DNA¹ has the nasty habit of getting tangled² and forming knots. Scientists study these knots to understand their function and learn how to disentangle them (e.g. useful for gene³ sequencing techniques). Cristian Micheletti, professor at the International School for Advanced Studies (SISSA) in Trieste and his team have been carrying out research in which they simulate these knots and their dynamics⁴（动力学）. In their latest paper, just published in the journal Soft Matter, Micheletti together with Marco Di Stefano, first author and PhD student at SISSA, and colleagues from Ljubljana and San Diego devised and tested a method based on the application of electric fields and "optical tweezers⁵." DNA is in fact an electrically charged molecule⁶ which reacts to the presence of opposing（相对的）hed-out" strand⁷ of DNA, with its ends secured by two optical tweezers that serve as anchors to keep them apart. We succeeded in moving the knot, inserted into the configuration⁸, by applying an electric field," explains Micheletti. "Try to imagine holding one end of a knotted rope so that the rope is suspended above the ground: by shaking it gently you can make the knot slide down with the help of gravity. Something like that takes place in our experiments."

 

"Our work," concludes Micheletti, "provides useful information for setting up new experiments where the movement of the DNA knots can be controlled from the outside." In fact, in studies of this kind carried out to date the movement of the knot was "stochastic," that is, produced by thermal⁹ noise, the random¹⁰ movement of atoms caused by the rise in temperature of the system, and never directly controlled by the investigator¹¹.

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