"To understand chemotherapy, the cell cycle must first be understood. Chemotherapy is effective because the drugs used effect some phase of the cell life cycle. Each cell goes through a four phase cycle in order to replicate itself.
The first phase called G1, is when the cell prepares to replicate its chromosomes.
The second stage is called S, and in this phase DNA synthesis occurs and the DNA is duplicated.
The next phase is the G2 phase, when the RNA and protein duplicate.
The final stage is the M stage, which is the stage of actual cell division.
In this final stage, the duplicated DNA and RNA split and move to separate ends of the cell, and the cell actually divides into two identical, functional cells. "