To understand how chemotherapy works, it is helpful to understand the normal life cycle of a cell, or the cell cycle. All living tissue is made up of cells. Cells grow and reproduce to replace cells lost during injury or normal "wear and tear." The cell cycle is a series of steps that both normal cells and cancer cells go through in order to form new cells.

This discussion is somewhat technical, but it can help you understand how doctors predict which drugs are likely to work well together and how doctors decide how often doses of each drug should be given.

There are 5 phases in the cell cycle, which are labeled below using letters and numbers. Since cell reproduction happens over and over, the cell cycle is shown below as a circle. All the steps lead back to the resting phase (G0), which is the starting point.

After a cell reproduces, the 2 new cells are identical. Each of the 2 cells made from the first cell can go through this cell cycle again when new cells are needed.

The Cell Cycle

• G0 phase (resting stage): The cell has not yet started to divide. Cells spend much of their lives in this phase. Depending on the type of cell, G0 can last for a few hours to a few years. When the cell gets a signal to reproduce, it moves into the G1 phase.

• G1 phase: During this phase, the cell starts making more proteins and growing larger, so the new cells will be of normal size. This phase lasts about 18 to 30 hours.

• S phase: In the S phase, the chromosomes containing the genetic code (DNA) are copied so that both of the new cells formed will have matching strands of DNA. S phase lasts about 18 to 20 hours.

• G2 phase: In the G2 phase, the cell checks the DNA and gets ready to start splitting into 2 cells. This phase lasts from 2 to 10 hours.

• M phase (mitosis): In this phase, which lasts only 30 to 60 minutes, the cell actually splits into 2 new cells.

This cell cycle is important to cancer doctors (oncologists) because many chemotherapy drugs work only on cells that are actively reproducing (not on cells in the resting phase, G0). Some drugs specifically attack cells in a particular phase of the cell cycle (the M or S phases, for example). Understanding how these drugs work helps oncologists predict which drugs are likely to work well together. Doctors can also plan how often doses of each drug should be given based on the timing of the cell phases.

When chemotherapy drugs attack reproducing cells, they cannot tell the difference between reproducing cells of normal tissues (those that are replacing worn-out normal cells) and cancer cells. The damage to normal cells can cause side effects. Each time chemotherapy is given, it involves trying to find a balance between destroying the cancer cells (in order to cure or control the disease) and sparing the normal cells (to lessen unwanted side effects).

Go back to Chemotherapy Principles

Last Medical Review: 06/17/2009
Last Revised: 06/17/2009