Genetics and Evolution- Molecular Basis of Inheritance - Repressible Operon

Slide 1

• Genetics and Evolution: Molecular Basis of Inheritance
  • Repressible Operon

Slide 2

• What is a repressible operon?
• Definition: A group of genes that are controlled by a single promoter and operator region, and their expression can be repressed when a regulatory protein binds to the operator.

Slide 3

• Components of a repressible operon:
  • Promoter: Initiates transcription of the genes
  • Operator: Regulatory DNA sequence that controls the accessibility of the promoter to RNA polymerase
  • Structural Genes: Code for enzymes or proteins with a specific function

Slide 4

• Example of repressible operon: The trp Operon
• Involved in the synthesis of tryptophan, an essential amino acid
• When tryptophan is abundant, the operon gets repressed and the synthesis of tryptophan is reduced

Slide 5

• Trp Operon:
  • Consists of five genes: trpE, trpD, trpC, trpB, trpA
  • Regulatory region contains promoter, operator, and regulatory gene

Slide 6

• Regulation of trp Operon:
  • Regulatory protein Trp repressor binds to the operator when tryptophan is present in excess
  • Binding of Trp repressor blocks the RNA polymerase from transcribing the structural genes

Slide 7

• Tryptophan Absent:
  • RNA polymerase can freely bind to the promoter and transcribe the structural genes
  • Enzymes for tryptophan synthesis are produced

Slide 8

• Tryptophan Present:
  • Excess tryptophan acts as a corepressor
  • Tryptophan binds to the Trp repressor protein, causing it to bind to the operator
  • This inhibits RNA polymerase from transcribing the structural genes

Slide 9

• Feedback Inhibition:
  • Tryptophan acts as a feedback inhibitor on the trp operon
  • When tryptophan levels are high, the operon is repressed to prevent unnecessary synthesis of tryptophan

Slide 10

• Importance of Repressible Operons:
  • Allows cells to regulate gene expression based on the availability of certain substances
  • Efficient utilization of resources
  • Avoids unnecessary synthesis of biomolecules.

Slide 11

• Mechanism of Repression:
  • Tryptophan acts as a regulator of the trp operon
  • When tryptophan is absent, the repressor protein is unable to bind to the operator region
  • RNA polymerase can freely transcribe the structural genes

Slide 12

• Mechanism of Repression (contd.):
  • When tryptophan is present, it binds to the repressor protein
  • The repressor-operator complex is formed and prevents RNA polymerase from transcribing the structural genes

Slide 13

• Anabolic Pathways:
  • The trp operon is an example of anabolic pathway regulation
  • Anabolic pathways are responsible for the synthesis of complex molecules from simpler precursors
  • Regulation is important to avoid unnecessary production of complex molecules

Slide 14

• Negative Control:
  • Repressible operons function through negative control
  • It involves the binding of a repressor protein to the operator region
  • Binding inhibits the transcription process

Slide 15

• Comparison with Inducible Operons:
  • Repressible and inducible operons are two types of gene regulation systems
  • Repressible operons are usually active and repression occurs as a response to excess product
  • Inducible operons are usually inactive, and transcription is induced in response to a specific molecule

Slide 16

• TrpR Gene:
  • trpR is the regulatory gene in the trp operon
  • It codes for the Trp repressor protein that binds to the operator
  • Expression of trpR is constitutive to ensure the presence of repressor protein

Slide 17

• Tryptophan Regulation in E. coli:
  • In E. coli, tryptophan biosynthesis is regulated by the trp operon
  • When tryptophan levels are low, the operon is active and tryptophan synthesis occurs
  • When tryptophan levels are high, the operon is repressed to avoid unnecessary synthesis

Slide 18

• Importance of Tryptophan:
  • Tryptophan is an essential amino acid required for various cellular processes
  • It serves as a precursor for the synthesis of important biomolecules such as proteins and neurotransmitters

Slide 19

• Regulation and Gene Expression:
  • Gene regulation is a fundamental process in molecular biology
  • It allows cells to adapt to different environmental conditions
  • Repressible operons play a crucial role in regulating gene expression

Slide 20

• Conclusion:
  • The trp operon is a prime example of a repressible operon
  • Tryptophan acts as a corepressor and regulates the transcription of tryptophan synthesis genes
  • Repressible operons help cells conserve resources by producing specific proteins only when needed

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Slide 21

• Lac Operon: An Inducible Operon
  • Controls the metabolism of lactose in bacteria
  • Composed of the promoter, operator, regulatory gene, and structural genes

Slide 22

• Regulation of the Lac Operon
  • Lac operon is usually inactive and needs to be activated by an inducer molecule, such as lactose
  • Lactose acts as an inducer by binding to the Lac repressor and preventing it from binding to the operator
  • RNA polymerase can then transcribe the structural genes for lactose metabolism

Slide 23

• Components of the Lac Operon
  • Promoter: Where RNA polymerase binds to initiate transcription
  • Operator: Regulatory DNA sequence that controls access to the promoter
  • LacI gene: Codes for the Lac repressor protein
  • Structural genes: lacZ, lacY, and lacA, code for enzymes necessary for lactose metabolism

Slide 24

• Lac Repressor Protein
  • LacI gene codes for the Lac repressor protein
  • In the absence of lactose, the repressor binds to the operator and inhibits transcription
  • When lactose is present, it binds to the repressor, inducing a conformational change and preventing it from binding to the operator

Slide 25

• Induction of the Lac Operon
  • In the absence of lactose, Lac repressor binds to the operator, blocking transcription of the structural genes
  • When lactose is present, it binds to the Lac repressor, causing it to detach from the operator
  • RNA polymerase can now bind to the promoter and transcribe the genes

Slide 26

• Positive Control of the Lac Operon
  • The Lac operon can also be positively controlled by the molecule cAMP
  • In the absence of glucose, cAMP levels increase and bind to the cAMP receptor protein (CRP)
  • CRP-cAMP complex enhances the binding of RNA polymerase to the promoter, leading to increased transcription

Slide 27

• Examples of Inducible Operons
  • Besides the Lac operon, other examples of inducible operons include:
    • Arabinose operon: involved in the metabolism of arabinose
    • Tryptophanase operon: involved in the metabolism of tryptophanase

Slide 28

• Importance of Inducible Operons
  • Inducible operons allow bacteria to respond to changes in their environment
  • They ensure the efficient utilization of available resources
  • By regulating gene expression, bacteria can adapt to various conditions and optimize their growth

Slide 29

• Comparison of Repressible and Inducible Operons
  • Repressible operons are usually active and get repressed in response to excess product
  • Inducible operons are usually inactive and get induced in response to a specific molecule
  • Both types of operons allow gene expression to be regulated based on need and availability of specific substances

Slide 30

• Summary
  • Repressible and inducible operons are essential in gene regulation
  • Repressible operons get repressed when the product is in excess
  • Inducible operons get induced in response to a specific molecule
  • Both types of operons ensure efficient utilization of resources and adaptation to changing environments

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