What are introns and exons?

• A. Introns are noncoding regions removed during processing; exons are coding sequences retained in mature mRNA.
• B. Introns are coding regions; exons are noncoding.
• C. Introns are promoters; exons are enhancers.
• D. Introns and exons refer to telomere regions.

Answer: (A)

In eukaryotic gene expression, the initial RNA transcript contains both exons and introns. Exons are the parts that will stay in the mature mRNA and typically encode the protein sequence. Introns are the intervening sequences that do not code for proteins; they are removed during RNA processing by the spliceosome, and the remaining exons are stitched together to form the mature mRNA that exits the nucleus for translation. So, exons carry the coding information that will be translated, while introns are noncoding segments discarded before the final message is read by the ribosome. The other statements don’t fit because promoters and enhancers are regulatory DNA elements, not the coding/noncoding distinction of RNA transcripts, and telomeres are the ends of chromosomes, not related to intron/exon structure.