16.3.216 · Inherited Change
Gene Control- Lac Operon — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions
5 min read 10 parts Lesson 16 of 18 1 quiz
Lesson summary
16.3.2 Gene Control: Lac Operon — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Explain how the lac operon controls the production of lactase in bacteria.Regulatory genes — control structural genes and the levels of protein they make.Several genes — a regulatory gen…
- Binding sites
- On the operator
- On lactose
- Low effector
- Effector molecule
- Gibberellin
Section 1 of 10Why control is needed
Regulatory genes
- Regulatory genes control structural genes and the levels of protein they make.
Several genes
- Several genes a regulatory gene may control several structural genes at once.
Operon
- Operon in prokaryotes, a cluster of genes controlled by the same promoter.
lac operon
- lac operon a well-known example that controls lactase production.
Section 2 of 10Role of lactase
Lactase
- Lactase also called β-galactosidase, breaks down lactose so it can be used for energy.
Inducible
- Inducible it is made only when lactose is present.
Benefit
- Benefit prevents waste of energy and materials.
Section 3 of 10Components of the lac operon
- Promoter for the structural genes.
- Operator.
- lacZ, coding for lactase.
- lacY, coding for permease, which allows lactose into the cell.
- lacA, coding for transacetylase.
- Upstream
- a promoter for the regulatory gene and the regulatory gene lacI, coding for the lac repressor protein.
Section 4 of 10The lac repressor
Binding sites
- Binding sites one binds the operator and one binds lactose, the effector molecule.
On the operator
- On the operator prevents transcription because RNA polymerase cannot attach to the promoter.
On lactose
- On lactose the repressor changes shape and can no longer bind the operator.
Section 5 of 10When lactose is absent
1
The regulatory gene is transcribed and translated, making the repressor.
2
The repressor binds the operator upstream of lacZ.
3
RNA polymerase cannot bind the promoter.
4
The structural genes are not transcribed.
5
No lactase is made.
Section 6 of 10When lactose is present
1
Lactose is taken up by the bacterium.
2
Lactose binds the repressor, which changes shape and cannot bind the operator.
3
RNA polymerase binds the promoter and transcription takes place.
4
The mRNA of all three structural genes is translated.
5
Lactase is made and lactose is broken down for energy.
Section 7 of 10Repressible enzymes compared
Effector
- Effector binds the repressor and helps it bind the operator.
Low effector
- Low effector the repressor cannot bind, so transcription goes ahead and the enzyme is made.
Section 8 of 10More from the specification
More from the specification
Beta-galactosidase catalytic function
- Beta-galactosidase catalytic function induction of the lac operon produces beta-galactosidase to hydrolyse lactose into glucose and galactose for cellular function.
Lac operon regulation
- In Escherichia coli cells, the lac repressor protein binds to the operator region to prevent RNA polymerase from transcribing the structural genes unless allolactose acts as an inducer.
Gibberellin and DELLA proteins
- In germinating cereal grains, the plant hormone gibberellin causes the breakdown of DELLA repressor proteins,
- allowing transcription factors to bind and promote gene expression.
Transcription factor activation of amylase
- Once DELLA repressor proteins are degraded, active transcription factor complexes bind promoter regions,
- allowing the gene that codes for alpha-amylase to be expressed to hydrolyse starch reserves.
Differential gene expression
- Even though specialized eukaryotic cells possess the same genetic material, differential expression is governed by specific transcription factors and epigenetic modifications regulating which genes are expressed.
Lac operon regulation
- In Escherichia coli cells, the lac repressor protein binds to the operator region to prevent RNA polymerase from transcribing the structural genes unless allolactose acts as an inducer.
Gibberellin and DELLA proteins
- In germinating cereal grains, the plant hormone gibberellin causes the breakdown of DELLA repressor proteins,
- allowing transcription factors to bind and promote gene expression.
Transcription factor activation of amylase
- Once DELLA repressor proteins are degraded, active transcription factor complexes bind promoter regions,
- allowing the gene that codes for alpha-amylase to be expressed to hydrolyse starch reserves.
Differential gene expression
- Even though specialized eukaryotic cells possess the same genetic material, differential expression is governed by specific transcription factors and epigenetic modifications regulating which genes are expressed.
Lac operon regulation
- In Escherichia coli cells, the lac repressor protein binds to the operator region to prevent RNA polymerase from transcribing the structural genes unless allolactose acts as an inducer.
Gibberellin and DELLA proteins
- In germinating cereal grains, the plant hormone gibberellin causes the breakdown of DELLA repressor proteins,
- allowing transcription factors to bind and promote gene expression.
Transcription factor activation of amylase
- Once DELLA repressor proteins are degraded, active transcription factor complexes bind promoter regions,
- allowing the gene that codes for alpha-amylase to be expressed to hydrolyse starch reserves.
Differential gene expression
- Even though specialized eukaryotic cells possess the same genetic material, differential expression is governed by specific transcription factors and epigenetic modifications regulating which genes are expressed.
Lac operon regulation
- In Escherichia coli cells, the lac repressor protein binds to the operator region to prevent RNA polymerase from transcribing the structural genes unless allolactose acts as an inducer.
Gibberellin and DELLA proteins
- In germinating cereal grains, the plant hormone gibberellin causes the breakdown of DELLA repressor proteins,
- allowing transcription factors to bind and promote gene expression.
Transcription factor activation of amylase
- Once DELLA repressor proteins are degraded, active transcription factor complexes bind promoter regions,
- allowing the gene that codes for alpha-amylase to be expressed to hydrolyse starch reserves.
Differential gene expression
- Even though specialized eukaryotic cells possess the same genetic material, differential expression is governed by specific transcription factors and epigenetic modifications regulating which genes are expressed.
Transcription factor
- Protein controlling transcription by binding DNA.
DELLA protein
- Repressor of PIF broken down by gibberellin.
Repressible enzyme
- Enzyme whose synthesis is stopped by a repressor.
Amylase
- Enzyme that breaks starch into maltose.
Gibberellin
- Hormone stimulating germination and stem elongation.
Specific
- Able to act on only one type of substrate.
Substrate
- The molecule that an enzyme acts on and which binds to the active site.
Section 9 of 10Key terms
| Term | Meaning |
|---|---|
| Operon | Cluster of genes controlled by one promoter. |
| Promoter | DNA region where RNA polymerase binds. |
| Operator | DNA region where the repressor binds. |
| Repressor | Protein that blocks transcription. |
| Lactase | Enzyme breaking lactose into glucose and galactose. |
| Permease | Protein bringing lactose into the cell. |
| Effector molecule | Molecule that binds and changes a regulatory protein. |
Section 10 of 10Exam technique
1
Saying lactose binds the operator — it binds the repressor.
2
Saying RNA polymerase binds when the repressor is on the operator — it is blocked.
3
Saying lacI is part of the operon — it is a separate regulatory gene with its own promoter.
4
Mixing lacY and lacZ — lacZ codes for lactase, lacY for permease.
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