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 strands can be joined again by which of the following enzymes? 
Select and explain. 
A. DNA polymerase, B. DNA ligase, C. DNA topoisomerase, D. DNA gyrase
B: DNA ligase
Answer: B: DNA ligase 
Explanation: DNA is cleaved into fragments by restriction endonucleases. After the fragments of DNA have base paired, the ends are covalently joined by the action of DNA ligase. Restriction enzymes in conjunction with DNA ligase can produce vector containing recombinant or hybrid or chimeric DNA. Ref: Essentials of Biochemistry By Naik, 2012, Page 335.