Lac Operon vs Trp Operon: AP Biology Guide

• Lac operon is inducible.
• Trp operon is repressible.
• Lac operon turns on when lactose is present.
• Trp operon turns off when tryptophan is abundant.
• AP questions usually test repressor, operator, transcription, and mRNA level logic.

An operon is a prokaryotic gene regulation system that controls related genes together. The promoter is where RNA polymerase binds. The operator is where a repressor can bind. If RNA polymerase can move through the structural genes, transcription increases.

For the full promoter–operator–repressor system, see the operons guide—this page focuses on lac vs trp comparison and AP exam reasoning.

The lac operon helps bacteria use lactose as an energy source when it is available in the environment.

• When lactose is absent, the repressor binds the operator
• RNA polymerase is blocked at the operator region
• Transcription of lactose-metabolism genes is low
• When lactose is present, allolactose inactivates the repressor
• The operator is open and RNA polymerase can transcribe
• mRNA and lactose-digestion enzyme levels increase

The trp operon helps bacteria synthesize tryptophan when the amino acid is scarce.

• When tryptophan is low, the repressor is inactive
• The operator is open and transcription occurs
• Structural genes for tryptophan synthesis are transcribed
• When tryptophan is high, tryptophan acts as a corepressor
• The active repressor binds the operator
• Transcription and mRNA for tryptophan-making enzymes decrease

Condition A: Lactose is present.
Condition B: Tryptophan is abundant.

• Lactose present inactivates the lac repressor; the operator opens; RNA polymerase transcribes lac genes; lac mRNA increases.
• Tryptophan abundant activates the trp repressor; the operator is blocked; RNA polymerase access drops; trp mRNA decreases.
• The two operons respond oppositely because lac responds to a substrate and trp responds to a final product.

Both operons show gene regulation because the DNA sequence usually does not change. Instead, the cell changes whether genes are transcribed. This changes mRNA levels and can change enzyme production without altering the genetic code.

See the broader gene regulation guide for how prokaryotic and eukaryotic cells control expression beyond lac and trp.

The central dogma shows DNA → RNA → protein. Lac and trp operons regulate the DNA-to-RNA step by changing whether RNA polymerase can transcribe structural genes.

Review the full central dogma flow, then connect how less transcription means less mRNA and often less protein at ribosomes during translation.

AP Biology questions may ask you to identify whether an operon is inducible or repressible, predict whether transcription increases or decreases, interpret lactose or tryptophan conditions, identify repressor activity, predict mRNA or enzyme levels, explain a mutation in the operator or repressor, and compare lac and trp logic in a table.

When a prompt asks about RNA synthesis mechanics, review transcription and RNA processing. For a side-by-side of transcription vs translation, see the transcription vs translation comparison.

Flip all 20 cards until you can trace signal → repressor → operator → transcription without hesitating.

Answer all 12 questions. Choices shuffle on reload—focus on repressor and operator logic, not letter memorization.

Open each card, draft your response, then reveal the rubric and sample answer.

Next, review operons for the full system, then use gene regulation and central dogma to trace how transcription changes mRNA and protein levels.