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🧬 Concept: DNA to hnRNA to mRNA

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🔹 1. Overview of the Flow

Gene Expression involves several key steps:

DNA (Gene) → hnRNA (pre-mRNA) → mRNA → Protein

This is part of the Central Dogma of molecular biology:
DNA → RNA → Protein

In eukaryotic cells, transcription produces a heterogeneous nuclear RNA (hnRNA), which must undergo processing to become mature mRNA.

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🔹 2. hnRNA (heterogeneous nuclear RNA)

• Also called pre-mRNA
• Directly transcribed from template DNA in the nucleus
• Contains both exons (coding regions) and introns (non-coding regions)
• Is not yet functional for translation

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🔹 3. Exons and Introns

Component	Description
Exons	Expressed sequences – remain in the final mRNA and code for proteins
Introns	Intervening sequences – non-coding, removed during RNA splicing

🧠 Think: EXons are EXpressed, INtrons are INtervening

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🔹 4. Steps: Processing of hnRNA to mRNA

✅ Step 1: 5′ Capping

• A 7-methylguanosine (m7G) cap is added to the 5′ end
• Protects RNA from degradation and helps ribosome binding

✅ Step 2: 3′ Polyadenylation

• A poly-A tail (~200 adenines) is added to the 3′ end
• Increases stability and facilitates nuclear export

✅ Step 3: Splicing

• Introns are removed, and exons are joined
• Catalyzed by the spliceosome (made of snRNPs and proteins)
• Sometimes alternative splicing occurs, producing different mRNAs from the same gene

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🔹 5. Final mRNA

• The processed mRNA now contains only exons, and is:
  • Capped (5′ m7G)
  • Spliced (no introns)
  • Polyadenylated (3′ poly-A tail)
• It is then exported to the cytoplasm for translation by ribosomes

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🔹 6. Diagram: Summary

DNA (gene with exons & introns)
         ↓ Transcription
     hnRNA / pre-mRNA
         ↓ 5′ cap + splicing + poly-A tail
         ↓
     Mature mRNA (only exons)
         ↓
     Translation → Protein

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🎓 Example

Let’s say a gene has this structure:

DNA:    Exon1 – Intron – Exon2 – Intron – Exon3

After transcription:

hnRNA:  Exon1 – Intron – Exon2 – Intron – Exon3

After processing:

mRNA:   Exon1 – Exon2 – Exon3

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💡 Additional Concepts

🔸 Alternative Splicing

• Same hnRNA can be spliced in different ways
• Leads to multiple proteins from one gene
• Increases protein diversity (seen in brain, immune cells, etc.)

🔸 Spliceosome

• A complex of snRNA + proteins (snRNPs)
• Recognizes splice sites at intron-exon junctions

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🧪 Summary Table

Term	Full Form	Location	Function
hnRNA	Heterogeneous nuclear RNA	Nucleus	Raw RNA transcript with introns
Exon	Expressed region	DNA/mRNA	Codes for protein
Intron	Intervening region	DNA/hnRNA	Removed before translation
mRNA	Messenger RNA	Nucleus → Cytoplasm	Final, translatable RNA

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DNA → hnRNA → mRNA

Transcription & Splicing Animation

Gene Expression Animation

DNA → hnRNA → mRNA