Final answer:
The question addresses eukaryotic mRNA processing, where introns are removed from pre-mRNA through splicing led by spliceosomes, followed by the addition of a 5' methylguanosine cap and poly-A tail to protect and transport the mature mRNA from the nucleus to the cytoplasm for protein synthesis.
Step-by-step explanation:
The question is addressing the coupling of processes within eukaryotic mRNA processing, focusing on mRNA splicing and export, and the role of Ref1 and the Exon Junction Complex (EJC). Splicing involves the precise removal of non-coding regions called introns from the primary mRNA transcript via protein complexes called spliceosomes, which include small ribonuclear proteins (snRNPs).
After splicing, eukaryotic pre-mRNAs receive a 5' methylguanosine cap and a poly-A tail, which protect the mRNA from degradation and facilitate its export from the nucleus to cytoplasm. The accuracy of splicing is critical; a single nucleotide error can lead to a dysfunctional protein. The spliced, capped, and polyadenylated mRNAs are recognized by nuclear transport receptors that help in the export process, ensuring only properly processed mRNAs reach the cytoplasm for protein synthesis.
Within eukaryotes, the process of mRNA splicing and export is intertwined. Splicing must be completed before mRNA can be exported, as only mRNAs that have been correctly spliced and processed can be effectively transported out of the nucleus. The inclusion of a methylguanosine cap and poly-A tail indicates the mRNA is mature and ready for export, where various proteins including Ref1 aid in recognition and transport through nuclear pores.