Cells do in fact "age" and many cell types in our bodies (and in those of other animals) actually die and are replaced on a regular basis. There are a few ways to determine the "age" of a cell. One way is to look at the chromosomes--they actually get shorter over time, reducing in size (at the ends) every time there is a round of DNA replication. Other ways involve looking at the basic machinery of the cell-- it's ability to create energy. This tends to decrease with age/time. Some cells in our body are very long-lived (that is, they don't turn over very rapidly) while other cells are constantly dying and being replaced. Can you determine what type of cells might have "short lives" and are being replaced regularly? Hints: If you cut yourself and lose 1 pint of blood, do you live the rest of your life with one pint less than you started with? How often do you get a haircut? Trim your nails?
Another aspect of this is something called "the cell cycle."
Basically, when a cell divides, it produces two daughter cells. These cells "grow" a bit (in a phase called G1), then they replicate their DNA (S phase), then they rest a bit, growing perhaps a bit more (G2 or gap 2 phase) as they prepare to divide, which is called Mitosis (M phase). Again, some cells go through a very fast, regular cell cycle, replacing dead cells. Other types may "arrest" (stop) at a certain point or just progress through the cycle very slowly.

Can you think of at least one human disease that might be cause by something going wrong with the cell cycle?

_____________________________________________________
For all of the questions: students interested in "cells and how they are studied" as well as "what is a cell biologist?" can contact the American Society for Cell Biology (ASCB) for free brochures and on-line information at:

http://www.ascb.org
--

We're made of cells, so whatever we're doing is because of what our cells are doing.But our cells don't live as long as we do - our old cells die, and new ones grow in their place. Scientists are very interested in your question, though - their name for cell death is 'apoptosis.' You could search for that on (http://www.google.com). A big question in science is whether we could live to be very old if we learned how to control the things that kill our cells.

Good question! Cells are always generally very tiny their entire lives. They don't really increase in size very much throughout their life and that is because of the way they are created. New cells are created by one of two processes; one is called mitosis and one is called meiosis. Meiosis involves mitosis, so I am just going to tell you about mitosis (also it is the more common of the two). Mitosis is basically when one cells clones itself and makes a new cell exactly like itself. Sort of like identical twin humans. During mitosis, one cell makes a copy of all of its internal workings, its DNA, its nucleus, and all of the other important cellular components. After this one cell has copied its entire insides, it then begins to increase its overall size slightly. As this takes place the cell also begins to separate the copied material from the original material at opposite ends of the enlarging cell. Over a few hours the middle of the cell begins to pinch in so that the cell starts to look like a bar bell. This process continues until finally the one cell is almost totally divided in half by this pinching in in the middle. Finally the process is complete and the cell that was once one, breaks apart into two new cells. Each of these two new cells are perfectly in tact, nearly exact copies of each other, and nearly the size of the
original cell when the whole process started. So the two new cells will increase in size a little bit, but very little. This whole process takes a few days.

Cells do get "old" too. Different types of cells can live for longer than others. Some cells live less than one day! Others can live for quite some time. As a cell gets old, generally its membrane gets leaky (the insides of the cell start leaking out) and other cellular processes start to fail (like when humans are dying and their organs begin failing). Eventually the cell's nucleus sends a message to the entire cell telling it to stop all processes and initiate the death sequence and shortly thereafter the cell dies and eventually disintegrates.