The length of the DNA coding region that codes for the toxin is longer than the mRNA used for translation. Explain why.

• A. DNA contains introns; mRNA contains exons
• B. mRNA contains introns
• C. DNA is duplicated
• D. The toxin gene is shorter in DNA

Answer: (A)

The difference comes from introns in DNA. In many genes, especially in eukaryotes, the DNA sequence includes noncoding regions called introns between coding segments (exons). When transcription happens, the initial RNA copy contains both exons and introns, but RNA splicing removes the introns, leaving a mature mRNA that contains only exons. Since the introns are not part of the translated message, the mature mRNA is shorter than the original DNA region that was transcribed. This is why the DNA coding region can be longer than the mRNA used for translation. The other ideas don’t fit because mature mRNA does not include introns, intron presence is not a statement about mRNA containing introns, and simply duplicating DNA or a shorter DNA toxin gene wouldn’t explain the observed length difference.