BERKELEY, CA — Most of us are familiar with the winding staircase image of DNA, the repository of a biological cell’s genetic information.  But few of us realize just how tightly that famous double helix is wound.  Stretched to its full length, a single molecule of human DNA extends more than three feet, but, when wound up inside the nucleus of a cell, that same molecule measures about one millionth of an inch across. Biologists have long believed that as a molecule of DNA is stretched, its double helix starts to unwind.  As much sense as this makes from an intuitive standpoint, a recent experiment proved it not to be the case.