When the DNA stretches..

DNA ¯ springy, stretchy and coiled ¯ is the cell's Slinky. And just like a Slinky, a DNA double helix can be stretched too far. The mechanics behind this process, called "overstretching," may be less cut-and-dried than scientists previously thought, a new study suggests.
Contrary to one prevailing theory, DNA molecules don't have to have loose-hanging single strands called free ends to overstretch, say researchers at the National Institute of Standards and Technology in Boulder, Colo. With or without free ends, the team reports in a paper to appear in the Journal of the American Chemical Society, DNA double helices spring to almost twice their length at the same elastic stretching point.
Like a Slinky, DNA plays nice under tiny forces, stretching as molecular theory predicts. But when scientists pull on these molecules hard enough using devices called optical traps, DNA seems to get extra elastic. At 65 piconewtons of force DNA elongates by 70 percent.
Source:Science News