What the exam asks
- Multiple Choice (Section I): Stimulus sets often present an unfamiliar prokaryotic operon or a bar graph of mRNA levels under different chemical treatments, asking you to predict the effect of a mutated operator, promoter, or regulatory gene.
- Free Response (Section II): Expect this topic in Question 4 (Conceptual Analysis) or Question 5 (Analyze Model/Visual Representation). You may be asked to explain how two cells with identical DNA produce different proteins, or predict and justify the phenotype of a bacterium when a specific metabolite is added or removed.
Core ideas
Regulatory sequences and expression patterns
Gene expression is primarily regulated at transcription through interactions between regulatory DNA sequences and regulatory proteins:
- Promoter: DNA sequence where RNA polymerase binds to initiate transcription.
- Operators / Enhancers / Silencers: Non-coding DNA sequences where regulatory proteins (repressors or activators) bind to inhibit or promote RNA polymerase attachment.
- Constitutive genes: Always transcribed at a relatively constant rate (e.g., genes for ribosomal RNA, essential metabolic enzymes, or regulatory repressor proteins).
- Inducible/regulated genes: Transcribed only when needed in response to internal or external signals.
Prokaryotic operons
Prokaryotes often group functionally related genes into an operon—a single promoter, an operator (DNA binding site for a repressor), and a cluster of structural genes transcribed together into one polycistronic mRNA. A separate regulatory gene (with its own promoter) encodes the repressor protein.
| Feature | Inducible Operon (e.g., lac operon) | Repressible Operon (e.g., trp operon) |
|---|---|---|
| Metabolic role | Catabolic (breaks down a nutrient when present) | Anabolic (synthesizes an essential molecule when absent) |
| Default state | OFF | ON |
| Repressor synthesized as | Active (binds operator alone, blocking RNA polymerase) | Inactive (cannot bind operator alone) |
| Small effector molecule | Inducer (e.g., allolactose): binds repressor conformational change inactivates repressor | Corepressor (e.g., tryptophan): binds repressor conformational change activates repressor |
| Effect of adding effector | Repressor detaches from operator; transcription turns ON | Repressor binds operator; transcription turns OFF |