Catenation
Catenation is the process by which two circular DNA strands are linked together like chain links. This occurs after DNA replication, where two single strands are catenated and can still replicate but cannot separate into the two daughter cells. As type II topoisomerses break a double strand, they can fix this state (type I topoisomerases could do this only if there were already a single-strand nick), and the correct chromosome number can remain in daughter cells. Linear DNA in eukaryotes is so long they can be thought of as being without ends; type II topoisomerases are needed for the same reason.
Read more about this topic: Type II Topoisomerase