ICE Bioassay: Isolating In Vivo Complexes of Enzyme to DNA

DNA topoisomerases I and II (topo I and topo II) are ubiquitous nuclear enzymes that regulate DNA topology by breaking and resealing one or both strands of a DNA duplex. Topoisomerase I makes a single-stranded break in a DNA duplex, mediates passage of the intact strand through the break, and then reseals it. Type II topoisomerases create transient breaks in both strands of a duplex, pass an intact DNA segment through that break, and then reseal the cleavage site. This ability of topoisomerases to catalyze concerted breaking and rejoining of DNA strands makes them biologically important enzymes that are involved in crucial cellular processes (1 –3 ).