap. NCERT states clearly that i does NOT mean inducer; it comes from the word 'inhibitor'. The i gene codes for the repressor protein, which inhibits (blocks) the operon. The inducer is a separate molecule (lactose/allolactose).
What is the role of the inducer in the lac operon?
The inducer is lactose (or its isomer allolactose). Its job is to bind the repressor protein and inactivate it. Once the repressor is inactivated, it cannot sit on the operator, so RNA polymerase can move and transcribe the genes. So the inducer turns the operon ON by removing the block.
What does the i gene code for?
The i gene codes for the repressor of the lac operon. The repressor is a protein that binds the operator region and stops RNA polymerase from transcribing the structural genes z, y and a.
How does lactose turn on the lac operon step by step?
1) The i gene keeps making repressor all the time. 2) In the absence of lactose the repressor binds the operator, so the operon is OFF. 3) When lactose enters (through permease, which is made at a very low baseline level), lactose/allolactose binds the repressor. 4) The repressor changes shape and lets go of the operator. 5) RNA polymerase now reaches the promoter and transcribes z, y, a. The operon is ON.
Is lac operon regulation positive or negative?
Regulation by the repressor is NEGATIVE regulation, because a protein binds and blocks transcription. NCERT says the lac operon is under positive regulation as well, but that part is 'beyond the scope' at NEET level. So for NEET, answer negative regulation (by repressor).
What happens if the i gene is mutated so its repressor cannot bind the inducer?
The repressor stays active and stays stuck on the operator even when lactose is present. RNA polymerase is blocked, so z, y and a are NOT transcribed and therefore NOT translated. The operon stays OFF permanently. This is a real NEET 2022 question.
Can glucose or galactose act as an inducer?
No. Only lactose (and allolactose) act as the inducer. NCERT specifically states glucose or galactose cannot act as inducers for the lac operon. Galactose is a product of lactose breakdown, not an inducer.
Do the i gene and the structural genes share one promoter?
No. The i gene has its OWN promoter and is expressed constitutively. Only the three structural genes z, y and a share a single common promoter. Confusing these is a classic trap.
⚠️ The NEET trap ✗ The 'i' in i gene stands for inducer, and lactose activates the repressor so it binds the operator. ✓ The i gene stands for 'inhibitor' and codes for the repressor. Lactose is the inducer and it INACTIVATES the repressor, so the repressor leaves the operator and the operon turns ON. The i gene is constitutively (always) expressed and has its own promoter. 🧠 NEET flips two words: i = inhibitor (not inducer), and lactose inactivates (not activates) the repressor.
Real NEET questions
NEET 2016 / 2019 / ReNEET 2026
Which of the following statements about the lac-operon is correct?
A · Gene i is constitutively expressed ✓
B · Lactose activates the repressor to bind to the operator
C · Genes i, z, y and a share a single common promoter
D · Galactose can act as an inducer of the lac operon
Solution: The regulatory gene i is expressed constitutively (always ON) to make the repressor. Lactose (its isomer allolactose), NOT galactose, is the inducer and it INACTIVATES the repressor, so the repressor binds the operator only in the ABSENCE of lactose. Only genes z, y, a share a common promoter; gene i has its own promoter. So only statement A is correct.
NEET 2016 Phase 1
Which of the following is required as inducer(s) for the expression of Lac operon?
A · Glucose
B · Galactose
C · Lactose ✓
D · Lactose and galactose
Solution: Lactose is the substrate of beta-galactosidase and acts as the inducer. It (or allolactose) inactivates the repressor, freeing the operator so RNA polymerase can transcribe the operon. NCERT states glucose or galactose cannot act as inducers.
NEET 2022
In an E. coli strain the i gene gets mutated and its product cannot bind the inducer molecule. If the growth medium is provided with lactose, what will be the outcome?
A · Only z gene will get transcribed
B · z, y, a genes will be transcribed
C · z, y, a genes will not be translated ✓
D · RNA polymerase will bind the promoter region
Solution: If the mutated repressor cannot bind the inducer (lactose/allolactose), it stays active and stays bound to the operator. RNA polymerase is blocked, so the z, y, a structural genes are not transcribed, and therefore not translated.
Solved Molecular Basis of Inheritance NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
An inducer is a molecule that switches a gene ON. In the lac operon the inducer is lactose (allolactose). It binds and switches off the repressor, which removes the block on transcription.
What is the repressor and where is it made?
The repressor is a protein made by the i gene. It binds the operator region and stops RNA polymerase, keeping the operon OFF when there is no lactose.
Why must a tiny amount of lac operon always be expressed?
Permease (made by the y gene) is needed to bring lactose into the cell. So a very low baseline level of the operon must always run; otherwise lactose could never enter to act as the inducer.
Who discovered the lac operon?
Francois Jacob (geneticist) and Jacques Monod (biochemist). It was the first transcriptionally regulated system to be explained. This pairing is a repeated NEET matching question.
Is the lac operon inducible or repressible?
It is an inducible operon. It is normally OFF and gets turned ON (induced) when the inducer lactose is present.
