Gene regulation in the operon
About this app
A bacterial cell in 3D with the lac and trp operons: drag lactose, glucose and tryptophan into the cell, repressor and CAP dock, the polymerase runs or stops and the enzyme level is plotted over time. Truth table, logic circuit and mutants as prediction tasks.
With explain mode: an AI tutor walks you through the app, points out what matters in the scene and answers your questions.
What an operon controls
A cell does not read its genes all the time. In bacteria, genes for one metabolic pathway often lie together as an operon: promoter, operator and several structural genes that are transcribed into one mRNA. A regulatory protein decides whether RNA polymerase gets through. In the lac operon the repressor sits on the operator by default. Lactose gives rise to allolactose, which acts as the inducer and removes it. This is substrate induction. The trp operon works the other way round: the aporepressor only becomes active with tryptophan. This is end-product repression.
What the app shows
You look into a bacterial cell with two DNA sections. The repressor is a clamp, the polymerase a ring around the DNA, and the mRNA grows across the strand. The truth table shows the four situations of lactose and glucose, and the logic circuit next to it shows that the operon only runs strongly with lactose AND NOT glucose. The enzyme level is plotted over time and follows the mRNA with a delay. The comparison view puts both operons side by side.
What you can try
Drag a substance from the top into the cell and watch who docks. Tap a row of the table to set up a situation. Insert mutants: lacI⁻ without a repressor, lacOᶜ with a defective operator, lacIˢ with a repressor that binds no inducer. In seven prediction tasks you first decide how much enzyme is made. Then the cell runs on and the app explains the result.
Frequently asked questions
- How does the lac operon work?
- Without lactose the repressor sits on the operator and blocks RNA polymerase. Lactose gives rise to allolactose, which binds to the repressor and removes it from the DNA. The genes are only read strongly if glucose is also missing, because then CAP-cAMP promotes the start of the polymerase.
- What is the difference between substrate induction and end-product repression?
- In substrate induction the substance to be broken down switches the genes on by inactivating an active repressor (lac operon). In end-product repression the product switches the genes off by activating an inactive aporepressor (trp operon).
- What happens in a lacI mutant?
- Without a working repressor the operator stays free. The operon is then read even without lactose, it is constitutive. If glucose is missing it runs strongly, with glucose only weakly, because CAP-cAMP is missing.
- Does the app show real measurements?
- No. The enzyme curve is a model with relative values and model time. The app shows the switching logic and the delayed rise; attenuation in the trp operon and the additional operators of the lac operon are left out.
Subject: Biology (Gymnasium)
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