Molecular Genetics Gene Control - In Depth Notes

Molecular Genetics Gene Control

Overview of Gene Expression

• The human genome includes hundreds of thousands of genes.
• Genes are responsible for genetic traits and cellular biochemistry.
• Not all genes are expressed in every cell; cells express only the necessary genes for their function.
• Differential Gene Expression: This allows specialization of cells.
• Gene Action Control: Occurs on two levels:
  • Short Term Control: Turning genes on/off based on immediate cellular needs.
  • Long Term Control: Permanent activation or deactivation of genes.

I. Prokaryotic Gene Control

1. The Inducible Operon

• Background: Established through studies by Jacob and Monod on E.coli.
• E.coli can utilize lactose (disaccharide) to produce glucose and galactose, needing the enzyme eta-galactosidase.
  • Reaction: $ext{lactose} + ext{water} <br /> ightarrow ext{glucose} + ext{galactose}$

Observations:

• With lactose present: approximately 3000 molecules of eta-galactosidase/cell.
• Without lactose: approximately 1 molecule of eta-galactosidase/cell.
• Conclusion: E.coli can regulate the production of eta-galactosidase based on its needs. Lactose serves as a gene inducer.

Operon Concept:

• Operon: A group of related genes on a DNA segment coding for enzymes in the same metabolic pathway.
• The lac operon consists of three genes needed to metabolize lactose, all under one promoter (one transcription unit).
  • Efficiency: Instead of transcribing each gene separately, one mRNA is produced for all related enzymes.

Control Switch - Operator:

• Operator: A DNA segment between coding genes and promoter which acts like an ON/OFF switch.
• In the absence of lactose, a repressor synthesized by a regulatory gene inhibits the lac operon genes.
• Repressor binds to operator, blocking RNA polymerase access, thus preventing transcription.

Lactose Presence Effect:

• When lactose (or isomer allolactose) is present, it binds to the repressor, preventing it from blocking the operator.
• Transcription of the lac operon genes occurs, allowing the production of lactose-digesting enzymes.
• As lactose is digested, its concentration decreases, allowing repressor to reattach and shut down the pathway.
  • Inducible Operon Summary:
  • Specific genes turned ON by the presence of an inducer.
  • Inducers can stimulate transcription or reactivate repressor function.
  • Positive control mechanism boosts gene expression.

2. The Repressible Operon

• Negative Feedback Mechanism: Presence of a repressor inhibits gene expression.
• Example: E.coli produces tryptophan when not available in the environment.

Tryptophan Operon Activation:

• Active when tryptophan is absent (RNA polymerase binds to promoter).

Repressor Action:

• Tryptophan Repressor: Inactive without tryptophan, preventing binding to operator. As tryptophan levels increase,
  • It binds to the repressor, activating it.
• Activated repressor binds the operator, blocking RNA polymerase and shutting down tryptophan production.
  • Repressible Operon Summary:
  • Gene expression is inhibited by repressor action.
  • Negative control mechanism decreases gene expression.

Final Note on Gene Regulation:

• Both positive and negative controls are critical for regulating gene expression in bacteria.

Cell Signaling and Gene Control

• Gene expression is further regulated by intercellular and intracellular signals.
• Example: Lac operon regulation influences are not solely from lactose but also from glucose levels.

Mechanism of CAP and cAMP:

• CAP (Catabolite Activator Protein): Stimulates transcription of lac genes when glucose is low.
• When glucose decreases, cyclic AMP (cAMP) levels increase. cAMP binds to CAP, activating it.
• CAP attaches upstream of the promoter, stimulating transcription.
• High glucose levels decrease cAMP, leading to less lac enzyme production.
• This represents positive gene regulation because transcription is stimulated based on regulatory signals.