Australian scientists say they've solved one of the DNA mysteries surrounding the way in which cells divide and replicate themselves. For more than 20 years scientists have tried to understand the final step in the copying of DNA in cells that are about to divide.A team of scientists led by Professor Nick Dixon at the Australian National University announced they had discovered why a small protein, TUS, stopped DNA duplication in some cases but not others. The replisome, a large "molecular machine", duplicates a cell's DNA by pulling the two DNA strands apart and copying each strand simultaneously.