16.3 Gene Control
- Syllabus
- 9700–2028–2029
- Topic
- 16.3
- Level
- A2
Learning objectives
16.3.1Structural genes vs regulatory genes• Structural genes vs regulatory genes16.3.2Repressible enzymes vs inducible enzymes• Repressible enzymes vs inducible enzymes16.3.3Genetic control in prokaryotes• Genetic control in prokaryotes: lac operon16.3.4Transcription factors in eukaryotes• Transcription factors in eukaryotes: bind to DNA, control gene expression by affecting transcription rate16.3.5Gibberellin and gene activation• Gibberellin activates genes by causing DELLA protein repressor breakdown
Structural genes make pathway products; regulatory genes make controllers
| Feature | Structural gene | Regulatory gene |
|---|---|---|
| Encoded product | polypeptide or functional RNA that performs a cellular role | RNA or regulatory protein that controls expression of other gene(s) |
| Main effect | directly supplies an enzyme, transporter or structural product | changes whether, when or how strongly target genes are expressed |
| lac example | lacZ and lacY encode proteins used in lactose uptake/metabolism | lacI encodes the repressor that controls the structural genes |
A promoter or operator is a regulatory DNA sequence, not a regulatory gene, because it does not encode the repressor. Both structural and regulatory genes are transcribed; their products differ in function.
Inducible and repressible enzymes have opposite default controls
| Feature | Inducible enzyme | Repressible enzyme |
|---|---|---|
| Default production | low/off when substrate is absent | on while an end product is scarce |
| Signal | substrate/inducer removes or prevents repression | accumulated end product/corepressor enables repression |
| Typical pathway logic | catabolic: make enzymes when the substrate is available | anabolic: stop making enzymes when enough product exists |
| Resource benefit | avoids producing substrate-use enzymes unnecessarily | avoids overproducing an end product |
Inducible does not mean permanently active, and repressible does not mean permanently absent. State the default expression and the metabolite that switches production.
Lactose removes lac repressor control so uptake and digestion genes are transcribed
The lac operon contains a promoter where RNA polymerase binds, an operator control site and structural genes including lacZ (β-galactosidase) and lacY (lactose permease). A separate regulatory gene, lacI, produces the repressor protein.
| Lactose absent | Lactose present |
|---|---|
| repressor has a shape complementary to the operator and binds it | lactose (inducer) binds the repressor and changes its shape |
| bound repressor blocks RNA polymerase from transcribing structural genes | repressor cannot bind the operator, so RNA polymerase transcribes lacZ and lacY |
| little/no permease or β-galactosidase is produced | permease increases lactose entry and β-galactosidase hydrolyses lactose |
Lactose does not bind the operator or directly switch on RNA polymerase. It binds the repressor. cAMP control is outside this syllabus objective.
Transcription factors bind DNA and change transcription rate
A transcription factor is a protein that binds to a specific DNA sequence and controls gene expression in eukaryotic cells by changing the rate of transcription.
- An activating transcription factor increases transcription, for example by helping RNA polymerase begin transcription.
- A repressing transcription factor decreases transcription, for example by preventing effective transcription initiation.
- Different cells can contain or activate different transcription factors, so the same genome can produce different patterns of gene expression.
A transcription factor is a DNA-binding protein, not RNA polymerase and not a DNA sequence. It can increase or decrease transcription; the name does not imply activation only.
Gibberellin activates genes by removing DELLA repression
- Gibberellin binds to its receptor in a target cell.
- The hormone–receptor interaction leads to breakdown of DELLA protein repressors.
- DELLA proteins normally inhibit factors that promote transcription; their removal releases these transcription-promoting factors.
- Transcription of target genes increases, so the cell produces proteins required for responses such as amylase production during germination or stem elongation.
This is activation by removing an inhibitor: gibberellin does not have to bind DNA itself. The amount of target mRNA and then protein rises because a repressive protein has been degraded.
Gibberellin is the signal, DELLA is the protein repressor, and transcription-promoting factors control gene expression. Gibberellin does not directly transcribe genes or act as amylase.