Some questions to consider: The two strands of the helix-shaped DNA molecule are

Question

Some questions to consider:

The two strands of the helix-shaped DNA molecule are held together by electrostatic forces. Assume that the net charge magnitude (due to electron sharing) indicated on H and N atoms is 0.2e and on the indicated C and O atoms is 0.4s. Assume also that atoms on each molecule are separeated by 1.0 x 10^-10 m.

Estimate the net forces between the following bases. For each bond (indicated by three dots) CONSIDER ONLY THE THREE ATOMS IN A LINE (two atoms on one molecule, one atom on the other). Note that you must evaluate both attractive forces and repulsive forces, so it will be easier to look at these separately before adding them.

a) a thymine and an adenine base pair

b) a cytosine and a guananine base pair

c) What would you estimate the total force to be for a DNA molecule containing 10^5 pairs of such molecules?

Explanation / Answer

F=k (q(1)q(2))/d^2 k=8.99E-9

O-H-N bond

O and N are negative, H is positive

O..........H---------N

Using our reading, I assumed I should consider them Q1 through Q3, each separated by r.

So we'd have:

Q1........Q2--------Q3

Since the bond (red dots) are between Q1 and Q2/Q3, I assumed I should find the net force on Q1

F12 = k[(Q1)(Q2)]/r^2 , where Q1 = .4e, Q2 = .2e, and r = 1.0 * 10^-10

Solving the equation I have F12 = 1.8432 * 10^-9 N

The same is then done for F13 (except that r = 2.0 * 10^10) , solving the equation I get F13 = 4.608 * 10^-10

As F12 is attractive and F13 is repulsive, I added the forces

F = 1.8432 * 10^-19 + (-4.608 * 10^-10) = 1.3824 * 10^-9


I did the exact same thing for the N----H.....N bond as well (N being negative, H being positive), but for this I calculated the forces being applied to Q3 instead of Q1

The results where

F = 6.912 * 10^-10

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