In the preface to The Double Helix, James Watson wrote of his intuition that DNA's structure would be "simple as well as pretty." While the double-helical molecule turned out to be elegant indeed, its structure poses complex problems. For example, how does DNA undergo replication - which involves unraveling and supercoiling - without tying itself into a mass of knots and tangles?

The solution to DNA's topological problems comes in the form of enzymes called topoisomerases that serve as "swivels" and allow the molecule to perform its requisite acrobatics. Type I topoisomerase breaks one strand of the DNA, and Type II topoisomerase breaks both strands of the helix.

Since 1980 there has been a notable increase in research on topoisomerases. The number of 1988 papers indexed in the Institute for Scientific Information's on-line database SciSearch having the word topoisomerase and variants in their titles was 10 times the number indexed...

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