MadSci Network: Cell Biology |
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
Try the links in the MadSci Library for more information on Cell Biology.