Single-molecule studies of the mechanical properties of individual double-stranded DNA have excited interest across
many scientific disciplines because of DNA’s fundamental role in biology and DNA’s remarkable overstretching
transition at higher forces. Here, we discuss a recent result on the overstretching transition of DNA and on the dynamics
of dye molecules intercalating into DNA under tension. Overstretching DNA is mechanical transition whereby DNA’s
extension increases by 70% at 65 pN. Notwithstanding more than a decade of experimental and theoretical studies, there
remains significant debate on the nature of overstretched DNA. We developed a topologically closed but torsionally
unconstrained DNA assay that contains no nicks or free ends. DNA in this assay exhibited the canonical overstretching
transition at 65 pN but without hysteresis upon retraction. Controlled introduction of a nick led to hysteresis in the force
extension curve. Moreover, the degree of hysteresis increased with the number of nicks. In the second study, we isolated
the effects of binding and intercalation of a DNA staining dye, by combining single molecule force spectroscopy with
simple buffer exchange. We showed that force-enhanced intercalation can occur from a reservoir of bound dye that was
not bis-intercalated, yet remained out of equilibrium with free dye for long periods (<5 min for YOPRO and <2 hr for
YOYO). Our work highlights that binding/unbinding and intercalation/de-intercalation are distinct processes that can
occur on very different time scales. Taken together, these works highlight ongoing discoveries based on a twenty year
old technique, force spectroscopy of single DNA molecules.
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