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The cell cycle

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 epithelial 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.

Cell Cycle

Phase		Description
Interphase	G₀	Resting phase (also called the quiescent or dormant phase) The cell performs all of its normal functions but is not preparing to divide. Some cells divide often and either are in G₀ for a short amount of time or skip G₀. Other cells do not divide very often and are in the G₀ phase for many years. When the cell receives the signal to divide, it moves into G₁ phase of the cell cycle.
	G₁	Gap 1 – first growth phase The cell prepares to undergo cell division. The cell still performs all of its normal functions, but starts to get bigger. The cell begins to make a copy of the cell parts (organelles). It also begins to make more protein to get ready to divide. The time for this phase varies from about 8 hours to several days, weeks or months.
	S	Synthesis phase The cell copies its DNA (DNA synthesis) to make 2 sets of chromosomes – one set for each new cell. This phase lasts about 6–8 hours.
	G₂	Gap 2 – second growth phase The cell makes more proteins in preparation for cell division. This phase lasts about 2–5 hours.
Prophase Metaphase Anaphase Telophase	M	Mitosis The cell divides into 2 new cells, which occurs in 4 stages (prophase, metaphase, anaphase and telophase). The mitosis phase lasts about 1–3 hours.
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.

Cell cycle and cancer

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 biopsy or undergoes surgery to remove a tumour, a pathologist 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.

Cell cycle and cancer treatment

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.

References

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