Operon Logic: Induction, Repression and Feedback on Transcription
Bacteria group related genes under one promoter and switch the whole set on or off with a repressor or activator that senses a small molecule. Whether the default is on or off depends on whether the pathway breaks something down or builds it.
What a learner can do afterwards
- Predicts expression from the presence of a substrate or a product for a stated operon.
- Explains why a catabolic operon is normally off and an anabolic one normally on.
- Works out the phenotype of a mutation in the repressor or in its binding site.
1 · Read
Picture bacteria keeping related genes on one switch to save energy. The enzymes of a pathway share a single starter with a nearby grip site, and a blocker protein decides whether the copying machine may pass. The blocker listens to a small molecule from the surroundings. No message is made until the cell truly needs the enzymes.
The two famous switches are mirror images, and the logic is pure thrift. Milk sugar digesting enzymes pay only when the sugar is present, so the blocker sits on by default and the sugar itself pries it off. Building blocks are always needed, so that switch runs by default and only shuts when the product piles up. Breakdown rests off, building rests on.
Now break the wiring and predict the outcome yourself. Snap the blocker so it cannot grip and the genes run nonstop. Snap the grip site so nothing can hold it and you get the same constant running. Every switch needs both a working protein and a working site; losing either locks it on.
Keep this contrast handy. Switching a resting off group on when its target appears is called induction. Switching a resting on group off when its product builds up is called repression. Name the default first, then the molecule, and the prediction writes itself.
You check the small molecule to predict whether the grouped genes run.
More to read
11.7 Gene Regulation: Operon Theory
Bacteria keep related genes on one switch. An operon groups the enzymes of a pathway under a single promoter with an operator site beside it, and a repressor protein decides whether RNA polymerase may pass. The repressor itself listens to a small molecule from the environment. This is regulation at the cheapest possible point: no mRNA is made until the cell actually needs the enzymes, so no energy is wasted.
16.2 Prokaryotic Gene Regulation
The lac and trp operons are mirror images, and the logic follows from economics. Lactose digesting enzymes pay off only when lactose is present, so the lac repressor sits on the operator by default and lactose itself pries it off: induction. Tryptophan is needed all the time, so the trp operon runs by default and only shuts down when tryptophan piles up and activates the repressor: repression. Catabolic pathways default off, anabolic pathways default on.
16.1 Regulation of Gene Expression
Mutations in this circuitry give clean, predictable phenotypes, which is why operons taught genetics how regulation works. Break the repressor so it cannot grip the operator and the genes run constantly. Break the operator so nothing can grip it and the result is the same: constitutive expression. The symmetry is the lesson. Every regulator needs both a working protein and a working binding site, and losing either one locks the switch.
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Where it sits
Learn first
Nothing; this is a starting point.
Optional connections
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.