In eukaryotic cells, the initial RNA transcript synthesized from DNA is known as heterogeneous nuclear RNA (hnRNA) or pre-mRNA. Before it can function as messenger RNA (mRNA), hnRNA must undergo several post-transcriptional modifications to ensure stability, nuclear export, and translational efficiency.
Steps of RNA Processing
| Step | Process | Location | Key Enzymes / Features | Clinical Relevance |
|---|---|---|---|---|
| 1. 5′ Capping | Addition of 7-methylguanosine cap to the 5′ end (co-transcriptional) | Nucleus | Guanylyltransferase, methyltransferase | Protects mRNA from degradation, assists ribosome binding |
| 2. 3′ Polyadenylation | Addition of a poly-A tail (~200 adenines) to the 3′ end | Nucleus | Poly-A polymerase (no template needed) | Mutation in AAUAAA signal → early mRNA degradation |
| 3. Splicing | Removal of introns and joining of exons | Nucleus | snRNPs (spliceosomes) | Defects → abnormal mRNA and disease (e.g., β-thalassemia) |
| 4. Transport | Mature mRNA exists nucleus → cytoplasm | Nuclear pores | — | Ready for translation in the cytoplasm |
Cytoplasmic Quality Control
After export, mRNA undergoes quality checks in processing bodies (P-bodies).
These contain:
- Exonucleases
- Decapping enzymes
- MicroRNAs
mRNAs in P-bodies may be stored for later translation or degraded if they are faulty.
Key Regulatory Sequences
| Sequence | Function | Clinical Note |
|---|---|---|
| AAUAAA (Polyadenylation Signal) | Signals poly-A tail addition | Mutation → unstable mRNA |
| Kozak Sequence (Surrounds AUG) | Aids ribosomal binding for initiation | Mutation → ↓ translation efficiency → ↓ protein synthesis |
Key Points to Remember
- hnRNA → mRNA conversion occurs in the nucleus
- All mature mRNAs are 5′ capped, 3′ polyadenylated, and spliced
- Poly-A polymerase adds the tail without a DNA template
- Kozak sequence helps initiate translation (analogous to Shine-Dalgarno in prokaryotes)
- P-bodies manage mRNA turnover and silencing in the cytoplasm
Learning Objective
By the end of this session, students should be able to:
Describe the major post-transcriptional modifications of eukaryotic hnRNA and their biological and clinical significance, including the role of Kozak and polyadenylation sequences.
