gene regulation

Prokaryotic Gene Regulation

  • Introduction to Prokaryotic Gene Regulation

    • Focus on a specific type: the lac operon in E. coli.

    • Comparison of two regulation types: positive and negative regulation.

The Lac Operon

  • Definition of an Operon

    • A DNA complex comprising a group of adjacent genes with related functions plus regulatory DNA sequences that control gene expression.

    • Primarily found in prokaryotes; exceptions exist in biology. Eukaryotes regulate each gene independently.

  • Structure of the Lac Operon

    • Promoter (green area): RNA polymerase binding site; initiates transcription.

    • Facilitates transcription of downstream genes.

    • Operator (red area): Acts as the switch, determining gene transcription. Default state is OFF; prevents RNA polymerase from transcribing genes in absence of lactose.

    • Genes (blue area): Three genes involved in lactose metabolism (not necessary to memorize gene functions).

  • Regulation Types

    • Operon is inducible: Default state is OFF; can turn on when needed (e.g., presence of lactose).

    • Constitutive Genes: Always expressed (e.g., DNA repair genes).

Negative Regulation of the Lac Operon

  • Mechanics of Negative Regulation

    • Default state: OFF when lactose is absent; genes for lactose digestion are not expressed to conserve energy.

    • When lactose is present, it induces operon activation by deactivating the repressor protein.

  • Repressor Protein Function

    • Binds to the operator to inhibit transcription when lactose is absent.

    • The repressor gene (lacI) is constantly expressed to facilitate regulation at all times, even when repressing gene action.

  • Mechanisms of Inducible Action

    • Lactose converts to allolactose (the inducer), binding to the repressor, inactivating it and allowing transcription to occur.

    • The repressor, when inactive, releases the operator, enabling RNA polymerase access.

  • Transcription Process

    • Transcription from the lac operon produces a single polycistronic mRNA encompassing multiple genes for lactose digestion. Translation yields three separate polypeptides.

Positive Regulation of the Lac Operon

  • Role of Positive Regulation

    • Activator protein (CAP) enhances transcription in the absence of glucose via cyclic AMP (cAMP).

    • Mechanism of Action:

    • When glucose levels drop, cAMP levels increase, stimulating CAP binding to the lac promoter, enhancing RNA polymerase affinity, leading to increased transcription rates.

  • Logic of Regulation

    • Negative regulation prevents energy waste in the absence of lactose.

    • Positive regulation ensures efficient use of lactose when glucose is scarce.

Eukaryotic Gene Regulation

  • Comparison with Prokaryotes

    • Eukaryotic regulation is more complex, allowing control at multiple levels: chromatin structure, transcription, mRNA modification, and translation.

Major Levels of Eukaryotic Regulation

  • Chromatin Structure Regulation

    • DNA can be loosely or tightly packed; tightly packed regions (heterochromatin) inhibit gene expression.

    • Methylation: Addition of methyl groups condenses DNA, inhibiting transcription in those regions (epigenetic regulation).

  • Transcription Initiation Regulation

    • Transcription Factors: Proteins that modulate the binding of RNA polymerase to DNA; can enhance or inhibit transcription of a gene.

  • Post Transcriptional Regulation

    • mRNA Modifications:

    • Addition of a poly(A) tail influences mRNA stability and translation frequency; longer tails mean longer availability in the cytoplasm.

    • Alternative Splicing: Produces different proteins from the same primary transcript by including or skipping certain exons.

  • Post Transnational Regulation

    • Modifications after translation, such as phosphorylation, can activate or deactivate proteins. This affects the activity of polypeptides leading to responses by the cell, allowing rapid adjustments to the cell's needs.

Summary

Eukaryotic gene regulation is multifaceted, incorporating development at chromatin structure, transcription factors, mRNA processing, and post-translational modifications. This layered complexity enables precise control of gene expression in response to cellular and environmental conditions.

  • Reminder about Exam: Cutoff for Exam 2 approaches. Review all concepts discussed, particularly between prokaryotic and eukaryotic gene regulations that highlight the complexity and mechanisms involved.