Almost all life on earth is based on DNA being copied, or replicated, and understanding how this process works could lead to a wide range of discoveries in biology and medicine. Now for the first time scientists have been able to watch individual steps in the replication of a single DNA molecule, with some surprising findings. For one thing, there’s a lot more randomness at work than has been thought.

“It’s a different way of thinking about replication that raises new questions,” said Stephen Kowalczykowski, distinguished professor in microbiology and molecular genetics at the University of California, Davis, and at the UC Davis Comprehensive Cancer Center. The work is published June 15 in the journal Cell with co-authors James Graham, postdoctoral researcher at UC Davis, and Kenneth Marians, Sloan Kettering Cancer Center.

Using sophisticated imaging technology and a great deal of patience, the researchers were able to watch DNA from E. coli bacteria as it replicated and measure how fast enzyme machinery worked on the different strands.