BERKELEY, CA ó Most of us are familiar with the winding staircase image
of DNA, the repository of a biological cellís 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.