Discussion
Which one of the following enzymes can alter the linking number of covalently closed circular form of DNA ?
Answer: D
The enzyme is Gyrase.
The linking number of a covalently closed circle can change only if a strand is cut, passed through and resealed. DNA gyrase, a type II topoisomerase, cuts both strands, passes a duplex segment through the break and reseals it, introducing negative supercoils and changing the linking number by 2 in each cycle, using ATP.
Primase makes RNA primers and DNA polymerase extends DNA; neither breaks and rejoins a closed circle. Ligase seals nicks, but a covalently closed circle has none to seal.
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