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Learning Goals

• Identify key sequences involved in transcription.
• Understand the difference between transcription in prokaryotes and eukaryotes.
• Describe co-transcriptional processing of eukaryotic mRNA.

Overview of Transcription

• Transcription: The process of synthesizing RNA from a DNA template, crucial for gene expression.
  • Involves the formation of mRNA from DNA.
  • Key phases of transcription: Initiation, Elongation, and Termination.

Transcription Process in Prokaryotes

• Components of Transcription:
  • Template Strand: DNA strand used as a guide to synthesize mRNA.
  • Coding Strand: DNA strand with the same sequence as the resulting mRNA (except for thymine (T) replaced by uracil (U) in RNA).
  • Promoter: DNA sequence where RNA polymerase binds to initiate transcription.

Key Sequences in Prokaryotic Promoters

• Consensus Sequences:
  • -35 Sequence: TTGACA
  • -10 Sequence: TATAAT (also known as the Pribnow box).
• Location of sequences:
  • -35 and -10 are usually around 10 base pairs upstream of the transcription initiation site (denoted as +1).

Phases of Prokaryotic Transcription

1. Initiation:

   • RNA polymerase binds to the promoter region with the help of a sigma factor.
   • Formation of the open promoter complex.
   • Transcription starts at the initiation site.

2. Elongation:

   • RNA polymerase travels along the DNA, unwinding the double helix and synthesizing mRNA by adding complementary RNA nucleotides.
   • Multiple mRNA transcripts can be made simultaneously from a single gene.

3. Termination:

   • Occurs when RNA polymerase encounters a termination sequence characterized by an inverted repeat followed by a run of adenines (A's).
   • Formation of a hairpin loop in the mRNA, signaling the end of transcription.

Transcription Process in Eukaryotes

• Eukaryotic transcription occurs in the nucleus and involves more complexity due to the presence of introns and exons.

Eukaryotic RNA Polymerases

• RNA Polymerase I: Transcribes rRNA (ribosomal RNA).
• RNA Polymerase II: Transcribes mRNA (messenger RNA).
• RNA Polymerase III: Transcribes tRNA (transfer RNA) and other small functional RNAs.

Key Sequences in Eukaryotic Promoters

• Promoter regions contain the TATA box (a consensus sequence) located about 30 base pairs upstream of the transcription initiation site.
• Other consensus elements include:
  • GC-rich box
  • CAAT box

Phases of Eukaryotic Transcription

1. Initiation:

   • Binding of the TATA binding protein to the TATA box.
   • Formation of a pre-initiation complex, including other transcription factors and RNA Polymerase II.

2. Elongation:

   • RNA Polymerase II separates from general transcription factors and synthesizes mRNA.

3. Termination:

   • Involves a specific termination signal sequence (e.g., AAUAAA) recognized by an endonuclease that cleaves the RNA.
   • The addition of a poly(A) tail occurs after the cleavage for mRNA protection and transport.

Co-Transcriptional Processing of Eukaryotic mRNA

• After transcription, eukaryotic mRNA undergoes several modifications:

  1. 5' Cap Addition: Protects the mRNA from degradation and is involved in ribosome recognition during translation.
  2. Polyadenylation: Addition of a poly(A) tail at the 3' end for stability and export from the nucleus.
  3. Splicing: Removal of introns and joining of exons to form a continuous coding sequence.

• The final mRNA product is then transported through nuclear pores to the cytoplasm for translation into proteins.

Summary of Differences: Prokaryotic vs Eukaryotic Transcription

• Location: Prokaryotes (cytoplasm), Eukaryotes (nucleus).
• RNA Polymerases: Prokaryotes have one, while eukaryotes have three.
• mRNA Processing: More complex in eukaryotes, including capping, polyadenylation, and splicing.
• Promoter Structure: Different sequences and upstream elements involved; eukaryotes' TATA box is farther from the start site than the Pribnow box in prokaryotes.