Which statement describes modifications to mRNA before it leaves the nucleus?

• A. Only a 5' cap is added; no splicing occurs.
• B. Introns are excised, exons are spliced together, a 5' cap is added, and a poly(A) tail is added to the 3' end.
• C. Ribosomes add amino acids to mRNA during transcription.
• D. Introns are translated into protein.

Answer: (B)

In eukaryotes, mRNA undergoes processing in the nucleus before export. The main steps are adding a 5' cap, splicing out introns and joining exons, and adding a poly(A) tail at the 3' end. The 5' cap protects the transcript and helps ribosomes recognize it for translation later. Splicing removes noncoding introns and links the coding exons together, which can also generate different protein isoforms through alternative splicing. The poly(A) tail increases mRNA stability and aids in export from the nucleus and efficient translation. Translation, where amino acids are added to form a protein, happens in the cytoplasm after the mRNA has left the nucleus. Introns are not translated; they’re removed during processing. The combination of these processing events described here is exactly what happens to mRNA before it leaves the nucleus.