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Look in the mirror. What do you see? Now, pretend you could look at yourself under a big microscope. If you could, you would see zillions and zillions of tiny cells. In fact, if one of your cells were the size of a dime, you would be the size of the Empire State Building! It's a good thing you have all those cells. Without them, there is no way you could do everything you do. You would just be one cell, so you'd be one big blob!
That is the way you, as well as every person, started out—as one tiny cell. That first cell had all the information and instructions to make you as you are now. All that information was stored in the middle of the cell in a secret code called DNA. DNA is a very long list of instructions that gives each cell in your body its own shape and job to do.
That very first cell divided into two identical cells in a process called mitosis. Mitosis is the simple duplication of a cell and all of its parts. The cell duplicates its DNA, and the two new cells have the same pieces and genetic code as the original one. It's like you told your friend a secret, and your friend told two friends, and then each of them told two friends—and friends were telling each other your secret 2 million times per second! Imagine how many people would know your secret!
In humans, cells have different roles to play. There are red blood cells that carry oxygen, skin cells that protect the body, bone cells that make the skeleton, and nerve cells that send messages around the body. Cells are always busy, working and dividing. Cells divide, or regenerate, to replace cells that are worn out and die. In fact, before you finish reading this, millions of cells in your body will have killed themselves—in the interest of keeping you alive.
Not all cells regenerate at the same rate. If all of the cells in a healthy human body had a race to see which could divide the fastest, the red blood cells in the bone marrow, which divide at a rate of 2 million per second, would be among the winners. So would the skin cells, which live only three weeks, and the cells in the intestine. The losers would be the muscle cells, which don't carry on mitosis after birth, the nerve cells, which lose their ability to divide once they reach maturity, and the liver cells, which will divide only if part of the liver is removed. Many experts also believe that over-consumption of alcohol can kill nerve and brain cells and that they can never be replaced or "grow back."
Understanding how quickly cells regenerate and die can lead to medical breakthroughs. If cells regenerate too quickly, it might indicate certain cancers are present. Too-rapid cell death appears to play a key role in many disorders, including heart disease, strokes, and Alzheimer's disease.
Okay, look in the mirror again. While you were reading this, 30,000–40,000 dead skin cells fell off your body! Gross!
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