Use the Lactose and Glucose switches to set the cell’s conditions and watch the operon respond. Drag to rotate and click any structure — promoter, operator, repressor, RNA polymerase, CAP, or a gene — to learn what it does.
A bacterium like E. coli can eat the sugar lactose, but building the enzymes to digest it costs energy. So it only makes those enzymes when lactose is actually around. The genes for the enzymes sit together on the DNA as the lac operon — three genes ($lacZ$, $lacY$, $lacA$) plus their control switches — all transcribed together. The big question the operon answers is simply: "should I switch these genes on right now?"
Two proteins decide. The lac repressor is a brake: by default it sits on the operator (a DNA sequence right after the promoter) and physically blocks RNA polymerase. So with no lactose, the operon is off. When lactose appears, a form of it (allolactose) binds the repressor, changes its shape, and knocks it off the operator — the brake releases. This is negative control: removing a block.
Releasing the brake is not the whole story. Bacteria prefer glucose, so a second protein, CAP, acts as an accelerator for positive control. CAP only works when bound to cAMP, and cAMP is high only when glucose is low. So CAP–cAMP binds near the promoter and helps RNA polymerase latch on strongly — but only when glucose is scarce. Put the logic together: the operon runs at full speed only when lactose is present AND glucose is absent. It is a biological AND gate — remove the block and add the boost.
Start at the default (no lactose, glucose present): the repressor sits on the operator and nothing is made. Flip Lactose on — the repressor falls off, but with glucose still present transcription is only weak. Now flip Glucose off — CAP–cAMP binds and RNA polymerase fires at full speed. Try all four switch combinations and watch the Operon status; only one gives full ON.
| Lactose | Glucose | Repressor | CAP–cAMP | Transcription |
|---|---|---|---|---|
| Absent | Present | On operator (blocks) | Inactive (low cAMP) | OFF |
| Absent | Absent | On operator (blocks) | Active (high cAMP) | OFF (still blocked) |
| Present | Present | Off operator | Inactive (low cAMP) | Weak (basal) ON |
| Present | Absent | Off operator | Active (high cAMP) | FULL ON |
When the operon is transcribed, one mRNA carries all three genes. lacZ codes for β-galactosidase, the enzyme that splits lactose into glucose and galactose (and also makes a little allolactose, the inducer). lacY codes for lactose permease, a membrane pump that brings lactose into the cell. lacA codes for a transacetylase whose role is minor. Because all three are on one mRNA, the cell makes the transporter and the digesting enzyme together, exactly when it needs them.
The lac operon is famous because it combines both logics. Negative control: the repressor must be removed (by lactose) for any transcription at all. Positive control: CAP–cAMP must be present (when glucose is low) for strong transcription. This double control means the cell only commits fully to lactose metabolism when lactose is available and its preferred fuel, glucose, has run out — an efficient, logical response to the environment. The system was deciphered by François Jacob and Jacques Monod, who shared the 1965 Nobel Prize for it.
An operon is a cluster of genes in bacteria that are controlled together as a single unit, transcribed into one messenger RNA from one promoter. The lac operon contains three genes ($lacZ$, $lacY$, $lacA$) for digesting lactose, plus the control sequences — the promoter and operator — that switch them on and off.
Key takeaway: an operon is a set of related genes controlled together from a single promoter.The lac repressor is a protein that binds to the operator, a DNA sequence just after the promoter. When it sits on the operator it physically blocks RNA polymerase from moving forward, so the genes are not transcribed. The repressor is the default "off switch": with no lactose around, it keeps the operon silent so the cell does not waste energy.
Key takeaway: the repressor binds the operator and blocks transcription, keeping the operon off by default.When lactose is present, a little of it is converted to allolactose, which acts as an inducer. Allolactose binds the repressor and changes its shape so it can no longer hold onto the operator. The repressor falls off, the operator is clear, and RNA polymerase is free to transcribe the genes. This is why the lac operon is called an inducible operon.
Key takeaway: lactose (as allolactose) binds and releases the repressor, removing the block so transcription can begin.Bacteria prefer glucose, so even when lactose is available they make lac proteins slowly if glucose is also present — this is catabolite repression. When glucose is low, the molecule cAMP rises and binds an activator protein called CAP. CAP–cAMP then binds near the promoter and helps RNA polymerase attach strongly. High glucose keeps cAMP and CAP activity low, so even with the repressor off, transcription is weak.
Key takeaway: glucose lowers cAMP and CAP activity, so the operon runs at full speed only when lactose is present and glucose is absent.The operon reaches its highest transcription only when two conditions are met at once: lactose is present (so the repressor is off the operator) and glucose is absent (so cAMP is high and CAP is helping RNA polymerase bind). With lactose but also glucose, you get only weak transcription; with no lactose, the repressor keeps it off no matter what.
Key takeaway: full ON requires lactose present and glucose absent — a logical AND.They are opposite kinds of control. The repressor is a negative regulator: when bound to the operator it turns transcription off. CAP is a positive regulator: when bound (with cAMP) near the promoter it turns transcription up. The repressor responds to lactose, CAP responds to glucose levels through cAMP. Together they let the cell sense both fuels.
Key takeaway: the repressor is a brake (negative control); CAP is an accelerator (positive control).The lac operon, worked out by Jacob and Monod in the early 1960s, was the first gene-regulation system ever understood in molecular detail, and it earned a Nobel Prize. It cleanly demonstrates the core ideas of gene regulation: genes are switched on and off, control can be negative (repressor) or positive (CAP), and small molecules act as signals. Almost every later discovery in gene regulation is compared back to it.
Key takeaway: it is the foundational, textbook model of how cells control which genes are expressed and when.