Exons in mRNA code for the amino acid sequence after processing.

• A. They are spliced out
• B. They code for the amino acid sequence
• C. They are never translated
• D. They signal transcription initiation

Answer: (B)

Exons are the parts of a gene that stay in the mature mRNA after processing and are used to build the protein. During RNA processing, introns are removed and the remaining exons are joined together to form the codon sequence that will be translated into amino acids. So exons directly determine the amino acid sequence of the resulting protein. The idea that exons are spliced out would apply to introns, not exons. Saying exons are never translated isn’t correct, since the coding information needed to specify amino acids lies in the exons. Transcription initiation is controlled by promoter and regulatory regions, not by exons themselves.