This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Replication, Subject name: Biochemistry
Supercoiling occurs in : 
Select and explain. 
A. Only Eukaryotes, B. Only Prokaryotes, C. Both, D. Only viruses
C: Both
Answer: C: Both 
Explanation: Supercoiling occurs in both Prokaryotes & Eukaryotes. Supercoiling is because of dsDNA & dsDNA is present in Both Prokaryotes & Eukaryotes. Supercoiling means a segment of dsDNA is wound nearly twice, causing supercoiling. Eukaryotes have closed, circular, dsDNA molecules in their mitochondria, as do plant chloroplasts. A prokaryotic organism typically contains a single, circular, dsDNA molecule. [Note: Circular DNA is "supercoiled", that is, the double helix crosses over on itself one or more times. Supercoiling can result in over winding (positive supercoiling) or under winding (negative supercoiling) of DNA.