CIE AS/A Level Biology -16.3 Gene control- Study Notes- New Syllabus
CIE AS/A Level Biology -16.3 Gene control- Study Notes- New Syllabus
Ace A level Biology Exam with CIE AS/A Level Biology -16.3 Gene control- Study Notes- New Syllabus
Key Concepts:
- describe the differences between structural genes and regulatory genes and the differences between repressible enzymes and inducible enzymes
- explain genetic control of protein production in a prokaryote using the lac operon (knowledge of the role of cAMP is not expected)
- state that transcription factors are proteins that bind to DNA and are involved in the control of gene expression in eukaryotes by decreasing or increasing the rate of transcription
- explain how gibberellin activates genes by causing the breakdown of DELLA protein repressors, which normally inhibit factors that promote transcription
Structural vs Regulatory Genes & Enzyme Control
Structural Genes
- Code for proteins/enzymes directly involved in cell structure or metabolism.
- Examples:
- Enzymes in respiration (e.g., hexokinase).
- Structural proteins like actin, tubulin.
- Enzymes in respiration (e.g., hexokinase).
- Always needed for cell survival and function.
Regulatory Genes
- Code for proteins (e.g., repressors or activators) that control the expression of structural genes.
- Do not code for enzymes used in metabolism directly.
- Example: The lacI gene in E. coli codes for a repressor protein that regulates the lac operon.
🧪 Repressible vs Inducible Enzymes
| Feature | Repressible Enzymes | Inducible Enzymes |
|---|---|---|
| Definition | Enzymes normally made by the cell but can be switched off (repressed) when not needed. | Enzymes not normally made, but can be switched on (induced) when substrate is present. |
| Default State | ON (active, being produced). | OFF (not produced). |
| Control Mechanism | Repressor protein is inactive → becomes active when product is abundant → stops transcription. | Repressor protein is active → substrate (inducer) inactivates repressor → transcription starts. |
| Example | trp operon in E. coli (for tryptophan synthesis). | lac operon in E. coli (for lactose metabolism). |
📌 Key Takeaway
- Structural genes → make proteins/enzymes used in metabolism or structure.
- Regulatory genes → control whether structural genes are expressed.
- Repressible enzymes → default ON, switched OFF by end product (feedback inhibition).
- Inducible enzymes → default OFF, switched ON by presence of substrate.
Genetic Control of Protein Production in Prokaryotes – The Lac Operon
🌱 What is the lac operon?
- Found in E. coli (a bacterium).
- Controls the production of enzymes needed to digest lactose.
- Example of an inducible system (normally OFF, switched ON in presence of lactose).
Components of the lac operon
- Structural genes:
- lacZ → codes for β-galactosidase (breaks lactose → glucose + galactose).
- lacY → codes for permease (transports lactose into the cell).
- lacA → codes for transacetylase (function less important for exams).
- Regulatory gene (lacI): Produces a repressor protein.
- Operator region (O): DNA sequence where the repressor binds to block transcription.
- Promoter region (P): Site where RNA polymerase binds to start transcription.
How it works
When lactose is absent
- The repressor (from lacI gene) binds to the operator.
- This blocks RNA polymerase from binding to the promoter.
- Result: No transcription of lacZ, lacY, lacA → enzymes not made.
- Saves energy because lactose isn’t available.
When lactose is present
- Lactose (actually its isomer allolactose) acts as an inducer.
- It binds to the repressor protein → changes its shape.
- Repressor can’t bind to the operator anymore.
- RNA polymerase binds to the promoter and transcribes lacZ, lacY, lacA.
- Enzymes are produced → lactose can be broken down.
📌 Key Features to Remember
- Inducible system → default OFF, switched ON by lactose.
- Controlled by a repressor protein (negative control).
- Efficient: enzymes only made when needed.
Transcription Factors in Eukaryotes
🌱 What are transcription factors?
- Proteins that bind to specific DNA sequences near genes.
- They control gene expression (whether a gene is turned ON or OFF).
🔹 Role in gene expression
- Increase transcription rate → called activators.
- Decrease transcription rate → called repressors.
🔹 How they work
- Bind to the promoter region or other regulatory DNA sequences.
- Help or block RNA polymerase from attaching to DNA.
- Control which proteins are made in a cell → determines the cell’s structure and function.
📌 Key points to remember
- Found in eukaryotes.
- Essential for cell differentiation and response to signals (e.g., hormones can activate certain transcription factors).
Can switch genes ON or OFF depending on conditions.
✅ In short: Transcription factors = proteins that regulate transcription in eukaryotes by increasing or decreasing the rate of transcription.
Gibberellin and Gene Activation
Normal role of DELLA proteins
- DELLA proteins = repressors.
- They block transcription factors that promote growth-related gene expression.
- Result → no transcription of genes needed for stem elongation.
Role of gibberellin
- Gibberellin binds to a receptor in the plant cell.
- This activates a pathway that leads to the breakdown of DELLA proteins.
- With DELLA removed, the transcription factors are free to bind DNA and activate target genes.
Outcome
- Genes for enzymes needed in stem elongation are switched ON.
- Leads to:
- Cell wall loosening → cells can expand.
- Cell division and elongation → stems grow taller.
✅ Summary:
Gibberellin promotes stem growth by causing the breakdown of DELLA repressors, which normally inhibit transcription factors. Once DELLA is removed, transcription factors activate growth genes.