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Gene Control — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 12 parts Lesson 15 of 18 3 quizzes

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision
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Lesson summary

16.3 Gene Control — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Distinguish structural and regulatory genes, explain the lac operon and describe transcription factors including gibberellin and DELLA.Same genes — the nucleus of every body cell contains the same gen…

  • Expression
  • Structural gene
  • Inducible enzyme
  • Repressible enzyme
  • Lactose absent
  • Lactose present

Part 1 of 10Control of gene expression

Same genes
  • Same genes the nucleus of every body cell contains the same genes.
Expression
  • Expression not every gene is expressed in every cell or all the time.
Purpose
  • Purpose ensures the correct genes act in the correct cell at the right time.
Regulatory genes
  • Regulatory genes are involved in this control.

Part 2 of 10Structural and regulatory genes

Structural gene
  • Structural gene codes for a protein with a function in the cell, such as F8 for factor VIII.
Regulatory gene
  • Regulatory gene codes for a protein that helps control the expression of another gene.
Position
  • Position structural and regulatory genes that work together are usually close together.

Part 3 of 10Inducible and repressible enzymes

Inducible enzyme
  • Inducible enzyme made only when its substrate is present, which starts transcription.
Repressible enzyme
  • Repressible enzyme made until a repressor binds to an operator and transcription stops.
Benefit
  • Benefit saves materials and energy that would be wasted making an enzyme without its substrate.
Example
  • Example the enzymes of the lac operon are inducible; tryptophan synthesis enzymes are repressible.

Part 4 of 10Structure of the lac operon

Structure of the lac operon
Operon
  • Operon a cluster of structural genes controlled by the same promoter, found in prokaryotes.
Order
  • Order promoter, operator, lacZ, lacY, lacA.
lacZ
  • lacZ codes for lactase, which breaks lactose into glucose and galactose.
    • (β-galactosidase)
lacY
  • lacY codes for permease, which brings lactose into the cell.
lacA
  • lacA codes for transacetylase.
lacI
  • lacI regulatory gene upstream with its own promoter, coding for the lac repressor.

Part 5 of 10Lactose absent and present

Lactose absent
  • Lactose absent repressor binds the operator, RNA polymerase cannot bind the promoter and there is no transcription.
Lactose present
  • Lactose present lactose binds the repressor, changing its shape so it cannot bind the operator.
Transcription
  • RNA polymerase binds the promoter and the structural genes are transcribed.
Result
  • Result lactase is made and lactose is used for energy.
Glucose
  • Glucose if glucose is also present, transcription stays low because cAMP is low.

Part 6 of 10Repressible control

Effector
  • Effector an effector molecule helps the repressor bind the operator.
Result
  • Result when the effector is scarce, the repressor cannot bind and the enzyme is made.
Contrast
  • Contrast the opposite of the lac operon.

Part 7 of 10Transcription factors

Definition
  • Definition proteins that control transcription of genes by binding to specific DNA regions.
Role
  • Role ensure genes are expressed in the correct cells, at the correct time and to the right level.
Number
  • Number about 10 % of human genes code for transcription factors.
Binding
  • Binding many bind the promoter, increasing or decreasing the rate of transcription.
Hormones
  • Hormones some hormones act through transcription factors, for example steroid hormones.
Example
  • PIF activates the amylase gene in plants.

Part 8 of 10Gibberellin and DELLA

1
DELLA protein is bound to PIF, so PIF cannot bind the promoter of the amylase gene.
2
Gibberellin binds a receptor and enzyme, starting the breakdown of DELLA.
3
PIF is freed and binds the promoter of the amylase gene.
4
Transcription of the amylase gene begins.
5
Amylase is produced.
  • Evidence
    • gibberellin applied to germinating seeds increases mRNA for amylase.

Part 9 of 10RNA analysis

Same DNA
  • Same DNA cells look similar and have the same DNA but express different genes.
RNA analysis
  • RNA analysis the mRNA present in a cell shows which genes are being transcribed.

Part 10 of 10More from the specification

Genetic control
  • Genetic control gene expression is regulated so that proteins are made only when needed
Lac operon
  • Lac operon in a bacterium, a repressor protein binds the operator and prevents transcription of structural genes when lactose is absent
Inducer
  • Inducer lactose binds the repressor, changing its shape so RNA polymerase can transcribe the genes
Eukaryotic control
  • Eukaryotic control transcription factors bind promoters, and several factors act together to switch genes on or off, as with oestrogen
Bacterium
  • A single-celled organism with no nucleus.

Key terms

TermMeaning
Structural geneGene coding for a functional protein.
Regulatory geneGene coding for a protein that controls another gene.
OperonCluster of genes controlled by one promoter.
OperatorRegion where the repressor binds.
Transcription factorProtein controlling transcription by binding DNA.
DELLA proteinRepressor of PIF broken down by gibberellin.
Inducible enzymeEnzyme made when its substrate is present.
Repressible enzymeEnzyme whose synthesis is stopped by a repressor.

Avoid these mistakes

1
Saying lactose binds the operator — lactose binds the repressor.
2
Saying the repressor is made only when lactose is absent — lacI is transcribed continuously.
3
Saying transcription factors are enzymes that copy DNA — they control binding of RNA polymerase.
4
Mixing inducible and repressible — inducible enzymes are made when the substrate is present.

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