Single molecule statistics and the polynucleotide unzipping transition

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Original entry: Hsin-I Lu, APPHY 225, Fall 2009

"Single molecule statistics and the polynucleotide unzipping transition"

D. K. Lubensky and D. R. Nelson, PRA 65, 031917 (2002)


This paper presents an extensive theoretical investigation of the mechanical unzipping of double-stranded DNA under the influence of an applied force. In the limit of long polymers, there is a thermodynamic unzipping transition at a critical force value of order 10 pN, with different critical behavior for homopolymers and for random heteropolymers. As the applied force approaches the critical value, the double-stranded DNA unravels in a series of discrete, sequence-dependent steps that allow it to reach successively deeper energy minima. Plots of extension versus force thus take the striking form of a series of plateaus separated by sharp jumps. Similar qualitative features should reappear in micromanipulation experiments on proteins and on folded RNA molecules.

Micromanipulation of Single Molecule

Micromanipulation experiments on single molecules provide the opportunities to measure entire distributions of molecular properties, without the requirement for averaging over a macroscopic sample. The system studied in the paper-the unzipping of double-stranded DNA (<math>ds</math>DNA) shows novel response to force on single molecule level. Fig. 1 shows the two single strands of a double-stranded DNA molecule with a randomly chosen base sequence are pulled apart under the influence of a constant force. Unzipping.jpg One of the single strands from a double-stranded DNA molecule is attached to a glass slide, and the other to a bead on which a constant force F is exerted. F could be created, for example, with magnetic tweezers, which have been used to exert constant piconewton-scale forces over hundreds of microns. Optical tweezers or atomic force microscopes (AFM) with appropriate feedback can create a similar effect. As a result of the applied force, the DNA partially ‘‘unzips,’’ breaking m bonds.