Re: How do cells know when to stop growing and what goes wrong in cancer?

Great question.  I'll give you a brief answer, and refer you to two of my 
previous answers, which will give you a more complete overview of cancer 
and the cell cycle and some interesting links.

Normally, cells in adults don't divide very much.  Some exceptions include 
blood cells, and cells lining the stomach and intestine, which are 
continually replaced.  Cells like nerve cells almost never divide.  These 
cells are "trapped" in a cell cycle stage called "G0".  Cells in G0 do not 
respond to hormones or outside growth factors.  They do not synthesize the 
correct proteins, such as Cyclin D and Cyclin E, proteins which drive the 
cell through the cell division cycle.  And, they express proteins such as 
p21, small inhibitory proteins that put the brakes on the cell cycle. Most 
cells in the body reside in G0, or G1, a very similar cell cycle stage. 
Please see my cell cycle answer for more information:
http://www.madsci.org/posts/archives/oct98/906391245.Cb.r.html

So, what goes wrong in a cancer cell?    Cancer cells have mutations in 
their DNA.  Mutations often have no effect on proteins, because much of the 
DNA in an organism doesn't encode proteins.  However, sometimes a DNA 
mutation can cause a mutant protein to be produced which has an abnormal 
function or cause no protein to be produced at all.  In order to become a 
cancer cell, several mutations have to accumulate.  Often, there are 
mutations in DNA repair proteins.  If these proteins are missing, whole 
sections of chromosomes can be lost, causing the loss of many genes.  

Normal cells have several important controls, which keep them from becoming 
cancer cells:

1) Cell cycle control (divide and make a new cell only at the right time)
2) Repair (fix mutations in DNA)
3) Contact inhibition (stay put next to neighbor cells to form a tissue)
4) Cell suicide program (if things get too bad, the cell dies to protect 		
   the rest of the organism)

In a cancer cell, each of these controls has to be disabled or mutated.  
Then, the cancer cell can make lots of copies of itself, tolerate broken, 
mutated DNA, escape from it's neighbors and travel around the body, and 
avoid cell suicide (which it would normally undergo if all that other stuff 
was wrong). 

For more information, please see my breast cancer answer:
http://www.madsci.org/posts/archives/oct98/908950863.An.r.html

or 
 http://www.oncolink.com/ 

a site with a lot of cancer information.

Hope that helps!!  

Erin Cram