Re: Why do cells die?

	I am sorry that is took so long to anwear your questions! They very 
interesting questions, and a lot of current reseach is devoted to 
understanding the mechanisms that underly cell aging/death and what makes 
cells different.  
	I will answear your last question first.  Cell differentiation is a 
term that aplies to multi-cellular organisms which have many different types 
of cells.  Although all cells in the organism all came from one single 
diploid zygote, (and, therefore, all cells have the same DNA) a multi-
cellular organism has many cells which look and function very differently.  
The process by which an early (embroyonic) cell matures into a cell with a 
very specialized function (called cell "fate") is called cell 
differentiation.   So, how do cells look/funtion differently if they have 
the same exact DNA?  They USE the DNA differently.  They use ("express") 
different PARTS (genes) in the DNA. 

	There are thought to be several ways  in which a cell that is perfectly 
healthy and supplied with nutrients can die.  One process is called 
"Apoptosis" or "Programmed Cell Death".  Apoptosis is a suicide mechanism in 
the cell- and many, many different signals can induce the onset of 
apoptosis.  A great deal of current research is focused on identifying the 
many different ways in which this system is turned on/off.  Many times the 
signal is coming from outside the cell "extra-cellular signal" (usually from 
signals produced from other types of cells nearby) or even from a self-
triggering mechanism in the cell itself ("intra-cellular signal").  You 
might ask, why would you ever want to kill one of your own cells?  The most 
simple explanation is that this system works primarily to kill sick cells.  
For example, cells that have become infected with a virus act differently 
and nearby cells can "sense" this unusual activity and send a signal for 
that infected cell to undergo apoptosis.  Another common example how a cell 
can become "sick  is that a cell's DNA (especially old cells) accumulate 
mutations and DNA damage through time (from "mutagens' such as UV light, 
chemical toxins, ect.)  When a cell acuumulates mutations in essential genes 
and those genes are now making mutated protein for example and this mutation 
activates a signal that turns on apoptosis.  
	 Another interesting mechanism in which cells die is due to DNA 
replication (which is required for cell division and making new cells).  
Because of the way DNA replicates, a cell can only replicate a limited 
amount of times.  With every round of Replication, the ends of the DNA 
chromosomes get shorter and shorter.  Eventually, replication will lead to 
chromosomes that have stopped replicating at a very important gene (an 
"essential" gene) and the cell cannot survive without this gene. 
Then, when replication proceeds the newly made daughter cell will die 
without it.  Some cells, however, (like sperm cells) have an amazing enzyme 
called Telomerase that is capable of extending the ends of every chromosome 
so that they are never shortened past that critical length (a lot of current 
research is working on understanding how telomersase works to allow 
indefinate replications or  "immortalized cells")
	So I have described to you three ways that healthy, well-fed cells can 
die:  DNA-mutation, apoptosis, and too many DNA replications without 
telomerase.  I hope that I have answeared your questions and, hopefully, 
that I have sparked some more questions!  I suggest trying to find some cell 
biology text books (Molecular Biology of the Cell is my favorite)- where you 
can find detailed diagrams and explanations.
Good Luck!
Elena Rodriguez
Grad Student: University of California, Berkeley
elenarod@yahoo.com