What is the role of exons and introns; what is alternative splicing?

• A. Exons encode sequence; introns are removed; alternative splicing generates multiple mRNA isoforms by including or excluding exons
• B. Introns encode protein; exons are removed; alternative splicing reduces gene count
• C. Exons encode noncoding sequences; introns are coding; alternative splicing happens after translation
• D. Introns are promoters; exons are terminators; alternative splicing is unrelated

Answer: (A)

The idea being tested is how a gene’s transcripts are processed and how alternative splicing expands protein diversity. Exons are the parts of a gene that remain in the mature mRNA and usually code for the protein sequence. Introns are transcribed but removed during RNA processing. The spliceosome joins the exons together to form the final mRNA that will be translated.

Alternative splicing lets different exons be included or skipped, or different splice sites used, producing multiple mRNA isoforms from a single gene. This is how one gene can give rise to several related proteins.

That is why the chosen statement fits best: exons encode the sequence, introns are removed, and alternative splicing generates multiple mRNA isoforms by including or excluding exons. The other options misstate the roles of exons and introns or when splicing occurs.