Causes of Cancer
We know earlier that a malignant tumor is a large aggregation of cancer cells, all of them descended from a single founder cell that was once a normal cell with normal functions in a particular tissue. What happens to the cell to free it from the general constraints on cellular growth? We can examine the cause of cancer mostly at two levels:
(1) The change that converts a normal cell to a cancer cell.
(2) The agents that trigger the change from normal to cancerous stage and
These are called carcinogens. For instance X-rays and ultraviolet rays in sunlight are both physical agents which cause cancer. Exposure to X-rays is a important cause of leukemia and brain cancer, whereas UV is known to cause skin cancer including the deadly type-melanoma. Similarly several chemicals are known carcinogens. For example tobacco is one of the most common agents related to cancer of the oral cavity. We also learnt that certain viruses can cause cancers in animals like cats, laboratory mice, birds and frogs, certain viruses have also been relates with human cancers. In all these cases we should realize that some change occurs at the level of genes that can be passed on at each mitosis, so giving a clone of cancer cells.