M03.03.006 Production of Eukaryotic Messenger RNA

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Learning Objective: Describe the structure, transcription, and processing of eukaryotic mRNA, including promoter recognition, transcription by RNA polymerase II, and post-transcriptional modifications such as capping, splicing, and polyadenylation.

Eukaryotic protein-coding genes contain exons (coding regions) interrupted by introns (non-coding regions). Both are transcribed into a primary transcript (hnRNA), which must undergo several processing steps to produce mature mRNA. All eukaryotic mRNA is monocistronic, meaning each mRNA encodes one protein. The mature mRNA is exported from the nucleus to → cytoplasm for translation.

Activity

Transcription of a Typical Eukaryotic Gene

1. Promoter Recognition (Initiation)

  • Transcription begins when RNA Polymerase II (RNAP II) binds to the promoter with transcription factors (e.g., TFIID).
  • Two key basal promoter elements:
    • TATA (Hogness) Box at ~–25
    • CAAT Box at ~–70
  • RNAP II unwinds a small region of DNA and begins synthesizing RNA.

2. Elongation

  • RNAP II reads the template DNA strand 3′→5′.
  • Synthesizes the primary transcript (hnRNA) 5′→3′.
  • Both exons and introns are transcribed.

3. Termination

  • Eukaryotic termination signals are not fully understood.
  • RNAP II eventually releases the transcript.

Activity

Processing of Eukaryotic Pre-mRNA (hnRNA)

1. 5′ Capping

  • A 7-methylguanosine (m⁷G) cap is added to the 5′ end during transcription.
  • Functions:
    • Protects RNA from degradation
    • Required for ribosome binding

2. Polyadenylation

  • A poly-A tail (~200 adenines) is added to the 3′ end.
  • Enhances:
    • Stability
    • Nuclear export
    • Translation efficiency

3. Splicing (Removal of Introns)

  • Performed by spliceosomes (snRNPs or snurps).
  • Introns are removed as lariat structures.
  • Exons joined to form the continuous coding sequence.

Clinical link:

Mutations in β-globin splice sites → β-thalassemia.

4. hnRNA Intermediates

  • All partially processed transcripts before final mRNA are collectively called hnRNA.

5. Export to Cytoplasm

  • After processing, mature mRNA is transported through the nuclear pore to the cytoplasm for translation.

Activity

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