A research team studying gene expression in bacteria observed that when they grew Escherichia coli in a medium containing ONLY lactose as the carbon source, the bacteria began producing β-galactosidase enzyme within minutes. When they switched the bacteria to a glucose-rich medium (removing lactose), β-galactosidase production stopped almost immediately. The team also noted that a mutant strain of E. coli, in which the operator region had a structural alteration, produced β-galactosidase continuously — even in the complete absence of lactose.
Read the following passage and answer the questions that follow:
A research team studying gene expression in bacteria observed that when they grew Escherichia coli in a medium containing ONLY lactose as the carbon source, the bacteria began producing β-galactosidase enzyme within minutes. When they switched the bacteria to a glucose-rich medium (removing lactose), β-galactosidase production stopped almost immediately. The team also noted that a mutant strain of E. coli, in which the operator region had a structural alteration, produced β-galactosidase continuously — even in the complete absence of lactose.
(a) Name the regulatory model that explains the above observation in wild-type E. coli. Who proposed this model? (1 mark)
(b) Explain, with reference to the structural components of this system, why β-galactosidase is produced when lactose is the ONLY carbon source but NOT when glucose is present. (2 marks)
(c) Account for the continuous production of β-galactosidase in the mutant strain even in the absence of lactose. (1 mark)
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(a) Name the regulatory model and its proposer: (1 mark)
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• The regulatory model is the lac operon (lactose operon). (½)
• It was proposed by François Jacob and Jacques Monod (1961). (½)
[1 × 1 = 1 mark]
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(b) Mechanism of β-galactosidase production in presence of lactose vs. absence (glucose present): (2 marks)
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STATE A — Lactose present as ONLY carbon source (β-galactosidase IS produced):
• The lac operon consists of: regulator gene (i) → promoter (p) → operator (o) → structural genes z (β-galactosidase), y (permease), a (transacetylase).
• In the ABSENCE of lactose: the regulator gene (i) produces an ACTIVE repressor protein that binds to the operator → blocks RNA polymerase from transcribing structural genes → NO β-galactosidase produced.
• When lactose is the ONLY carbon source: lactose is converted to ALLOLACTOSE (the inducer) by a small amount of pre-existing β-galactosidase. (1)
• Allolactose binds to the active repressor → repressor changes shape (allosteric change) → repressor can NO LONGER bind the operator → operator is free → RNA polymerase transcribes z, y, a genes → β-galactosidase (and permease, transacetylase) are synthesised. (1)
STATE B — Glucose present (β-galactosidase production STOPS):
• When glucose is available, E. coli preferentially uses glucose (catabolite repression).
• Glucose lowers cAMP levels → CAP (catabolite activator protein) cannot bind the promoter → even if operator is free, transcription efficiency is very low → β-galactosidase production effectively stops.
[Award 1 mark for correct explanation of allolactose as inducer releasing repressor from operator; 1 mark for stating glucose/catabolite repression stops transcription. Accept explanation of either mechanism for 1 mark each.]
[1 + 1 = 2 marks]
DIAGRAM (embedded, labelled — for full credit):
STATE A: lac operon WITH lactose (INDUCED — ON state)
Regulator Promoter Operator z y a
gene (i) (p) (o) (β-gal) (permease) (transacetylase)
───────────────────────────────────────────────────────────
↓
Repressor
protein
(inactive —
bound to
allolactose)
[Operator FREE]
↓
RNA polymerase → transcribes z, y, a
↓
mRNA → β-galactosidase + permease + transacetylase
Allolactose ──binds──→ Repressor (inactive form — cannot bind operator)
STATE B: lac operon WITHOUT lactose (REPRESSED — OFF state)
Regulator Promoter Operator z y a
gene (i) (p) (o) (β-gal) (permease) (transacetylase)
───────────────────────────────────────────────────────────
↓
Active
Repressor ──────────────→ BINDS OPERATOR
↓
RNA polymerase BLOCKED
↓
NO transcription → NO β-galactosidase
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(c) Continuous β-galactosidase production in the operator-mutant strain: (1 mark)
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• In the mutant strain, the operator region has a structural alteration (mutation) → the active repressor protein CANNOT recognise or bind to the mutated operator. (½)
• Since the repressor can never block the operator, RNA polymerase transcribes the structural genes (z, y, a) CONSTITUTIVELY — continuously and regardless of whether lactose is present or absent. (½)
• This is called a constitutive mutant (operator-constitutive mutation).
[1 × 1 = 1 mark]
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TOTAL: 4 marks
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NOTE TO EXAMINER:
• Award full marks for any biologically equivalent correct explanation.
• For part (b): if student explains only one state (lactose present OR glucose present) correctly, award 1 out of 2.
• For part (c): accept 'repressor cannot bind mutated operator' as the complete answer for 1 mark.
