This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Biochemistry
After digestion by restriction endonucleases DNA bands can re-joined by which of the following substance? 
Select and explain. 
A. DNA polymerase, B. DNA ligase, C. DNA topoisomerase, D. DNA gyrase
B: DNA ligase
Answer: B: DNA ligase 
Explanation: DNA ligase is used to join the DNA bands formed after digestion by restriction endonuclease. DNA gyrase: Its a special type of topoisomerase, has the unusual propey of being able to introduce negative supercoils into resting DNA. This facilitates the future replication of DNA because the negative supercoils neutralise the positive supercoils introduced during opening of the double helix.