Gene Regulation in Prokaryotic vs. Eukaryotic Cells

Prokaryotic Gene Regulation

• Regulation at transcription level
• Example: Lac operon
  • Uses transcription factors like CAP or repressor proteins
  • Activated or repressed under different environmental conditions

Eukaryotic Gene Regulation

• More complex than prokaryotes
• Multiple mechanisms not present in prokaryotes
• Involves regulation from transcription to protein degradation

Gene Products

• Genes encode proteins or functional RNA
• Regulation affects protein presence and functionality
• Can happen at multiple stages:
  • Transcription
  • Translation
  • Protein lifespan and structure

Constitutive Expression vs. Regulated Expression

• Housekeeping Genes:
  • Constitutively expressed (e.g., glycolytic enzymes, cytoskeleton components)
  • About 4,000 out of 20,000 human genes
• Regulated Genes:
  • Expressed variably depending on cell type and environment
  • 70% regulated at transcription initiation

Mechanisms of Regulation

• Alternative Splicing:
  • Produces different proteins from the same gene
• Nuclear Export Regulation:
  • mRNA export is regulated and signal-dependent
• mRNA Stability:
  • Varies; can be rapidly degraded or have a long half-life
• mRNA Localization and Translation Regulation:
  • mRNAs can be transported and tethered in specific locations
  • Interaction with ribosome regulates translation initiation

Regulatory Sequences

• In untranslated regions of mRNA
• Bind RNA-binding proteins for regulation

Protein Level Regulation

• Processing:
  • Proteins may be inactive until cleaved
• Degradation:
  • Example: Cyclin protein degradation during mitosis

Transcription Initiation in Eukaryotes

• Much regulation at this step
• Different cell types express different genes due to regulatory transcription factors

Regulatory vs. General Transcription Factors

• General Transcription Factors:
  • Required for any gene expression
  • Assemble at promoter, recruit RNA polymerase
• Regulatory Transcription Factors:
  • Bind to specific sequences, influence gene-specific expression
  • Activation is more common than repression
• Combinatorial Code:
  • Combination of transcription factors regulate specific genes

Chromatin and Gene Expression

• Chromatin Structure:
  • DNA wrapped around histone proteins
  • Euchromatin (open) vs. Heterochromatin (closed)
  • Dynamic changes can regulate gene expression

DNA Methylation

• Cytosine Methylation:
  • Long-term or permanent gene silencing
  • Recruitment of proteins that remodel chromatin

Histone Modification

• Histone Acetylation (HATs):
  • Leads to less condensed chromatin, more transcription
• Histone Deacetylation (HDACs):
  • Leads to more condensed chromatin, less transcription
• Recruitment by Transcription Factors:
  • Regulate through chromatin remodeling

Summary

• Eukaryotic gene regulation involves complex interplay of factors
• Regulation occurs at multiple stages, with chromatin structure playing a key role
• Regulation is essential for differential gene expression in various cell types