Cancer (medicine), any of more than 100 diseases characterized by excessive, uncontrolled growth of abnormal cells, which invade and destroy other tissues. Cancer develops in almost any organ or tissue of the body, but certain types of cancer are more lethal than others. Cancer is the leading cause of death in Canada and second only to heart disease in the United States.
A normal cell has a life span of about 40 cell divisions. This life span is controlled in part by telomeres, protective segments at the ends of the cell’s DNA. Telomeres shorten with each cell division until they can no longer protect the DNA. At this point cell division severely damages the DNA, ultimately killing the cell. This normal process ensures that older cells, which may have accumulated mutations, no longer reproduce. Cancer cells escape this protective mechanism by producing a protein called telomerase. Telomerase extends the length of telomeres indefinitely, rendering the cells immortal and capable of never-ending cell division.
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