The shape of the lesson
By the end of the lesson, learners can
- The lac operon: an inducible system: Start with no lactose and read the REPRESSOR and β-GALACTOSIDASE readouts.
- Inducible and repressible enzymes: β-galactosidase is switched on by its substrate.
- Transcription factors and DELLA proteins: Choose lele and LeLe, and tick Spray with gibberellin on the dwarf plant.
- Repressor binds the operator; RNA polymerase binds the promoter.
- Lactose binds the repressor: an inducible system.
- Transcription factors are proteins that raise or lower transcription rate.
- Gibberellin causes DELLA breakdown, freeing transcription factors.
One picture every idea returns to
Structural genes code for proteins with a function in the cell, such as enzymes; regulatory genes code for proteins, such as repressors and transcription factors, that control the expression of other genes. An inducible enzyme is made only when its substrate is present (β-galactosidase in E. coli); a repressible enzyme is made until its product builds up and switches its synthesis off (the enzymes that make tryptophan). In the lac operon, the regulatory gene lacI codes for a repressor that binds the operator, so RNA polymerase cannot bind the promoter and the structural genes lacZ, lacY and lacA are not transcribed. Lactose binds the repressor, changing its shape so it leaves the operator, and the genes are transcribed. In eukaryotes, transcription factors are proteins that bind to DNA and increase or decrease the rate of transcription. Gibberellin binds a receptor and causes DELLA proteins to be broken down; DELLA proteins normally inhibit transcription factors, so their removal lets transcription of growth genes begin.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Switching genes on and off.. A bacterium does not waste energy making enzymes for a sugar that is not there, and a pea seedling does not grow until gibberellin tells it to. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: The lac operon: an inducible system | Start with no lactose and read the REPRESSOR and β-GALACTOSIDASE readouts. Project tab 6 of the lab (Inheritance lab, the lac operon) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Inducible and repressible enzymes | β-galactosidase is switched on by its substrate. | Pairs |
| 21 to 30 min | Teach and check: Transcription factors and DELLA proteins | Choose lele and LeLe, and tick Spray with gibberellin on the dwarf plant. Project tab 7 of the lab (Inheritance lab, gibberellin and DELLA proteins) and let a student drive it while the class predicts. | Frayer Model |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 19 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Transcription factors and DELLA proteins, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Frayer Model
Pairs fill a four-box grid for the key term: definition, characteristics, examples and non-examples. The non-examples box is where the misconceptions surface, so give it the most time.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| The repressor binds the operator, not the promoter, and lactose binds the repressor, not the DNA. | The repressor is coded for by the regulatory gene lacI, which is not one of the structural genes. |
| Gibberellin does not bind to DNA and is not a transcription factor itself. | It acts indirectly: it causes the breakdown of DELLA, and that frees the transcription factors. |
Gene control: worksheet
6 questions, 19 marks. Written in the style of the papers; no past paper question is reproduced.
Distinguish between a structural gene and a regulatory gene, giving an example of each from the lac operon.
Explain how the lac operon allows E. coli to make β-galactosidase only when lactose is present.
Compare inducible and repressible enzymes.
Predict, with reasons, whether β-galactosidase is made in each case: (a) a mutant operator to which the repressor cannot bind, no lactose; (b) a mutant repressor that cannot bind lactose, lactose present; (c) a promoter mutation that prevents RNA polymerase binding, lactose present.
Explain how gibberellin causes stem elongation by activating genes.
State what is meant by a transcription factor and where transcription factors act.
Answer key and marking notes
- Structural gene: codes for a protein used in the cell's metabolism or structure, for example lacZ (β-galactosidase) or lacY (lactose permease).
- Regulatory gene: codes for a protein that controls the expression of other genes, for example lacI (the repressor).
- The regulatory gene lacI is transcribed and translated to make a repressor protein.
- Without lactose the repressor binds the operator, so RNA polymerase cannot bind the promoter / cannot transcribe the structural genes.
- When lactose is present it binds to the repressor, changing its shape so it cannot bind the operator.
- RNA polymerase binds the promoter and transcribes lacZ, lacY and lacA as one mRNA, which is translated into β-galactosidase, permease and transacetylase.
- When lactose is used up the repressor binds the operator again; this saves energy and materials.
- Inducible enzymes (e.g. β-galactosidase) are made only when their substrate is present; the substrate inactivates the repressor.
- Repressible enzymes (e.g. tryptophan synthesis enzymes) are normally made, and their synthesis stops when their product accumulates.
- In a repressible system the product binds and activates the repressor, which then binds the operator.
- (a) Made: the repressor cannot block the operator, so the genes are transcribed all the time.
