I don't think that there would be too much benefit gained by taking a Polical Science course, unless you are genuinely interested in the material. The purpose of the writing sample is to test your literary skills in creating a cohesive, and persuasive essay. By taking a Polysci course, you would probably gain a fair amount of historic material, and perhaps some writing practice; however, what course would you have to give up instead?
That time could be spent taking a Biochem course, or perhaps an English course or something that would directly improve your MCAT scores. On the other hand, a knowledge of politics, and the theories that drive them go a long way in life, both in your interviews, and your interactions with other people. I'd take the course if I was generally interested in the material, but not just for the sake of the MCAT.
Regarding the Biochemistry, each MCAT is different, and there are several versions of the MCAT distributed at each sitting. I don't remember my MCAT being particularly heavy on Biochem, but that doesn't mean yours won't. However, I feel that if you are able to work through all the sections in an MCAT prep book on Biochem, you should be able to do reasonably well.