Smith S B, Cui Y, Bustamante C
Institute of Molecular Biology, University of Oregon, Eugene 97403, USA.
Science. 1996 Feb 9;271(5250):795-9. doi: 10.1126/science.271.5250.795.
Single molecules of double-stranded DNA (dsDNA) were stretched with force-measuring laser tweezers. Under a longitudinal stress of approximately 65 piconewtons (pN), dsDNA molecules in aqueous buffer undergo a highly cooperative transition into a stable form with 5.8 angstroms rise per base pair, that is, 70% longer than B form dsDNA. When the stress was relaxed below 65 pN, the molecules rapidly and reversibly contracted to their normal contour lengths. This transition was affected by changes in the ionic strength of the medium and the water activity or by cross-linking of the two strands of dsDNA. Individual molecules of single-stranded DNA were also stretched giving a persistence length of 7.5 angstroms and a stretch modulus of 800 pN. The overstretched form may play a significant role in the energetics of DNA recombination.
双链DNA（dsDNA）单分子通过测力激光镊子进行拉伸。在约65皮牛顿（pN）的纵向应力下，水性缓冲液中的dsDNA分子会经历高度协同的转变，形成一种稳定形式，每碱基对上升5.8埃，即比B型dsDNA长70%。当应力放松至低于65 pN时，分子会迅速且可逆地收缩至其正常轮廓长度。这种转变受到介质离子强度和水活性的变化或dsDNA两条链交联的影响。单链DNA的单个分子也被拉伸，其持久长度为7.5埃，拉伸模量为800 pN。过度拉伸形式可能在DNA重组的能量学中起重要作用。
