What features constitute the eukaryotic mRNA processing pathway and why is it important?

• A. 5' capping; splicing; 3' polyadenylation; required for mature mRNA export and translation
• B. mRNA processing is unnecessary in eukaryotes
• C. Only 5' capping occurs; splicing does not
• D. 3' polyadenylation inhibits export

Answer: (A)

Eukaryotic mRNA must be processed after transcription to become stable, export-ready, and translatable. The three key features are 5' capping, splicing, and 3' polyadenylation. The 5' cap is added early to the nascent transcript and serves to protect the RNA from 5' exonucleases and to recruit the cap-binding complex, which is essential for efficient translation initiation in the cytoplasm. Splicing removes introns and joins exons via the spliceosome, producing a mature, contiguous coding sequence and also influencing mRNA export and regulation. The 3' poly(A) tail is added after cleavage by poly(A) polymerase; this tail protects the mRNA from degradation, facilitates export through interactions with poly(A)-binding proteins, and, together with translation factors, promotes efficient translation by helping circularize the mRNA.

Together, these processing steps are required for mature mRNA to be exported from the nucleus and efficiently translated. The other statements misrepresent the process: processing is not unnecessary, splicing does occur, and polyadenylation supports rather than inhibits export.