科学家发现了当双链DNA拉伸超过了其极限的时候会熔化破碎，就仿佛遇到了高温。这些结果可能为理解DNA-蛋白质的相互作用以及DNA的热动力学——它们对于包括细胞分裂和转录在内的一些细胞过程至关重要——提供一个基础。
Erwin Peterman及其同事“过度拉伸”了这种核酸螺旋，把每一个分子链的一端与一个固体微球相连接，从而研究了DNA的弹性。这组科学家指出，拉伸最初可能始于腺嘌呤和胸腺嘧啶丰富的区域，而且双链DNA需要一个最低程度的力量才能开始分解。这种过分拉伸始于末端的绽裂，而且一直向两个方向运动，直到这些螺旋状损伤的DNA链破碎形成单链。这组作者说，然而，当DNA的两条链都固定于一个固态基底的时候，熔化这些链所需的力量几乎加倍了。（生物谷Bioon.com）
生物谷推荐原始出处：
PNAS October 19, 2009, doi: 10.1073/pnas.0904322106
Unraveling the structure of DNA during overstretching by using multicolor, single-molecule fluorescence imaging
Joost van Mamerena,1,2, Peter Grossa,1, Geraldine Fargea, Pleuni Hooijmana, Mauro Modestib, Maria Falkenbergc, Gijs J. L. Wuitea,1,3,4 and Erwin J. G. Petermana,1,3,4
aDepartment of Physics and Astronomy and Laser Centre, Vrije Universiteit, De Boelelaan 1081, 1081 HV, Amsterdam, The Netherlands;
bCentre National de la Recherche Scientifique, Unité Propre de Recherche 3081, Genome Instability and Carcinogenesis Conventionné par l'Université d'Aix-Marseille 2, 13402 Marseille Cedex 20, France; and
cDepartment of Medical Biochemistry and Cell Biology, G?teborg University, Box 440 SE-40530 G?teborg, Sweden
Single-molecule manipulation studies have revealed that double-stranded DNA undergoes a structural transition when subjected to tension. At forces that depend on the attachment geometry of the DNA (65 pN or 110 pN), it elongates ≈1.7-fold and its elastic properties change dramatically. The nature of this overstretched DNA has been under debate. In one model, the DNA cooperatively unwinds, while base pairing remains intact. In a competing model, the hydrogen bonds between base pairs break and two single DNA strands are formed, comparable to thermal DNA melting. Here, we resolve the structural basis of DNA overstretching using a combination of fluorescence microscopy, optical tweezers, and microfluidics. In DNA molecules undergoing the transition, we visualize double- and single-stranded segments using specific fluorescent labels. Our data directly demonstrate that overstretching comprises a gradual conversion from double-stranded to single-stranded DNA, irrespective of the attachment geometry. We found that these conversions favorably initiate from nicks or free DNA ends. These discontinuities in the phosphodiester backbone serve as energetically favorable nucleation points for melting. When both DNA strands are intact and no nicks or free ends are present, the overstretching force increases from 65 to 110 pN and melting initiates throughout the molecule, comparable to thermal melting. These results provide unique insights in the thermodynamics of DNA and DNA-protein interactions.
