Control of Gene Expression

An Organism’s DNA and Gene Expression

  • Overview of DNA Function:

    • DNA encodes RNA and protein molecules essential for cell construction.

    • Knowing an organism’s DNA sequence is insufficient to reconstruct its functionality (similarity to reconstructing Shakespeare play from words).

    • Key concerns: Conditions for gene product synthesis and their functions post-synthesis.

  • Purpose of Chapter: Focus on mechanisms enabling selective gene expression in cells.

The Differentiation of Cell Types

  • Cell Differentiation:

    • Diverse cell types in multicellular organisms demonstrate significant structural and functional differences.

    • Example: Comparison of mammalian neurons and liver cells.

  • Myth of Gene Loss:

    • Earlier belief that cells lose genes upon differentiation has been debunked; DNA sequences remain intact in differentiated cells.

  • Evidence from Experiments:

    • Frogs:

    • Experiment: Injection of a differentiated frog cell nucleus into an enucleated egg results in a normal tadpole.

    • Conclusion: Differentiated cells retain all necessary DNA sequences for organism development.

    • Plants:

    • Differentiated plant tissue can regenerate entire adult plants from single cells, reaffirming DNA integrity across cell types.

    • Mammals:

    • Cloning experiments (sheep, goats, etc.) confirm that differentiated cells maintain full genetic information.

Gene Expression: Commonalities and Differences

  • Common Gene Products:

    • Cells share many common gene products, including structural proteins of chromosomes, ribosomal proteins, and central metabolic enzymes (e.g., actin).

  • Unique Gene Products:

    • Some RNAs/proteins found only in specialized cells (e.g., hemoglobin in red blood cells, tyrosine aminotransferase in liver).

  • Percentage of Active Genes:

    • Typical human cells express about 30-60% of ~30,000 genes at any given time, with approximately 21,000 protein-coding and 9,000 noncoding RNA genes.

  • Gene Expression Variability:

    • Most genes show varied expression levels across cell types; even common genes differ in expression intensity.

  • Post-Transcriptional Regulation:

    • Final protein production patterns can differ significantly due to regulation after RNA production.

Environmental Signals and Gene Expression

  • Response to External Signals:

    • Specialized cells can alter gene expression patterns in reaction to extracellular signals.

    • Example: Glucocorticoid hormone exposure in liver cells increases protein production.

    • Response varies: Fat cells reduce tyrosine aminotransferase production under glucocorticoids.

Control of Gene Expression Steps

  1. Transcriptional Control: Determines when/how often a gene is transcribed.

  2. RNA Processing Control: Involves splicing and processing of RNA transcripts.

  3. RNA Transport and Localization Control: Regulates mRNA export and localization.

  4. Translational Control: Manages which mRNAs are translated by ribosomes.

  5. mRNA Degradation Control: Selectively degrades specific mRNA molecules.

  6. Protein Activity Control: Activating, inactivating, degrading, or localizing proteins post-synthesis.

  • Key Role of Transcriptional Control:

    • Most significant control point; prevents unnecessary intermediate synthesis.

Transcription Regulators and Gene Control

  • Transcription Regulators: Proteins that bind to specific DNA sequences (cis-regulatory sequences) controlling transcription based on their binding.

  • Efficiency of Binding:

    • Transcription regulators recognize short DNA sequences via structural motifs, and typically act as dimers to enhance binding affinity and specificity.

  • Key Mechanisms of Action:

    • Recognition of DNA features through the major and minor grooves of the double helix.

    • Cooperative binding significantly boosts recognition precision and transcription regulation.

Dimerization Effects

  • Homodimers and Heterodimers: Enhances the length and affinity of the corresponding cis-regulatory sequences recognized.

E. coli Gene Regulation: Specific Examples

  • Tryptophan Operon: A model of bacterial gene regulation, controlling synthesis in response to tryptophan levels.

    • Mechanism: A transcriptional repressor that binds the operator when tryptophan is abundant, halting transcription

  • Lac Operon: Encodes proteins for lactose metabolism, with dual regulation by the Lac repressor and CAP activator based on glucose and lactose presence.

Eukaryotic Gene Regulation Complexity

  • Multifactorial Control: Eukaryotic genes engaged by numerous transcription regulators.

  • Transcription Factors Involvement: Major transcriptional regulators require cooperation of multiple factors including coactivators and general transcription factors for effective expression.

Gene Regulatory Networks

  • Insulator sequences play a critical role in keeping regulatory domains distinct, preventing inappropriate interaction between genes.

Summary of Gene Expression Regulation

  • The overall framework for gene regulation in eukaryotes is elaborate, necessitating many proteins and regulatory sequences that allow for tightly controlled gene expression in response to diverse stimuli.