Define exon and intron and their roles in gene expression.

• A. Exons are coding sequences retained in mature mRNA; introns are noncoding sequences removed during RNA splicing.
• B. Exons are noncoding; introns are coding.
• C. Exons become introns in mature mRNA.
• D. Exons are promoters; introns are terminators.

Answer: (A)

RNA processing and splicing determine which parts of a gene are kept in the final message used to make protein. The pre-mRNA transcript includes both exons and introns. During RNA processing, the spliceosome removes introns (noncoding sequences) and joins exons (coding sequences) together to form mature mRNA. The exons provide the actual coding information that will be translated into the amino acid sequence of a protein, while introns are removed and do not contribute to the protein sequence (though they can have regulatory roles in expression).

That’s why the statement identifying exons as coding sequences retained in mature mRNA and introns as noncoding sequences removed during RNA splicing correctly describes their roles. The other options misstate the relationships: exons being noncoding, introns coding; exons becoming introns in mature mRNA; or exons and introns defined as promoters and terminators.