How do cell mutations cause cancer?

Mark A. O'Rourke, MD
Hematology & Oncology
The human body is made up of small units called cells. The lungs are a collection of lung cells. The stomach is a collection of stomach cells. The skin is a collection of skin cells. Every day old cells wear out and are removed by the body. From the remaining cells, new cells are formed by cell division.
Good health requires a balance of new cells replacing old cells.  Some mutations in the cell’s DNA cause the old cells to be abnormal and to keep on living.  These abnormal cells pile up.  When the pile of cells grows and does not stop (because of the mutation), it is a cancer. For more information, I recommend the American Cancer Society website for the public,
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Dr. Michael Roizen, MD
Internal Medicine
When deoxyribonucleic acid (DNA) in human body cells gets damaged, pieces of the instructions on the genes can get knocked out or changed. When this occurs during the process of cell division, when the DNA is being copied to the new cell, a mistake or mutation occurs.

There are three types of cell mutations -- harmless mutations, lethal mutations, and a tumor-causing type. The tumor-causing mutations are the rare cancer-causing mutations that tell the cell to begin growing and dividing uncontrollably. Your body has a regulatory system that keeps the number of cells in your body at a more or less constant level. The genes that regulate this process are known as cell-cycle genes, because they tell the cell when to divide, to grow, and to divide again. Some of these cell-cycle genes are proofreader genes. They scan the DNA when it replicates, ensuring that no mutations have been acquired. If a mutation has occurred, the proofreaders either fix it or kill the cell.

A few of these cell-cycle and proofreader genes are also known as oncogenes (cancer genes), because mutations in these genes are tied to the development of cancers. If a gene that is supposed to tell a cell to stop growing stops working -- that is, mutates -- then the cell grows uncontrollably, dividing faster than it should. Moreover, its daughter cells also inherit the mutation and grow out of control themselves. The effect multiplies, and soon there is a mass of rapidly dividing, quickly growing cells, a tumor, "the Big C."
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