Why are 5' caps and 3' poly(A) tails important for eukaryotic mRNA?

• A. They anchor mRNA to ribosome
• B. The 5' cap and 3' poly(A) tail protect from degradation, aid export, and enhance translation
• C. They signal transcription termination
• D. They are required for transcription initiation

Answer: (B)

They serve to protect mRNA and regulate its use after transcription. The 5' cap is added early and shields the 5' end from exonucleases, helps export the transcript from the nucleus, and provides a docking site for translation initiation factors, which makes ribosome recruitment efficient. The 3' poly(A) tail protects the RNA from 3' → 5' degradation, aids export by interacting with export proteins, and enhances translation by promoting a closed-loop structure with initiation factors that improves ribosome recycling and overall translation efficiency. The tail’s length also influences how long the message remains stable in the cytoplasm. These features are processing outcomes, not signals for transcription termination or initiation, and they’re specific to eukaryotic mRNA processing.