When it comes to cell division, cancer cells break just about all the rules!
Cancer cells can divide without appropriate external signals.
This is analogous to a car moving without having pressure applied to the gas pedal. An example would be the growth of a breast cancer cell without the need for estrogen, a normal growth factor. Some breast cancer cells actually lose the ability to respond to estrogen by turning off expression of the receptor for estrogen within the cell. These cells can still reproduce by bypassing the need for the external growth signal.
Cancer cells do not exhibit contact inhibition.
While most cells can tell if they are being 'crowded' by nearby cells, cancer cells no longer respond to this stop signal. The continued growth leads to the piling up of the cells and the formation of a tumor mass.
Cancer cells can divide without receiving the 'all clear' signal.
While normal cells will stop division in the presence of genetic (DNA) damage, cancer cells will continue to divide. The results of this are 'daughter' cells that contain abnormal DNA or even abnormal numbers of chromosomes. These mutant cells are even more abnormal than the 'parent' cell. In this manner, cancer cells can evolve to become progressively more abnormal.
Continued cell division leads to the formation of tumors. The genetic instability that results from aberrant division contributes to the drug resistance seen in many cancers. Mutations in specific genes can alter the behavior of cells in a manner that leads to increased tumor growth or development. The next chapter will examine a few of the best studied examples of these genes.