DNA Transcription Lecture Notes

Introduction

• Purpose: Understanding DNA transcription.
• Definition: Conversion of DNA to RNA.
• Requirements: Proteins/enzymes to facilitate the process.

Transcription Differences Between Prokaryotic and Eukaryotic Cells

Prokaryotic Cells

• Enzyme Required: RNA polymerase holoenzyme.
  • Components:
    • Core enzyme: 2 alpha, beta, beta prime, omega subunits.
    • Sigma subunit for DNA binding.
• Promoter Region:
  • Allows RNA polymerases and transcription factors to bind.
• RNA Types Produced: mRNA, rRNA, tRNA by one RNA polymerase.

Eukaryotic Cells

• Multiple RNA Polymerases:
  • RNA Polymerase I: Produces rRNA.
  • RNA Polymerase II: Produces mRNA and snRNA.
  • RNA Polymerase III: Produces tRNA and small amounts of other RNAs.
• Promoter Regions:
  • TATA Box, CAAT Box, GC Box.
• General Transcription Factors Required:
  • Help RNA polymerases bind to DNA.

Key Processes in Transcription

Initiation

• Prokaryotic:
  • RNA polymerase holoenzyme binds to promoter.
• Eukaryotic:
  • RNA polymerase II and transcription factors bind to promoter.

Elongation

• Process:
  • RNA polymerase reads DNA template (3’ to 5’) and synthesizes RNA (5’ to 3’).
• DNA Strand Types:
  • Template (antisense) strand.
  • Coding (sense) strand.

Termination

• Prokaryotic Mechanisms:
  • Rho-dependent: Rho protein displaces RNA polymerase.
  • Rho-independent: Formation of hairpin loop in RNA.
• Eukaryotic Mechanism:
  • Polyadenylation signal triggers enzyme-mediated RNA cleavage.

Post-Transcriptional Modifications (Eukaryotic Cells Only)

5’ Capping

• Enzyme: RNA triphosphatase, guanyl transferase, methyltransferase.
• Purpose:
  • Initiates translation.
  • Prevents degradation.

3’ Poly-A Tail

• Enzyme: Poly-A polymerase.
• Purpose:
  • Initiates translation.
  • Prevents degradation.
  • Aids in nuclear export.

Splicing

• Components: snRNPs (snurps).
• Process:
  • Removal of introns.
  • Joining of exons.
• Errors: Can lead to diseases like spinal muscular atrophy, beta-thalassemia.

Regulation of Transcription

Eukaryotic Gene Regulation

• Enhancers: Increase transcription rate.
• Silencers: Decrease transcription rate.
• Specific Transcription Factors: Bind enhancers and silencers, altering DNA conformation.

Alternative RNA Splicing

• Concept: Generates protein variants from a single gene.
• Examples:
  • Immunoglobulins.
  • Dopamine receptors.
  • Tropomyosin variants in muscle types.

RNA Editing

• Example: Apo B protein differentiation.
  • Process: Cytidine deaminase edits CAA to UAA, creating a stop codon.
  • Result:
    • Apo B100 produced in hepatocytes.
    • Apo B48 produced in enterocytes.

Conclusion

• Major Takeaways:
  • Differences in transcription between cell types.
  • Importance of RNA polymerases and transcription factors.
  • Role of post-transcriptional modifications in mRNA maturation.