Adapted from J. Am. Chem. Soc. Artist’s rendition of a nick-free, freely rotating double-stranded DNA molecule (blue and red), which is anchored to a glass coverslip (left) at one end and to a bead (pink) via biotin on the other.

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There’s more than one way to overstretch DNA, a new experiment suggests (J. Am. Chem. Soc., DOI: 10.1021/ja108952v). The finding recasts a 15-year-old debate on double-stranded DNA’s mechanical properties and might make it easier to calibrate instruments that measure small forces.

DNA experiences many forces during transcription and other biological events, but one particular force has become the subject of scientific fascination. “If you pull really hard on DNA, you think it’s about to break. But at 65 piconewtons of force, something amazing happens—it almost doubles in length,” says Thomas T. Perkins of JILA, a precision physics lab run jointly by the National Institute of Standards & Technology (NIST) and the University of Colorado, Boulder. A single pico­new­ton is approximately the force exerted by the mass of 100 Escherichia coli bacteria, Perkins says.

What DNA looks like in that stretched-out, or “overstretched,” state is controversial, but several teams’ work suggests that nicks or breaks in the DNA always make the double-stranded molecule peel open to single-stranded DNA, Perkins says.

Perkins’ postdoctoral colleague, D. Hern Paik, tested that hypothesis with a piece of DNA containing no nicks or free ends and designed to freely rotate, an important characteristic for probing stretching in the desired force range. The pair’s results suggest that overstretched DNA can form in other ways than just peeling, Perkins says.

Researchers don’t know whether DNA experiences stretching forces of this magnitude in living things. But because the peculiar overstretching happens at a characteristic force, NIST is pursuing a piconewton-scale force standard based on DNA that could be used to calibrate instruments that measure all kinds of biological and chemical forces, Perkins says.

This study “may explain why it has been so difficult to unequivocally determine the structure of overstretched DNA,” says Mark C. Williams, a biophysicist at Northeastern University.