In mRNA processing in eukaryotes, which components are removed and which are joined?

• A. Introns are removed; exons are spliced together
• B. Exons are removed; introns are spliced together
• C. Introns are translated into protein
• D. Exons exit the nucleus

Answer: (A)

In eukaryotic mRNA processing, introns are removed and exons are joined to create a continuous coding sequence. The pre-mRNA contains both introns (noncoding regions) and exons (coding regions); the spliceosome recognizes intron–exon boundaries, removes the introns, and ligates the exons together. The resulting mature mRNA is then capped at the 5' end and polyadenylated at the 3' end, and it is exported from the nucleus to the cytoplasm for translation. Exons do not exit the nucleus separately, and introns are not translated. This arrangement allows a single gene to produce a functional mRNA composed solely of exons, and also enables alternative splicing to generate different proteins from the same gene.