- (b) Not made: lactose cannot change the repressor's shape, so it stays on the operator.
- (c) Not made: RNA polymerase cannot bind the promoter, so the structural genes are not transcribed even though the repressor has left.
- Gibberellin (GA1) binds to a receptor protein in the cell.
- This causes DELLA proteins to be broken down.
- DELLA proteins normally bind to and inhibit transcription factors needed to transcribe growth genes.
- Once DELLA is removed, the transcription factors bind to DNA (the promoter) and increase transcription; the proteins made cause cell elongation in the stem.
- A protein that binds to DNA (a specific sequence, often near the promoter) and controls gene expression.
- It increases or decreases the rate of transcription of a gene in eukaryotes (in the nucleus).
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
In the lac operon, where does the repressor protein bind when lactose is absent?
- Athe operator
- Bthe promoter
- Cthe lacI gene
- Dthe lacZ gene
Which gene in the lac operon is a regulatory gene?
- AlacI
- BlacZ
- ClacY
- DlacA
Why is β-galactosidase described as an inducible enzyme?
- Ait is made only when its substrate, lactose, is present
- Bit is made continuously and switched off by its product
- Cit is activated by binding to a coenzyme
- Dit is produced only when glucose is present
Which statement about eukaryotic transcription factors is correct?
- Athey are proteins that bind to DNA and can increase or decrease the rate of transcription
- Bthey are lengths of DNA where RNA polymerase attaches
- Cthey are mRNA molecules that leave the nucleus
- Dthey are enzymes that join amino acids during translation
How does gibberellin increase the transcription of genes for stem elongation?
- Ait causes DELLA proteins to be broken down, releasing transcription factors
- Bit binds directly to the promoters of the growth genes
- Cit activates DELLA proteins, which promote transcription
- Dit is translated into a transcription factor
E. coli with a mutant lac operator to which the repressor cannot bind is grown with no lactose. What is the result?
- Aβ-galactosidase and permease are made even though there is no lactose
- Bno β-galactosidase is made, even when lactose is added later
- Cthe repressor is not made because lacI is switched off
- Donly lacI is transcribed, and only when lactose is present
In the trp operon, the repressor is inactive until tryptophan binds to it. What happens when a cell has plenty of tryptophan?
- Athe repressor-tryptophan complex binds the operator and the tryptophan-synthesis genes are switched off
- Btryptophan removes the repressor from the operator and the genes are switched on
- Ctryptophan binds the promoter and speeds up transcription
- Dthe genes are switched on because the enzymes are inducible
A dwarf mutant plant makes normal amounts of GA1 but does not grow tall even when sprayed with extra gibberellin. Which mutation best explains this?
- Aits DELLA protein cannot be broken down in response to gibberellin
- Bit is homozygous for the le allele
- Cits DELLA protein is missing altogether
- Dits Le enzyme is more active than normal
Quiz key
The repressor binds the operator, preventing RNA polymerase from binding the promoter. The promoter is the RNA polymerase binding site. lacI codes for the repressor but is not where it binds. lacZ is a structural gene, downstream of the operator.
lacI codes for the repressor, which controls the other genes. lacZ (β-galactosidase), lacY (permease) and lacA (transacetylase) are structural genes coding for proteins that the cell uses.
Lactose induces its synthesis by inactivating the repressor. Being switched off by the product describes a repressible enzyme. Coenzyme activation is about enzyme activity, not gene expression. Glucose does not switch on the lac genes; in this topic lactose is the trigger.
Transcription factors are proteins that bind DNA and regulate how fast genes are transcribed. A length of DNA where RNA polymerase attaches is a promoter. mRNA is a product of transcription. Joining amino acids is the job of the ribosome in translation.
Gibberellin binds a receptor and triggers DELLA breakdown, freeing transcription factors to bind DNA. Gibberellin does not bind DNA itself. DELLA proteins are repressors, so activating them would reduce transcription. Gibberellin is not a protein and is not translated.
With nothing bound to the operator, RNA polymerase transcribes the structural genes all the time, so the enzymes are made without lactose. They would also be made with lactose, not prevented. lacI is still transcribed; its repressor simply cannot bind. lacI is transcribed continuously whether or not lactose is present.
Tryptophan activates the repressor, which binds the operator and blocks transcription: a repressible system. Removing the repressor describes the lac (inducible) system. Tryptophan acts through the repressor, not the promoter. The trp enzymes are repressible, not inducible.
A DELLA that resists breakdown keeps inhibiting the transcription factors whatever the GA level. A lele plant would make little GA1 and would respond to spraying. With no DELLA at all, the growth genes would be on and the plant would be tall. A more active Le enzyme would make more GA1, not less growth.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.