Before I get to the answer, I would recommend that you post biology questions under the biology category. You should get your answers more quickly that way :)
There are two things that happen before a parent cell divides:
1) DNA in the nucleus of the parent cell must replicate (this happens in the S phase of the cell cycle). If you're looking for a single answer to your question, then this is probably it.
2) Also, the cell must grow and some organelles (like the mitochondria) must also replicate before the parent cell divides. If organelles like the mitochondria did not replicate, then you would eventually end up with daughter cells that don't have enough mitochondria to support themselves.
