Deoxyribonucleic acid (DNA) is a double helical molecule that stores genetic information. When DNA is damaged, for instance by ultraviolet radiation, often two DNA molecules combine to form a four-way junction as part of the repair process. This four-way DNA junction is also called a "Holliday junction," named after the articulator of the concept. Although its static structures have been known for more than a decade, the dynamics of the Holliday junction have remained a mystery. The difficulty lies with the fact that it is impossible to coax the molecules into one state, much like the impossibility of getting a herd of physicists to march in step like a marching band.

Instead, Sean McKinney, a graduate student in the Department of Physics of the University of Illinois at Urbana-Champaign, used state-of-the-art single-molecule techniques to detect, for the first time, the motions of individual Holliday junction molecules. This work, performed in the laboratory of Professor Taekjip Ha in collaboration with the Lilley laboratory in Scotland, revealed surprising new insights into the molecular mechanisms of the Holliday junction (S.A. McKinney, A.C. Declais, D.M.J. Lilley and T. Ha, Nature Structural Biology 10, 93–97 [2002]).

The figure above is an artist's rendering of Holliday junctions attached to a glass surface via protein pedestals. Depending on the state of the molecule, either the green sphere (dye) or the red sphere lights up, giving clear signatures for the molecular dynamics. Actual time records of the green and red signals are superimposed onto the surface. Funding for this work was provided by the National Institutes of Health, the National Science Foundation, and the Searle Scholars Program. Further information is available from Professor Ha.