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OperonBiology
AP BiologyOlympiad

Lac Operon

Repressor, operator and CAP–cAMP logic, including merodiploids that separate cis-acting from trans-acting mutations.

Operons: the lac operonMutationsTranscription, RNA processing & translation

1. Glucose sensing: cAMP–CAP

Cytoplasm

The operon asks two questions. The first is "Is glucose scarce?" Low glucose raises cAMP, and CAP–cAMP then activates the weak lac promoter.

  • Glucose transport keeps adenylate cyclase inactive (low cAMP)
  • CAP binds cAMP, then the CAP site upstream of lacP
  • CAP–cAMP recruits RNA polymerase: positive control
  • Without CAP, expression stays low even when induced

lac-operon · stage 1/5

CytoplasmChromosomeF′ plasmidGlucosecAMPCAP–cAMPLactoseRepressorInducerallolactoseβ-galactosidaseLac permeaselacII⁺lacP · lacOO⁺lacZZ⁺lacYY⁺lacIlacP · lacOlacZlacY
  • Carbon / material flow
  • Electron transfer
  • Activation
  • Inhibition
  • Feedback
  • ▲ 12 change vs. baseline (percentage points)
β-galactosidase
100% WT
baseline 100% WT
Lac permease
100% WT
baseline 100% WT
Chromosomal lacZYA
100%
baseline 100%
F′ lacZYA
0%
baseline 0%
CAP–cAMP
90%
baseline 90%

Impact

No changes from baseline.

Change a variable or apply a scenario. This panel then shows what moved, which direction the effect travelled, and why.

Inspect a step

Select any node on the diagram to see what it does, its inputs and outputs, and how it compares with baseline.