View source for Single molecule statistics and the polynucleotide unzipping transition

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)

==Summary==
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.
[[Image: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.