Lecture on DNA Transcription

Jun 7, 2024

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DNA Transcription Lecture Notes

Lecture Introduction

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Definition of Transcription

  • Basic Definition: Transcription is converting DNA to RNA.
  • Organisms: Occurs in both eukaryotic (e.g., human cells) and prokaryotic cells (e.g., bacteria).
  • Essential Enzymes: RNA polymerases and transcription factors.

Promoter Region

  • Definition: Nucleotide sequence in DNA where proteins bind to start transcription.
  • Key Proteins: RNA polymerases and transcription factors.

Prokaryotic Transcription

  • Key Enzyme: RNA polymerase holoenzyme (core enzyme + sigma factor).
  • Core Enzyme Subunits: Two alpha units, Beta, Beta prime, Omega unit.
  • Sigma Factor: Binds to promoter, enables core enzyme to transcribe DNA.
  • RNA Types Produced: mRNA, tRNA, rRNA all by a single RNA polymerase.

Eukaryotic Transcription

  • Multiple Polymerases: RNA Polymerase I, II, III.
  • Requirement: Each type requires specific general transcription factors.
  • Types of RNA Produced:
    • RNA Polymerase I: rRNA
    • RNA Polymerase II: mRNA, snRNA
    • RNA Polymerase III: tRNA, snRNA, and some rRNA

Eukaryotic Gene Regulation

  • Enhancers: DNA sequences increasing transcription rate through specific transcription factors.
  • Silencers: DNA sequences decreasing transcription rate.
  • Mechanism: Specific transcription factors bind enhancers/silencers, modify DNA conformation, influencing promoter regions.

Initiation of Transcription

  • Stages:
    • Prokaryotes: Promoter (-35, -10, +1 regions), RNA polymerase holoenzyme.
    • Eukaryotes: Promoter (TATA box, CAAT box, GC box), RNA Polymerase II, Transcription factor IID.

Elongation Phase

  • Mechanism: RNA polymerase reads DNA from 3' to 5' and synthesizes RNA from 5' to 3'.
  • Functionality: RNA polymerase opens, stabilizes, and unwinds DNA, potentially lacking proofreading functions.

Termination Phase

  • Prokaryotic Termination:
    • Rho-Dependent: Rho protein detaches RNA polymerase from DNA.
    • Rho-Independent: Formation of hairpin loops in RNA triggers detachment.
  • Eukaryotic Termination: Polyadenylation signal (AAUAAA) activates cleavage enzymes.

Post-Transcriptional Modification

  • Occurs in Eukaryotes: Converts hnRNA to mature mRNA.
  • 5' Capping: Addition of a 7-methylguanosine cap (RNA triphosphatase, guanylyl transferase, methyl transferase).
  • 3' Polyadenylation: Addition of poly(A) tail by poly(A) polymerase.
  • Splicing: Removal of introns, joining exons by snRNPs (snurps).
  • Alternative Splicing: Produces different mRNA variants from the same hnRNA.
  • RNA Editing: Modifying RNA sequences (e.g., ApoB100 -> ApoB48 by cytidine deaminase).

Cell-Specific Examples and Relevance

  • Gene Regulation: Enhancers and silencers modulate transcription rates.
  • RNA Polymerase Inhibitors: Amanitin (eukaryotes), Rifampicin (prokaryotes).
  • Diseases Related to Splicing Errors: Spinal muscular atrophy (SMA), Beta thalassemia due to improper splicing.

Conclusion

  • Main Takeaway: Understanding the process and regulation of transcription is crucial for grasping genetic expression and related disease mechanisms.

End Note: Encourages engagement with future content on transcription and gene regulation.