AP Biology · Unit 6 · Topic 6.5
Two operons with opposite logic. Toggle the sugars and read the whole truth table, rather than memorising which one is which.
Learning objective 6.5.A Describe the types of interactions that regulate gene expression, and how the location of regulatory sequences relates to their effect.
The lac operon is usually summarized as “lactose turns it on”. Set both sugars independently and watch what actually happens — there are four environments, and they do not give two answers.
| Lactose | Glucose | Repressor | CAP | Transcription |
|---|
Look down the table. Knowing only the lactose gives you two different answers, and knowing only the glucose gives you two different answers. Neither input on its own determines the state, which is the definition of two independent controls acting on the same promoter.
Glucose is metabolised more directly than lactose. A cell with both sugars available gains almost nothing by making the lactose enzymes at full rate, so the second control is not a redundancy — it is the cell choosing between two usable foods rather than deciding whether it has food at all.
Three genes, one promoter, one switch. That is the arrangement that makes it possible to control three enzymes of the same pathway with a single decision — and it is why prokaryotic regulation can be so fast.
The trp operon has a repressor too. The difference is that it is made in a form that does not work until tryptophan binds it — which reverses everything.
Reduced to one input each — is the small molecule present? — the two operons answer every question in exactly opposite ways.
| Small molecule present? | lac operon | trp operon |
|---|
These are not two mechanisms to memorise separately. They are the two possible shapes a one-input truth table can have: off unless something starts it, or on unless something stops it.
The defaults follow from what the genes are for, so you can work out which kind an unfamiliar operon is without being told: ask whether its enzymes break something down or build something up.
A eukaryotic gene is switched on by proteins bound at sequences that may be thousands of bases away. The DNA loops, and the distance stops mattering.
| Gene | Needs | Expressed now? | States it is on in |
|---|
| Factors present | Genes expressed |
|---|
Independent assortment in Topic 5.2, alternative splicing in Topic 6.3, and combinatorial control here are all the same piece of arithmetic: n independent two-way choices give 2ⁿ outcomes. It is worth recognizing the shape, because it is how biology gets large numbers out of small ones.
Answer these in your own words before you open the answers.
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