Sciatica is a symptom, not a disease and consists of pain in the buttocks radiating down the leg, mainly the back of the leg.
Causes are many.
In answer to your question, it does not affect bones, but may indeed cause muscle weaknwess.
It's to do with inflammation of the Sciatic Nerve, which as Squad says runs from the buttocks and down the leg. Strangely enough it is often the left leg. It often shows itself during very cold weather.
It doesn't affect the calf muscles/bones in any way but it can be felt right down to the foot and/or sweeping around the back to the front of the calf. It can also feel like the pain is actually in the bone.
When I'm suffering, I get muscle spasms down my whole leg. I know it's sciatica because the doctor diagnosed it. His concrete test was to get me to lie on my back and lift the offending leg without pointing the toe. If it's sciatica, it'll hurt like hell.