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As far as I know it's because they mutate into a type of cell which simply does not know when it's enough to self replicate, they also have many features different than a typical cell biologically. One of the causes are free radicals in a our blood which transferred to our cells which can damage our DNAs in our healthy cells and might cause to mutate them into cancerous cells, that's why there is a whole nutrition market for anti-oxidants, which helps reduce the free radicals by binding with them to form harmless molecules otherwise harmful forms which can bind or remove parts of our DNA and eventually cause mutation.
It is also possible for cells to become cancerous if they are unable to go through normal cell processes- one of the most important being Apoptosis or programmed cell destruction. This process would normally destruct the cell upon abnormal behavior. However, in the case of growing cells that simply do not stop, because the cell is unable to perform this programmed cell death, it will continue growing until it is massive and ... cancerous.
wow it's a very fascinating topic I think we have first to distinguish between a "stimulus" that may cause cancer, and an "alteration" of a cellular component that lead or privilege undergoing cancer a stimulus: may be chemical products, viruses, x-rays, and all what you want these stimuli induce and "alteration" (that may be genetic or not), it may be a mutation, mislocalization of a protein. The most important thing to say about these alterations, is that they are qualitative or quantitative (it means that they can induce a change in the "quality" of an important cellular component to be active all the time "if it's an inductor of cell cycle", or to be inactive all the time "if it's an inhibitor of the cell cycle". Or a "quantitative" alteration that increase the expression of an inductor of the cell cycle, or decrease the expression of an inhibitor of the cell cycle)
Now, I have to tell you about the actors that induce or inhibit the cell cycle (as i said the alteration of these factors is the main mechanism in oncogenesis "i.e: cancer formation" these factors are generally implicated in cell cycle, differentiation, apoptosis, reparation of DNA alterations, cellular senescence But, what you have to know also is that "cancer is a multi-factorial and a multi-steps event" it means that the induction of cancer require 2 or more alterations, and pass by a number of steps Finally, if you have other questions, i'll be pleased to answer you (if i can of course) enjoy Biology have a great day
Cancer happens when the normal process of cell division, growth, or death is affected by some form of change, resulting in excessive number of cells which work abnormally. Usually this follows a mutation either in the genetic sequence or the triggering pathway. Mutations can remain dormant for long periods before they express.
Defected or mutated genes within the cell are responsible for a normal cell to become a cancerous cell. When a normal cell turns into cancerous, it wont stop dividing thus turning the unstoppable mass of multiplied cancerous cell to become a tumor.
Cells can mutate and too many cells become infected