Where do cells come from?

Sometimes you accidentally bite your lip or skin your knee and you can see that in a matter of days, the wound heals.  Is it magic, or is there another explanation for what happens? 

Every day, every hour, every second one of most important events in life is going on in your body- cells are dividing. The ability of cells to divide into two living cells is unique for living things. 

The process of cell division is called mitosis. A single cell divides to make two cells. These two cells then further divide to form 4 cells and so on.

You and I began as a single cell, or what you would call an egg. By the time we are adults we have trillions of cells. Take a look at the movie on the right to see a single animal cell divide into two cells.

If think it would take a long time to build all those cells try this out. Let's pretend a single penny is a cell. Calculate the number of days it would take to reach a million dollars if you double the amount of pennies each day.  The answer can be found here.

Do cells know when they have to divide?

All cells follow a routine. Cells know when they have to take in food, when to grow and in the same way they know when it is time for them to divide. This routine is called the cell cycle. 

The life of a cell begins when a parent cell divides by mitosis. This process is very well controlled by the cell. When they are dividing, cells produce special proteins that help them in this process. These special proteins are called cyclins. They are called by this name because these proteins are produced only during a particular time of the cell cycle.

Sometimes cells lose control over their cell cycle and they start dividing uncontrollably and end up with a huge number of cells in a very short amount of time. This type of unusual behavior by cells is called cancer.

  What happens to old cells?

Just like us, cells grow old and die. When old cells die, new ones replace them. For example, a blood cell in our body lives for about 120 days. Another example is our skin cells. We shed our skin cells about every 35 days. Did you know our skin is also the largest organ in our body? Don't worry, we do not shed all our skin cells at the same time like snakes do. In human beings, only the skin cells that are old are shed, others are not. 

Here is a amazing fact for you, if your skin cells are replaced every 35 days, by the time you are 20 years old you would have replaced your skin cells roughly 200 times. You can see how we solved this puzzle below*.

* How'd you figure that? In each year there are 365 days (except for leap year when there are 366 days).  If we divide the number of days it takes to replace your skin cells (35) into the number of days in a year (365) you can see that the skin is replaced about 10 times. 365/35 = 10.428 Now if you replace your skin on average 10 times each year for 20 years you find that you have worn about 200 skins! 10 X 20 = 200 Now it's your turn. How many skins have you had? How many skins will you have had by the time you are 35 and 50 years old?