Question

Which of the following statements about topoisomerases is FALSE?

Type I AND type II topoisomerases can both increase or decrease L.

In the reaction catalyzed by type I topoisomerases, the enzyme changes conformation to provide the energy required to change the linking number of the DNA.

Type I topoisomerases use a tyrosine side chain in their active site to catalyze a nucleophilic attack.

Type I topoisomerases do not use ATP in their reaction mechanism.

Type II topoisomerases can increase writhe because they use ATP in their reaction mechanism.

Answer 1

Topoisomerases are the enzymes which initiate and help in the unwinding of the intertwined double helix of the DNA strands. During DNA replication and transcription, DNA becomes overwound ahead of the replication fork.

Topoisomerases bind to DNA and cut the phosphate backbone of either one or both the DNA strands. This break allows the DNA to untangle and at the end of these processes, the DNA backbone is sealed again. All through this, the overall chemical composition and connectivity of the DNA does not change. This has given the name Topoisomerases to these enzymes.

Among the above statements, the only false statement is the second one i.e. in the reaction catalysed by topoisomerase I , the enzyme changes conformation to provide the energy required to change the linking number of the DNA.

All the other statements are true. Topo I is energy independent except the gyrase. Topo II requires energy to cleave both the strands of the DNA, which it obtains by hydrolysis of ATP. In Topo I the energy is obtained from the torque created when one strand of the DNA is cleaved.

As for changing the L, Topo II can increase or decrease the number by a unit of 2 while Topo I does so by unit of 1 or 1[n].