Journal
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Volume 110, Issue 10, Pages 3865-3870Publisher
NATL ACAD SCIENCES
DOI: 10.1073/pnas.1213740110
Keywords
DNA bubble; magnetic tweezers
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Funding
- National Research Foundation Singapore through the Singapore-MIT Alliance for Research and Technology's BioSystems and Micromechanics research programme
- National Research Foundation Singapore through the Mechanobiology Institute at the National University of Singapore
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Double-stranded DNA (dsDNA) unconstrained by torsion undergoes an overstretching transition at about 65 pN, elongating the DNA to about 1.7-fold. Three possible structural transitions have been debated for the nature of DNA overstretching: (i) peeling apart of dsDNA to produce a peeled ssDNA strand under tension while the other strand coils, (ii) inside-strand separation of dsDNA to two parallel ssDNA strands that share tension (melting bubbles), and (iii) B-to-S transition to a novel dsDNA, termed S-DNA. Here we overstretched an end-opened DNA (with one open end to allow peeling) and an end-closed (i.e., both ends of the linear DNA are covalently closed to prohibit peeling) and torsion-unconstrained DNA. We report that all three structural transitions exist depending on experimental conditions. For the end-opened DNA, the peeling transition and the B-to-S transition were observed; for the end-closed DNA, the inside-strand separation and the B-to-S transition were observed. The peeling transition and the inside-strand separation are hysteretic and have an entropy change of approximately 17 cal/(K.mol), whereas the B-to-S transition is nonhysteretic and has an entropy change of approximately -2 cal/(K.mol). The force-extension curves of peeled ssDNA, melting bubbles, and S-DNA were characterized by experiments. Our results provide experimental evidence for the formation of DNA melting bubbles driven by high tension and prove the existence of nonmelted S-DNA. Our findings afford a full understanding of three possible force-driven structural transitions of torsion-unconstrained DNA and the resulting three overstretched DNA structures.
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