Consider (negatively charged) DNA in an ionic solution (assume equal numbers of

Question

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 dont 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.

Explanation / Answer

A) In electrically neutral DNA there is no space for the positive ions to bind. All the available ions are neutralized

B) The electric field magnitude will be smaller due to the potential drop in the electric double layer where the electric field is vanishing.

C) The presence of ions in the solution make it easier for the two piece of floating DNA around the solution since they aggregate with each other in ionic solution.

D) In the presence of salt concentration and increased temperature the positive ions in the solution attaches to the -ve ly charged molecule. But it does not neutralize the DNA the week positive ion attracts other positive ions and forms a cluster. Where there is a potential drop in electric field. The Debye length is found to increase with temperature and salt concentration.

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