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. Both depend on proteins that bind to DNA and decide whether RNA polymerase gets to work.
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.
Fig. 1: the lac operon of E. coli with and without lactose
Section 01
The lac operon: an inducible system.
Start with no lactose and read the REPRESSOR and β-GALACTOSIDASE readouts. Then choose lactose added and watch the repressor leave the operator, RNA polymerase move along the structural genes and enzyme molecules appear.
Part
What it is
Role
lacI
regulatory gene, with its own promoter
always transcribed; codes for the repressor protein
promoter (P)
a length of DNA before the structural genes
where RNA polymerase binds to start transcription
operator (O)
a length of DNA next to the promoter
where the repressor binds, blocking RNA polymerase
lacZ
structural gene
codes for β-galactosidase, which hydrolyses lactose to glucose and galactose
lacY
structural gene
codes for lactose permease, which transports lactose into the cell
lacA
structural gene
codes for transacetylase
Operon: a group of structural genes under the control of one promoter and operator, transcribed together as a single mRNA.
No lactose: the repressor binds the operator, so RNA polymerase cannot bind to the promoter; the structural genes are not transcribed and no energy or amino acids are wasted on enzymes that are not needed.
Lactose present: lactose binds to a different site on the repressor, changing its shape so it can no longer bind the operator. RNA polymerase binds the promoter and transcribes lacZ, lacY and lacA; the enzymes are made and lactose is taken up and used.
Lactose used up: the repressor is free again, binds the operator, and the genes switch off.
Examiner trap
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.
Section 02
Inducible and repressible enzymes.
β-galactosidase is switched on by its substrate. Other enzymes work the opposite way: they are switched off by their product.
Inducible enzyme
Repressible enzyme
example
β-galactosidase (lac operon)
enzymes that synthesise tryptophan (trp operon)
normally
not made: the repressor is active and bound to the operator
made: the repressor is inactive and cannot bind the operator
switch
the substrate (lactose) inactivates the repressor, so the genes are switched on
the product (tryptophan) binds the repressor and activates it, so the genes are switched off
benefit
enzymes made only when there is substrate to use
enzymes stop being made when enough product is present
Structural genes code for proteins used in the cell's metabolism or structure (lacZ, lacY, lacA, the trp enzyme genes).
Regulatory genes code for proteins that control whether other genes are expressed (lacI and its repressor; genes for transcription factors).
Section 03
Transcription factors and DELLA proteins.
Choose lele and LeLe, and tick Spray with gibberellin on the dwarf plant. Watch gibberellin (GA1) arrive at its receptor, the red DELLA protein being removed, and the blue transcription factor moving onto the growth genes.
Transcription factors: proteins that bind to specific DNA sequences (often near the promoter) and control gene expression in eukaryotes by increasing or decreasing the rate of transcription, for example by helping or preventing RNA polymerase binding.
DELLA proteins are repressors: they bind to and inhibit transcription factors that would promote the transcription of genes for growth, such as stem elongation.
Gibberellin's action: GA1 binds to a receptor protein; the gibberellin-receptor complex binds DELLA proteins and causes them to be broken down.
Result: the transcription factors are released, bind to the promoters of their target genes and increase transcription; the proteins made cause cell elongation and the stem grows.
Link to Le: lele plants make little GA1, so DELLA proteins stay intact, the growth genes stay switched off and the plant is dwarf.
Examiner trap
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.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Quick check+10 XP
In the lac operon simulator with no lactose, what do the readouts show?
Quick check+10 XP
After lactose is added in the simulator, why does the repressor leave the operator?
Quick check+10 XP
In the gibberellin simulator, why is the lele plant dwarf?
Quick check+10 XP
When the dwarf plant is sprayed with gibberellin in the simulator, what happens to the DELLA protein?
Stage 2 · Exam
Exam-style questions
Unlocks once the four checks above are done. Worth more XP, written to A Level Paper 4 standard.
Finish the four checks above to unlock the exam questions
Exam style+20 XP
A mutation in lacI produces a repressor that cannot bind to the operator. What is the effect?
Exam style+20 XP
Which statement describes a repressible enzyme?
Exam style+20 XP
Which describes a transcription factor?
Exam style+20 XP
A mutant plant makes DELLA proteins that cannot be broken down. What is its likely phenotype, even when sprayed with gibberellin?
Skill unlocked
Gene control, mastered.
This skill is now recorded on your profile. Keep the chain going.