AP Physics Problem?

[Brian Z]

A molecule of DNA is 2.17 micrometers long. The ends of the molecule become singly ionized---negative on one end, positive on the other. The helical molecule acts as a spring and decompresses 1.00% upon becoming charged. Determine the effective spring constant of the molecule.

Any suggestions on how to go about this? I've been stuck on it for awhile now and I'm close to giving up hope. I've tried assuming the ends are charged to 1 and -1 since it's looking for a constant but had no luck. I'm given the answer as I'm just supposed to prove I can solve the problem and have understanding. Answer is 2.25x(10^9) N/m

Anybody think they can help me? Even a place to start would be greatly appreciated.