Antitumour drugs impede DNA uncoiling by topoisomerase I

Opoisomerases are enzymes that act to relax supercoiling, a form of built-up strain in DNA - see one of our previous covers for Nature in 2005. This is a prime example of what these enzymes can actually do: as illustrated on the cover, positive DNA supercoils accumulate due to the action of the drug (shown red) Such overwinding of the DNA hinders an advancing DNA polymerase and may play a role in the stalling or collapse of a replication fork, ultimately leading to cell death. This may be an effective way to stop unwanted cell replication.

A nice detail is the background: this is an actual image of the yeast used by the researchers in the lab. We love to incorporate the real imagery as much as possible in the final result.