Topoisomerases are the enzymes that wind and unwind the DNA that makes up the chromosomes. These enzymes work by altering the linking number of DNA strands in a 3-step process:
  1. they break (cleave) one or both DNA strands
  2. they pass a DNA segment through the break
  3. they reseal the break.
Topoisomerases are important because the chromosomes must be unwound in order for the cell to use the genetic information to synthesize proteins and to undergo replication, recombination and transcription.

Thus, compounds like camptothecin that inhibit topoisomerase are useful in treating cancer. Topoisomerase inhibitors keep the chromosomes wound tight (the DNA is said to be supercoiled), and so the cell can't make proteins. As a result, the cell stops growing. Because cancer cells grow and reproduce at a much faster rate than normal cells, they are more vulnerable to topoisomerase inhibition than are normal cells.

This is based on work I did for the science dictionary at