DNA Transcription Lecture Notes

Introduction

• DNA Transcription: Conversion of DNA into RNA within cells (both eukaryotic and prokaryotic).
• Importance: Essential for protein synthesis and cellular function.

Transcription in Prokaryotic Cells

Key Components

• Promoter Region: Specific nucleotide sequence in DNA where transcription factors and RNA polymerase bind.
• RNA Polymerase Holoenzyme: Enzyme complex crucial for transcription.
  • Core Enzyme: Composed of 2 alpha, beta, beta-prime, and omega subunits.
  • Sigma Factor: Binds to promoter region to initiate transcription.

Process

• Reads DNA from 3' to 5' to synthesize RNA from 5' to 3'.
• Produces all types of RNA (mRNA, tRNA, rRNA) using a single enzyme.

Transcription in Eukaryotic Cells

RNA Polymerases

• RNA Polymerase I: Synthesizes rRNA.
• RNA Polymerase II: Synthesizes mRNA and snRNA.
• RNA Polymerase III: Synthesizes tRNA and some snRNA, rRNA.

General Transcription Factors

• Required for RNA polymerases to bind and read DNA.
• Facilitate binding to promoter regions (TATA box, CAAT box, GC box).

Gene Regulation

• Enhancers: Increase transcription rate by spatially bringing promoter and transcription machinery closer.
• Silencers: Decrease transcription rate.

Stages of Transcription

Initiation

• RNA polymerases and transcription factors bind to promoter regions.

Elongation

• RNA polymerase synthesizes RNA by reading the template strand.

Termination

• Prokaryotes: Row-dependent or row-independent termination.
• Eukaryotes: Polyadenylation signal triggers cleavage of RNA.

Post-Transcriptional Modification (Eukaryotes Only)

Capping and Tailing

• 5' Cap: Addition of 7-methylguanosine for stability and translation initiation.
• 3' Poly-A Tail: Adds a chain of adenines for stability, translation initiation, and nuclear export.

Splicing

• Removal of introns (non-coding regions) and joining of exons (coding regions) to form mature mRNA.
• SNRPs: Complexes involved in removing introns and splicing exons.

Alternative RNA Splicing

• Produces different mRNA variants from the same gene, leading to different protein isoforms.

RNA Editing

• Specific nucleotide modifications that can alter RNA and resulting protein products.
• Example: Editing of APOB100 to APOB48 in enterocytes via cytidine deaminase.

Key Takeaways

• Prokaryotes vs. Eukaryotes: Different transcription mechanisms and requirements.
• RNA Types and Functions: Connection between structure and role in cells.
• Gene Regulation: Enhancers and silencers play crucial roles in transcription control.
• Importance of Post-Transcriptional Modifications: Essential for functional mRNA and protein synthesis.

Clinical Relevance

• Errors in splicing or transcription can lead to diseases like spinal muscular atrophy and beta-thalassemia.

Remember: RNA polymerases 1, 2, 3 correspond to rRNA, mRNA, and tRNA production, respectively.