Cancer ; Cell division , like all other biological processes, is under genetic control. Certain genes must regulate the processes of cell growth and division in response to intracellular, intercellular and environmental single . Modern scientists have identified and characterized about twenty allelic pairs of such genes in the genomes of different vertebrates, including humans . These genes are called proto- oncogenes, If during DNA replication, a proto oncogene happens to change (= mutate), the affected cells is likely to exhibit uncontrolled growth and division ( neoplastic cell) , Further mutations in such cells usually cause cancer.
A mutated proto oncogen is called oncogene. In tissues other than haemopoetic and lymphoid tissues , the uncontrolled growth and proliferation of an oncogene containing cell results into the formation of a cell cass called tumor, such a tumor may remain harmless ( bengn tumor ) for indefinite time and , if it caused some physical discomfort it can be removed surgically. If on the other hand, further oncogenic mutations occur in certain cells of such a tumor , the becomes then the progeny of mutant cells start breaking loose from the tumor and invading through blood circulation, other tissues and forming secondary tumors ( a process called metastasis ) in different tissues. This is the disease of Cancer.
Cancerous tumors arising from epithelial cells carcinomas, and those arising from connective tissues or muscle cells are called sarcomas. Cancerous mutation in myelogenous cells of bone marrow , or lymphogenous cells of lymploid tissues result into uncontrolled production of white blood corpuscles, This is called blood cancer or leukemia.
Cancer, sooner or later kill the patients because cancerous cells complete with and deprive normal tissues of essential nutrients , so that normal tissues gradually meet a starvation death.
Probability of cancer generating oncogenic mutations have presently increased manyfold due to the effects of a number of environmental mutagenic agents. The latter include ionizing radiations (e.g. radiations from Hiroshima and Nagasaki bombings ) pollutant chemicals ( carcinogens ) of which cigarette smoke is said to be most potent carcinogen, certain physical irritants , and hereditary tendencies, That is why cancer is progressively becoming one of the most potent fatal diseases of man ,It is feared that roughly one person in five specially in prosperous countries, would die of cancer in the future.