Lecture on DNA Transcription

Introduction

• Topic: DNA Transcription
• Definitions:
  • Transcription: Conversion of DNA to RNA in both eukaryotic and prokaryotic cells.
• Important Elements:
  • Requires proteins/enzymes to facilitate the process.
  • Key differences between transcription in prokaryotic and eukaryotic cells.

Transcription in Prokaryotic Cells

• RNA Polymerase Holoenzyme:
  • Composed of two parts:
    • Core Enzyme: Consists of 5 subunits (2 alpha, beta, beta prime, omega).
    • Sigma Subunit: Binds to promoter region to initiate transcription.
• Promoter Region:
  • Specific nucleotide sequence within DNA.
  • Allows RNA polymerase and transcription factors to bind to DNA.
• RNA polymerase in prokaryotes synthesizes all types of RNAs (rRNA, mRNA, tRNA).

Transcription in Eukaryotic Cells

• RNA Polymerases:
  • RNA Polymerase I: Makes rRNA.
  • RNA Polymerase II: Makes mRNA and small nuclear RNAs (snRNAs).
  • RNA Polymerase III: Makes tRNA and some rRNA.
• Promoter Regions:
  • Different promoters for each RNA polymerase.
• General Transcription Factors:
  • Required for RNA polymerases to bind to DNA.

Gene Regulation

• Enhancers: Increase transcription rate.
• Silencers: Decrease transcription rate.
• Specific transcription factors can bind to enhancers or silencers to modulate gene expression.

Stages of Transcription

1. Initiation:

   • RNA polymerase binds to promoter with the help of transcription factors.
   • In prokaryotes, RNA polymerase holoenzyme and sigma factor are used.
   • In eukaryotes, RNA polymerase II binds with transcription factor 2D.

2. Elongation:

   • RNA polymerase reads the DNA template and synthesizes RNA.
   • Moves 3' to 5' on DNA and synthesizes RNA 5' to 3'.

3. Termination:

   • Prokaryotes:
     • Rho-dependent: Rho protein displaces RNA polymerase.
     • Rho-independent: Formation of a hairpin loop.
   • Eukaryotes:
     • Polyadenylation signal (AAUAAA) triggers cleavage.

Post-Transcriptional Modifications

• 5' Capping:
  • Addition of 7-methylguanosine.
  • Initiates translation and prevents degradation.
• 3' Poly-A Tail:
  • Addition of adenine nucleotides.
  • Initiates translation, prevents degradation, and aids in transport.
• Splicing:
  • Introns removed, exons joined.
  • Performed by snRNPs (snurps).

Alternative RNA Splicing

• Allows for different protein variants from the same gene.
• Example: Antibodies, dopamine receptors, tropomyosin in different muscles.

RNA Editing

• Cytidine Deaminase: Modifies mRNA in enterocytes to create different proteins (e.g., Apo B100 to Apo B48).

This extensive lecture covered all aspects of DNA transcription, including the processes in prokaryotic and eukaryotic cells, gene regulation, stages of transcription, and post-transcriptional modifications. The lecture concluded with examples of alternative splicing and RNA editing, emphasizing their biochemical and clinical significance.