Human Type I DNA Topoisomerase: Regulation of Expression and Function

The replication of DNA requires two fundamental steps, namely an unwinding of the parental DNA double helix and the utilization of the emerging single strand sequences as templates for the synthesis of progeny DNA strands. The unwinding of the double helix is catalyzed by DNA helicases breaking the hydrogen bonds between complementary DNA strands at the expense of ATP hydrolysis (reviewed by Matson and Kaiser-Rogers, 1990). Inevitably, this reaction causes topological strain in circular or long linear DNA molecules. This strain is due to alterations in the helical twist and writhe of the double helix and must be relaxed by DNA topoisomerases as an essential condition for the progression of the replication fork.