Gene Expression and Regulation in Eukaryotes and Prokaryotes

Learning Objectives

  • Understand that all genes are present in every cell, but not all genes are expressed at the same time or in every cell.

  • Differentiate between eukaryotic and prokaryotic gene expression.

  • Explain the mechanisms regulating gene expression and their importance.

Case Study on Vitamin A Deficiency

  • Vitamin A is crucial for gene expression involved in cell differentiation.

  • Consider implications for client health if vitamin A is deficient.

Eukaryotic Gene Expression

  • Cells in multicellular organisms have the same genetic material but express different genes.

  • Gene expression is highly regulated, allowing cells to turn genes on or off as needed.

  • This regulation is essential for normal cell function and response to environmental changes, development, and disease.

Analogy for Gene Expression

  • Gene expression can be compared to adjusting the volume of sounds in a song, where each gene represents a sound.

  • The volume can be turned on or off and adjusted to create different outcomes (songs).

Factors Influencing Gene Expression

  • Gene expression can vary due to:

    • Development

    • Reproduction

    • Response to energy sources

    • Stress responses

    • Diseases (e.g. cancer)

  • The evolutionary advantages include adaptation to environmental changes and conservation of energy.

Prokaryotic Gene Expression

  • Prokaryotic gene expression is simpler than eukaryotic as it often involves operons: groups of genes controlled by a single promoter.

  • This allows simultaneous expression of genes that perform related functions.

Transcription in Prokaryotes

  • Polycistronic mRNA: In bacteria, multiple genes can be transcribed into a single mRNA transcript which encodes multiple proteins.

    • Each protein has its own start and stop codons.

  • Operator Mechanism: Gene expression is usually controlled at the transcription level by the ability of RNA polymerase to bind to the promoters.

    • Repressors inhibit transcription in a mechanism called negative control.

    • Inducers can deactivate repressors allowing for transcription to proceed.

The lac Operon

  • Example of inducible gene expression where E. coli can utilize lactose in the absence of glucose.

  • Lactose metabolite serves as an inducer releasing the repressor, leading to transcription of genes needed to metabolize lactose.

Eukaryotic Gene Regulation

  • Multi-layered regulation involving:

    • Gene accessibility

    • RNA polymerase binding

    • Processing of mRNA

    • mRNA stability and translation regulation

    • Protein folding and stability

  • Changes in chromatin structure can influence transcription levels.

Chromatin Remodeling

  • DNA is tightly packaged around histones in eukaryotes.

  • Due to tight packing, RNA polymerase cannot access promoters when DNA is in this state.

  • Histone Acetylation: Acetylation by HATs relaxes DNA-histone interactions while deacetylation by HDACs tightens them.

DNA Methylation

  • Methylation is an epigenetic mechanism that generally silences gene expression and is important for cell differentiation.

  • Patterns of DNA methylation can be inherited, impacting long-term gene expression.

RNA Polymerase Binding Control

  • Binding of RNA polymerase is regulated by multiple proteins that stabilize the complex and facilitate DNA opening for transcription.

  • TATA-binding protein & Enhancers: Help RNA polymerase to bind efficiently and initiate transcription.

Prokaryotic vs Eukaryotic Comparison

  • Prokaryotic: Uses operons for coordinated gene expression.

  • Eukaryotic: Genes are individually regulated, with enhancers influencing transcription independently.

Hormonal Control of Gene Expression

  • Hormones like cortisol and thyroid hormones can activate or repress transcription.

  • Cortisol (steroid hormone) binds to a receptor, allowing it to enter the nucleus and promote transcription.

  • Thyroid hormones replace repressor complexes with activators to enable transcription.

DNA Binding Proteins

  • Transcription factors contain DNA binding motifs that allow them to recognize and bind specific DNA sequences, which is crucial for gene regulation.

Alternative Splicing

  • Alternative splicing allows for the production of different proteins from the same gene, depending on tissue-specific needs.

  • Stability of mRNA in the cytosol can influence protein production longevity.

Protein Lifespan

  • Proteins have variable lifespans, on average lasting around 7 hours, significantly impacting cellular functions and potential implications for diseases like cancer due to uncontrolled growth.

Additional Case Studies

  • Discusses impact of vitamin A and iron deficiency on gene expression and cell differentiation crucial for functions like hemoglobin production.

  • Explores mechanisms by which abnormal gene expression leads to diseases such as cancer.