Which statement best describes exons?

• A. The coding regions of a gene.
• B. The noncoding introns.
• C. Regions that regulate expression.
• D. The promoters that start transcription.

Answer: (A)

Exons are the coding portions of a gene that remain in the mature mRNA and are translated into protein. In the initial gene transcript, exons and introns are both present, but during RNA processing the introns are removed and the exons are joined together to form the continuous coding sequence used to make the protein. The other options describe parts of the gene that aren’t coding sequences: introns are noncoding segments that are spliced out; regulatory regions control when and where a gene is expressed; promoters are where transcription begins but are not part of the coding sequence.