8.3.P4

Consider (negatively charged) DNA in an ionic solution (assume equal numbers of positive and negative ions in the solution), as illustrated in the cartoon below (taken from Phillips, et al.,  Physical Biology of the Cell).

A. Why don’t positive ions attach all along the length of the DNA until the DNA is electrically neutral?  What, if anything, opposes that process?

B. The positive cloud of ions surrounding the negatively charged DNA extends a distance λ_D, the “Debye length.” Just outside the Debye length, is the electric field magnitude smaller or larger than it would be if not for the positive ionic cloud? Why?

C. Does the presence of the ions in solution make it harder or easier for two pieces of DNA floating around in solution to get close together (compared to the case of pure water)? Why?

D. Explain how the Debye length changes with temperature and salt concentration and give an argument as to why these dependencies are plausible.

Catherine Crouch, Wolfgang Losert, and Joe Redish 3/3/13