⚡ Core Concept: Post-Transcriptional Gene Silencing (PTGS) via Small RNAs
Discovered by Andrew Fire and Craig Mello in 1998 in Caenorhabditis elegans (Nobel Prize 2006), RNA Interference (RNAi) is an evolutionary conserved post-transcriptional gene silencing mechanism. Small non-coding double-stranded RNAs (20–24 nucleotides, including miRNAs and siRNAs) guide the RNA-Induced Silencing Complex (RISC) containing Argonaute endonucleases to sequence-specifically degrade target mRNAs or repress their translation.
1. Biogenesis Pathway of MicroRNAs
- Transcription: RNA Polymerase II transcribes primary microRNA (pri-miRNA) displaying a stem-loop hairpin with 5′ cap and poly-A tail.
- Nuclear Processing: The Microprocessor complex (the RNase III endonuclease Drosha + DGCR8 cofactor) cleaves the flanking ssRNA arms to release a ~70-nt pre-miRNA hairpin.
- Nuclear Export: Exportin-5 in complex with Ran-GTP transports pre-miRNA across the nuclear pore complex into the cytoplasm.
- Cytoplasmic Cleavage: The cytoplasmic RNase III enzyme Dicer cleaves the terminal loop, generating a ~22-bp miRNA/miRNA* duplex.
- RISC Assembly: The duplex is loaded onto an Argonaute (Ago) protein. The passenger strand (miRNA*) is discarded, while the guide strand is retained in the active RISC.
Figure 1: MicroRNA Biogenesis and RISC-Mediated Gene Silencing
RNA Biology
Nuclear processing of pri-miRNA by Drosha/DGCR8, Exportin-5 transport across the nuclear pore, Dicer processing in the cytoplasm, and incorporation into the Argonaute-containing RISC complex.
2. Comparison: MicroRNA (miRNA) vs. Small Interfering RNA (siRNA)
| Attribute | MicroRNA (miRNA) | Small Interfering RNA (siRNA) |
|---|---|---|
| Origin | Endogenous (transcribed from non-coding genes or introns in genomic DNA). | Primarily exogenous (viral RNA infection, synthetic laboratory introduction). |
| Precursor Structure | Single-stranded RNA folding into a hairpin stem-loop with imperfect base-pairing. | Long, fully complementary double-stranded RNA (dsRNA). |
| Target Specificity | Imperfect pairing with target 3′ UTR (exact match required only at 2–8 nt seed region); one miRNA regulates hundreds of genes. | 100% perfect Watson-Crick sequence complementarity to target mRNA. |
| Primary Mechanism | Translational repression and mRNA deadenylation / decay. | Direct endonucleolytic cleavage (slicing) of target mRNA by Ago2. |
🎓 High-Yield Exam Checkpoint
- Seed Sequence: Nucleotides 2–8 at the 5′ end of the mature miRNA represent the critical “seed” sequence determining target mRNA 3′ UTR recognition.
- Therapeutic Applications: Patisiran (Onpattro) was the first FDA-approved RNAi drug (2018), utilizing lipid nanoparticles delivering siRNA to silence mutant transthyretin (TTR) in hereditary amyloidosis.
