Monday, December 11, 2017

The Physics of DNA

I have always been interested in biology, hence why I am a molecular biology major, but taking this intro physics class has made me wonder more about the physics behind biology. I have just accepted that many of the biological processes that occur, and have never really thought about what drives them: spoiler, it’s physics. So I decided to do some digging into the physics behind biology, specifically DNA.
Researchers from the Pratt School of Engineering at Duke University investigated the double helix structure of DNA. One of the hallmarks of DNA is its stability, which is largely due to its double helix structure. The double helix structure seems to be the result of a balance between many forces: “stacking forces” occur between bases across the length of the DNA strand, and “pairing forces” are observed between complementary base pairs. These physicists devised a system that allowed them to focus on stacking forces, while previous studies struggled to separate stacking and complementary forces.
Different bases have different bonding properties: adenine forms strong and regular stacking bonds, while thymine forms the weakest and more disorganized stacking bonds. Strands of just adenine bases showed complex elasticity. I wonder if these bonds were studied/approximated as springs. They stretched the bonds with increasingly greater force and made a surprising discovery: the measurements plateaued off in not one but two places (23 and 113 pico-Newtons). The leveling off indicates breaking of bonds and that the helix unfolds. Thymine strands show no plateauing and do not resist stretching. I am really curious what it is about the structure of the bonds that causes them to show different elasticities and resistance to forces; this is an area that is being studied more now.
A physicist's view of DNA describes DNA through a physics based lens. The DNA double helix is much like a spring, in the sense that is can be stretched but has a resistance to stretching. Proteins input energy/force and “walk along” the DNA; this “melts” the DNA by supercoiling and bending it. These forces cause the DNA to denature and melt. When placed in a fluid, DNA tends to  fold in on itself, while it resists being pulled. DNA has intrinsic forces which resist stretching and deviation from a double helix. There is something about the structure of a double helix which allows for balance between pairing forces, stacking forces, and other forces that DNA comes into contact with in biological systems. I’m sure that this is an oversimplified view of the physics of DNA, but I think it’s pretty interesting that DNA has adopted a complex structure based on physics that allows it to best balance all of the forces which it experiences.

Duke University: Pratt School of Engineering. (2007). Unraveling the physics of DNA's double

Mashaghi, A., & Katan, A. (2013). A physicist's view of DNA. De Physicus, De Physicus,

(24e,3), 59-61.

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