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
After digestion by restriction endonucleases, DNA strands can be joined again by 
Select and explain. 
A. DNA polymerase, B. DNA ligase, C. DNA topoisomerase, D. DNA gyrase
B: DNA ligase
Answer: B: DNA ligase 
Explanation: In mammals, after many Okazaki fragments are generated, the replication complex begins to remove the RNA primers, to fill in the gaps left by their removal with the proper base-paired deoxynucleotide, and then to seal the fragments of newly synthesized DNA by enzymes referred to as DNA ligases. DNA ligase Seals the single strand nick between the nascent chain and Okazaki fragments on the lagging strand.Ref.: Harpers Illustrated Biochemistry; 30th edition; page no: 383