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Gene Expression in Organisms

Overview of DNA and Gene Expression

• DNA and its Role: Encodes RNA and protein molecules needed for cell formation.

• Importance: Knowing the full DNA sequence isn’t enough, similar to understanding a language without knowing how words combine (e.g., Shakespeare's plays).

• Gene Expression: The process through which genes are selectively turned on or off, allowing cells to adapt and respond to their environments.

Gene Regulation in Single-Celled vs. Multicellular Organisms

• Single-Celled Organisms: Can flexibly switch genes on and off to adapt to various food sources (e.g., bacteria).

• Multicellular Organisms: Have complex gene expression regulation during development:

  • Specialized Cell Types: Multiple cell types arise from a single fertilized egg, each with distinct structures and functions (e.g., nerve cells vs. white blood cells).

  • Gene Expression Control: All cells typically contain the same genomic DNA; differentiation is due to selective gene expression.

Evidence of Gene Retention in Differentiation

• Nuclear Transplantation Studies: Experiments (e.g. frog eggs) show differentiated cells can still develop normally, proving no critical genes are lost.

• Plant Regeneration: Single cells can regenerate entire plants, indicating full genetic instructions remain.

• Gene Expression Variability: Different cell types express different genes despite sharing the same genome.

Levels of Gene Regulation

Proteins and RNA Diversity in Cells

• Protein Composition Differences: Differentiated cells show varying protein expression (e.g., liver vs. heart cells).

• Methods of Analysis: Techniques such as two-dimensional gel electrophoresis and mass spectrometry allow for detailed protein expression profiling.

Response to External Signals

• External Cue Responses: Cells sometimes alter gene expression in response to external signals (e.g., liver cells respond to cortisol).

• Hormonally Induced Changes: For example, cortisol increases production of specific proteins in liver cells under stress conditions.

Steps of Gene Expression Regulation

• Regulatory Steps: Gene expression can be controlled at multiple stages:

  1. Transcription Activation: Control over when and how often a gene is transcribed.

  2. RNA Processing: Control over how RNA transcripts are spliced and modified.

  3. mRNA Transport: Regulation of mRNA export from nucleus to cytoplasm.

  4. mRNA Stability: Control of mRNA degradation rates.

  5. Translation Regulation: Specific mRNAs selected for translation into proteins by ribosomes.

  6. Protein Stability and Activity: Control over how proteins are degraded or activated after synthesis.

Importance of Transcriptional Control

• Primacy of Transcription Regulation: Most significant control occurs at the transcription level, minimizing unnecessary synthesis of intermediates.

Mechanisms of Transcriptional Regulation

Transcriptional Switches

• Transcription Regulators: Proteins that interact with DNA to toggle gene expression on or off.

• Importance in Eukaryotes: Regulation is more complex due to chromatin structures compared to prokaryotes.

Regulatory DNA Sequences

• Promoter Regions: Regions where RNA polymerase binds; regulatory sequences control access.

• Operons in Bacteria: Coordination of gene clusters for efficient transcription in response to environmental changes.

• Transcription Factors: Proteins that bind to specific DNA sequences to regulate transcription initiation.

  • Dimerization: Many transcription regulators operate as dimers to enhance specificity and strength of binding.

Case Study: Tryptophan Operon in E. coli

• Operon Functionality: The tryptophan operon consists of genes arranged to construct the amino acid tryptophan; regulated by the tryptophan repressor.

• Repressor Mechanism: Active when tryptophan is abundant, blocking RNA polymerase access to prevent unnecessary protein synthesis.

• Rapid Response: Repressor protein is always present, allowing quick adjustment to tryptophan levels.

Activators and Repressors

• Roles of Proteins: The tryptophan repressor inhibits gene expression, while activators enhance it.

• Activation Process: Some promoters require activator proteins to facilitate RNA polymerase binding for effective transcription.

• Lac Operon Example: Controlled by both a repressor and an activator, showcasing the complexity of gene regulation.