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In short; Malignant and uncontrollable growth of the cells in the body forming tumors and effecting the cells functions, due to radiation, viruses etc
down here in bio, questions seem easier than their answers!
the tar is. the nicotine isn't! ain't it ?
nicotine is addictive, tar clogs the lungs, fairly sure its a combo of things and chemicals in the cigarette
Smoking irritates the lining of the lungs, which causes the cells lining the lungs to keep replacing the killed cells. As the cells are damaged by the smoke and tar, they get replaced faster and faster, which increases the chances of out of control growth, ie cancer Nicotine itself doesn't cause cancer, AFAIK but it causes other problems, such as heart disease, high blood pressure, stroke, etc.
All in all, smoking = cancer, regardless of whats in it at this stage it is a stupid risk to start
different cancers have different causes but the fact is that cancerous cells proliferate uncontrollably forming tumors
The uncontrollable proliferation of cancer cells is caused by mutations in the cell's DNA that causes them to not respond to normal "death signals". All normal cells are regulated by growth and death signals and the balance between them. So basically anything that can cause random DNA mutations (chemicals, radiation, viruses, etc) can cause cancer, provided that the mutations occur in or near genes that are involved in cell death, proliferation, or cell cycle regulation.