DNA twists and flips

摘要

DNA can shape itself into many forms to achieve its purposes in life. The crystal structure of the junction between two of its forms provides insight into how DNA might accomplish some of these acrobatics. Crick and Watson's famous structure of DNA has become an icon of our age, somewhat eclipsing DNAs other structural forms. Imagine unravelling the familiar helix (termed B-DNA) and then twisting it up the other way around. The result — Z-DNA — is not so visually appealing, and not so stable, but it has some intriguing biology. On page 1183 of this issue, Ha et al.' present the X-ray crystal structure of a junction between B- and Z-DNA. They show that a single base pair is broken and flipped out of the DNA double helix, thereby facilitating the juxtaposition of a left-handed Z-DNA helix with the right-handed B-DNA helix. This deceptively simple observation, however, has ramifications for DNA structure, the dynamics of DNA structural transitions and Z-DNA biology. Natures design of the B-Z junction maximizes helix integrity and base pairing, thereby minimizing the impact of the dramatic alteration in helix structure where Z-DNA forms in a chromosome.