Put all your answers in the answer box labeling each one by

Put all your answers in the answer box, labeling each one by its number and putting a blank line between each answer. 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 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.

Solution

A)

The positive ions surrounding the negatively charged DNA are attracted by the negative ions in the solution. This attractive force opposes the process of making the DNA electrically neutral.

(B) Just outside the debye length, the charge enclosed by a surface is increased due to partial neutralization of the negative charges of the DNA , so electric field will be less than that would be without positive ion cloud.

(C)

The presence of ions in solution makes it easier for two pieces of DNA floating around in the solution to get closer since the effective charge of each DNA decreasesand the repulsion also decreases.

(D)

Debye length will increase with temperature as the ions will have more velocity and will be more mobile decreasing the shielding effect. With increase in salt concentration, debye length will decrease since since availability of more ions will increase the shielding.