A cell that spends to much or too little time in a phase might not successfully complete the job of that phase. For example, a cell might produce too many proteins or not enough enzymes. The abnormal cell continues along the cell cycle. Each abnormal cell is capable of producing two new abnormal cells. Those two cells produce four abnormal cells, four produce eight, until there are enough abnormal cells to form a mass or lump-called a tumour.
Chemotherapy is effective when it stops abnormal cells from going through the cell cycle.
Actively dividing cells are the most vulnerable to chemotherapy. Most tumour cells and some normal cells fall into that category. The effect of chemotherapy on normal cells causes unwanted side-effects. Chemotherapy is usually given in cycles, and the cycles are repeated over a specific period of time. The cycle schedule is designed to allow sufficient time for effected normal cells to recover between treatments.