Operon concept — structural, regulatory, and operator genes explained

Question

Describe the lac operon model in E. coli. Explain the roles of the structural genes, operator gene, promoter, and regulatory gene (i gene). How does lactose act as an inducer to switch on the operon?

(NCERT Class 12, one of the most repeated NEET questions — appeared in 2019, 2021, 2023)

Solution — Step by Step

The lac operon in E. coli consists of:

Structural genes — $z$ (codes for $\beta$ -galactosidase), $y$ (codes for permease), $a$ (codes for transacetylase). These three genes are transcribed as a single polycistronic mRNA.
Promoter (p) — the site where RNA polymerase binds to start transcription.
Operator (o) — a short DNA segment between the promoter and structural genes. Acts as the ON/OFF switch.
Regulatory gene (i gene) — located upstream, codes for the repressor protein. This gene has its own promoter and is expressed constitutively (always on).

The i gene produces the repressor protein continuously. This repressor binds to the operator region, physically blocking RNA polymerase from moving past the promoter to transcribe the structural genes.

Result: genes $z$ , $y$ , and $a$ are NOT transcribed. No $\beta$ -galactosidase, no permease, no transacetylase is made (except in very small basal amounts).

The repressor acts like a gate — when it sits on the operator, the road is blocked.

Lactose enters the cell (small amounts get in even without permease). Inside the cell, a small amount of $\beta$ -galactosidase converts lactose into allolactose — the actual inducer molecule.

Allolactose binds to the repressor protein and changes its shape (allosteric change). The altered repressor can no longer bind to the operator.

With the operator free, RNA polymerase slides through and transcribes all three structural genes into a single polycistronic mRNA. The enzymes are produced, lactose is metabolised, and the cell gets energy.

Once all lactose is consumed, no more allolactose is formed. The repressor protein returns to its original shape, binds back to the operator, and blocks transcription again.

This is a beautiful example of negative regulation — the default state is OFF, and the inducer removes the block. The system is self-regulating: it only makes the enzymes when the substrate (lactose) is actually present.

Why This Works

The lac operon is a model of gene regulation in prokaryotes, proposed by Jacob and Monod in 1961 (they won the Nobel Prize for this in 1965).

The logic is energy-efficient. Why would a bacterium waste resources making lactose-digesting enzymes when there’s no lactose around? The operon system ensures enzymes are made only on demand. This is called inducible expression — the genes are normally silent and are induced only when the substrate appears.

The polycistronic mRNA is a prokaryotic feature — all three enzymes needed for lactose metabolism are produced from a single mRNA in coordinated amounts. Eukaryotes don’t have operons; they regulate genes individually (monocistronic mRNA).

NEET loves asking: “Lac operon is an example of which type of gene regulation?” Answer: negative inducible. Negative because the regulator (repressor) inhibits transcription. Inducible because the operon is normally OFF and is turned ON by the inducer.

Compare with the trp operon — that’s negative repressible (normally ON, turned OFF when tryptophan accumulates and acts as co-repressor).

Alternative Method — Flow Diagram Approach

For quick revision and CBSE diagrams, think of it as a flow:

Lactose absent: i gene $\rightarrow$ repressor $\rightarrow$ binds operator $\rightarrow$ RNA polymerase blocked $\rightarrow$ NO transcription

Lactose present: Lactose $\rightarrow$ allolactose $\rightarrow$ binds repressor $\rightarrow$ repressor falls off operator $\rightarrow$ RNA polymerase transcribes $z$ , $y$ , $a$ $\rightarrow$ enzymes made $\rightarrow$ lactose metabolised

For CBSE boards, always draw the labelled diagram showing all components (i gene, promoter, operator, z, y, a genes, repressor, RNA polymerase, mRNA). This diagram alone can fetch you 2-3 marks. Label the inducer (allolactose) clearly — many students write “lactose” instead of “allolactose” as the inducer, which costs marks.

Common Mistake

The single biggest error: writing that lactose is the inducer. It’s not — allolactose (an isomer of lactose) is the actual inducer. Lactose is the substrate; allolactose is what binds to the repressor. NEET has specifically tested this distinction.

Another frequent mistake: confusing the operator with the promoter. The promoter is where RNA polymerase binds. The operator is where the repressor binds. The repressor does NOT bind to the promoter — it binds to the operator, which lies between the promoter and the structural genes. Getting this spatial arrangement wrong leads to incorrect diagrams and lost marks.

Also remember: the i gene is NOT part of the operon. It has its own separate promoter and is transcribed independently, constitutively producing the repressor protein.

Question

Solution — Step by Step

Why This Works

Alternative Method — Flow Diagram Approach

Common Mistake

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