This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Metabolism of nucleic acids, Subject name: Biochemistry
Intron is not found in which DNA? 
Select and explain. 
A. Nuclear DNA, B. Mitochondrial DNA, C. B DNA, D. Z DNA
B: Mitochondrial DNA
Answer: B: Mitochondrial DNA 
Explanation: Introns enable alternative splicing, which enables a single gene to encode multiple proteins that perform different functions under different conditions. For example, a signal the cell receives could cause an exon that is normally included to be skipped, or an intron that is normally spliced out to be left this would not be possible, would not be possible, or at least would be much more difficult, without the presence of introns.