Question
Explain the Lac Operon model in bacteria. Describe the structural components and how the system works in the presence and absence of lactose.
(NEET 2024 — this model has appeared consistently for the past 5 years, making it a near-certain question in any NEET paper)
Solution — Step by Step
The lac operon has three structural genes — lacZ (β-galactosidase), lacY (permease), and lacA (transacetylase) — arranged in sequence on the bacterial chromosome. Before these genes, there’s a promoter (where RNA polymerase binds) and an operator (the on/off switch). Separately, upstream, sits the regulator gene (i-gene) which codes for the repressor protein.
When lactose is absent, the i-gene is continuously transcribed into an active repressor protein. This repressor has high affinity for the operator region and binds to it, physically blocking RNA polymerase from moving forward. Result: no transcription of lacZ, lacY, lacA. The cell doesn’t waste energy making enzymes it doesn’t need.
Lactose enters the cell (via already present basal permease) and gets converted to allolactose — the actual inducer. Allolactose binds to the repressor protein, changing its shape (allosteric change). The repressor can no longer bind the operator and falls off.
With the operator free, RNA polymerase moves past it and transcribes all three structural genes as a single polycistronic mRNA. This mRNA is then translated to produce β-galactosidase (breaks lactose into glucose + galactose), permease (pumps more lactose into the cell), and transacetylase (whose exact role is still debated, but it’s part of NEET MCQs).
Once lactose is fully consumed, allolactose levels drop. The repressor reassembles into its active form, binds the operator again, and transcription stops. This is a classic negative feedback loop — the product (glucose from lactose breakdown) indirectly removes the inducer, switching the system off.
Why This Works
The lac operon is the textbook example of gene regulation in prokaryotes — specifically, an inducible system (genes are OFF by default, switched ON by the inducer). This is the opposite of a repressible system (like trp operon, which is ON by default and switched OFF by the co-repressor tryptophan). NEET frequently asks you to distinguish between these two.
The beauty of this system is economy. E. coli lives in environments where glucose is the preferred carbon source. Making lactose-digesting enzymes when there’s no lactose around would be wasteful. The operon model ensures genes are expressed only when the substrate is present.
Jacob and Monod proposed this model in 1961 for work on E. coli — they won the Nobel Prize for it. NEET has asked the full names, the organism, and the year, so know all three.
Alternative Method (Remembering the Components)
For MCQs, the fastest approach is to memorise the operon map as a left-to-right sequence:
i-gene — Promoter — Operator — lacZ — lacY — lacAUse the mnemonic: “I Promise Our Zoo Yields Animals” (i, P, O, Z, Y, A). Every component in order, nothing skipped.
NEET often gives a diagram with a component labelled and asks you to identify it. If you have this sequence memorised, you can answer such questions in under 10 seconds.
When lactose is absent: repressor (active) → binds operator → genes OFF. When lactose is present: allolactose → inactivates repressor → genes ON.
Two sentences. That’s the whole system for MCQ purposes.
Common Mistake
Students write “lactose binds to the repressor” — this is wrong. Lactose itself is not the inducer. It first gets converted to allolactose by the small amount of β-galactosidase always present in the cell, and allolactose is what binds the repressor. NEET 2022 had a direct MCQ on this distinction. If you write lactose instead of allolactose, you lose the mark.
A second slip: confusing the i-gene with part of the operon. The i-gene is not part of the lac operon — it’s a separate regulatory gene. The operon consists only of the promoter, operator, and structural genes (Z, Y, A). The i-gene just happens to be located nearby on the chromosome.