Tuesday, August 24, 2010

DNA and viscous drag

One of the themes in Biological Physics has been: how does a complex molecule such as DNA maintain it's function under constant bombardment by thermal forces? One of the consequences of this thermal motion at the nanometre scale is the viscosity experienced at low Reynolds numbers.

Section 5.3.5 considers the viscous drag at the DNA replication fork. During DNA replication the double-helix must be unwound, this begs the question: how significant is the viscous drag caused by DNA rotation?

Rotational forces are expressed as torque for DNA:
Torque = -const x omega*eta*radius-squared*length
T = -C*w*n*L*r^2 - with units of Nm

To find the work done per turn multiply the torque by the rotation rate, w.
Wfrict = 2piC x wnLR^2

DNA polymerase has a frequency of 600 Hz, plugging this into the equation gives the energy needed to turn DNA by one rotation. This gives a value of ~ 4.7x10^-17 Jm^-1 times the length. Compare this to the energy of ATP = 8.2x10^-20 J. Considering the length of the DNA molecule being turned is tiny the energy from ATP is enough to overcome this viscous force.

1 comment:

  1. What's also important is that during replication is the occurence of supercoiling, which can be overcome through the use of topoisomerase. During this process, there's extra surface area interacting with the water and experiencing viscous forces, but the high presence of energy from ATP can be used to also overcome this.