What are exons?

• A. The DNA nucleotides that code for the protein.
• B. Noncoding regions of DNA.
• C. Segments of DNA that regulate gene expression.
• D. Introns that are removed during RNA processing.

Answer: (A)

Exons are the portions of a gene that are kept in the RNA and ultimately used to make a protein. When a gene is transcribed, the initial transcript includes both coding segments (exons) and noncoding segments (introns). The introns are removed during RNA processing, and the exons are joined together to form mature messenger RNA. This mature mRNA carries the coding sequence that determines the amino acid sequence of the protein, so the DNA nucleotides in exons are the parts that code for the protein. Noncoding regions of DNA include introns and other sequences, segments that regulate gene expression refer to promoters and enhancers, and introns are the parts removed during RNA processing.