Every cell in the body goes through a life cycle. Cells grow and divide to replace cells that are lost because of normal wear and tear or injury. Different cells grow and die at different rates. Some cells, such as epithelialepithelialA thin layer of epithelial cells that makes up the outer surfaces of the body (the skin) and lines hollow organs, glands and all passages of the respiratory, digestive, reproductive and urinary systems. cells, reproduce quickly. Other cells, like nerve cells, grow slowly. Both normal cells and cancer cells go through a sequence of steps, or phases, when they form new cells. This is called the cell cycle.
Resting phase (also called the quiescent or dormant phase)
Gap 1 – first growth phase
Gap 2 – second growth phase
After mitosis, a cell either re-enters the G1 phase or goes into the resting phase (G0) where it may later re-enter the cell cycle.
Normal tissue is made up of cells that are in the resting phase (G0) and cells that are in the process of dividing or dying. There is a balance of dividing cells and dying cells in normal tissue. Cancer can occur when there is no longer a balance between dividing and dying cells. The tissue can start to grow to form a tumour made up of abnormally growing and dividing cells. Cancer cells usually can’t enter G0 and therefore begin to divide uncontrollably.
After a person has a biopsybiopsyThe removal of cells or tissues for examination under a microscope. or undergoes surgery to remove a tumour, a pathologistpathologist1. The study of disease, including causes, development and effects on the body. 2. The symptoms, processes or conditions of a disease. will look for cells in the tissue sample that are in the process of dividing. These cells are called "mitotic figures." The number of mitotic figures seen under the microscope helps a pathologist determine the mitotic rate, which is used to diagnosis cancer.
The cell cycle is important in cancer treatment because some therapies work best when cells are actively or quickly dividing. For example, some chemotherapy drugs work by attacking cells in a particular phase of the cell cycle (such as G1, S or G2 phase), while radiation therapy seems to be most effective when cells are undergoing cell division (in the M phase). Knowledge about the cell cycle helps doctors decide on the best treatment, including what combinations of drugs to use, and in what order and how often to give them.