Which feature best distinguishes mRNA processing in eukaryotes from prokaryotes?

Explanation

## Eukaryotic vs Prokaryotic mRNA Processing ### Key Structural Differences **Key Point:** The most definitive discriminator between eukaryotic and prokaryotic mRNA is the presence of post-transcriptional modifications in eukaryotes: 5' 7-methylguanosine cap and 3' poly(A) tail. ### Comparative Table | Feature | Eukaryotes | Prokaryotes | |---------|-----------|-------------| | **5' Cap** | Present (7-methylguanosine) | Absent | | **3' Poly(A) tail** | Present (~200 adenines) | Absent | | **Introns** | Present (require splicing) | Absent | | **Transcription-Translation coupling** | Spatially separated (nucleus/cytoplasm) | Coupled (cytoplasm) | | **mRNA stability** | Hours to days | Minutes | | **Promoter elements** | TATA box, CAAT box, GC box | −10 (Pribnow) and −35 boxes | ### Why These Modifications Matter 1. **5' Cap function:** - Protects mRNA from degradation by 5' exonucleases - Facilitates ribosome binding and translation initiation - Aids in mRNA export from nucleus 2. **3' Poly(A) tail function:** - Protects from 3' exonuclease degradation - Enhances translation efficiency - Increases mRNA half-life **High-Yield:** The cap and tail are added *after* transcription in eukaryotes; prokaryotic mRNAs lack both and are translated while still being transcribed. **Clinical Pearl:** Defects in capping or polyadenylation enzymes lead to mRNA instability and can cause genetic diseases; this principle is exploited in antisense oligonucleotide therapy (e.g., nusinersen for SMA). ### Why Other Options Are Incorrect - **Option 1 (coupling):** Prokaryotes couple transcription-translation; eukaryotes do not. This is a difference, but the cap/tail is the most specific structural marker. - **Option 2 (introns):** While eukaryotes have introns and prokaryotes do not, the question asks about *mRNA processing*, and the cap/tail are the hallmark *processed* features. - **Option 3 (sigma factor):** Prokaryote-specific; not a discriminator of mRNA itself.