What has to happen before mitosis begins?

Mitosis is simply a stage in a cells life cycle, which could be broken down further into stages of mitosis. The rest (so not mitosis) is know as Interphase and is where the cell does its normal function, so if it's the cell is in a region of muscle it would contract/relax as normal etc.

Immediately before mitosis (or M phase) is what's know as G2 phase, where the G stands for growth and is where particular gene pathways are expressed to promote growth of the volume of DNA (chromosomes are duplicated, so from 46 to 92). At the end of this stage is a checking process where the DNA is scrutinised for any errors, if all is okay then the cell can proceed to mitosis, if not then the process is stopped so that errors in copying the DNA can be fixed. This is know as a restriction point and the cell must meet the requirements to pass. This is often seen as a way cells can prevent damage being replicated and therefore interfering with normal cell processes. When cells are cancerous they are able to override this and therefore divide and spread the damaged DNA.