Cell division

Oh, are you alright, Leon? That's just a scratch. It'll heal in a couple of days. The damaged skin cells will be replaced by new ones: Do you know where they come from? No, it's not magic.

The answer is -- that cells divide. Most cells divide. Single celled organisms, like bacteria, reproduce through cell division. And in multiple cell organisms, like large animals and plants, cell division happens all the time. Growth results from cell division...

All cells die, and are replaced by new ones, that appear through cell division. Like when Leon's scratched knee heals. In the body, cells die around the clock, even though we're not hurt nor ill. There are cells that only last a few days, and others, like certain nerve cells that can last a hundred years. And most of them divide.

This is one way cell division can work. Firstly, the chromosomes in the cell nucleus are duplicated and doubled. The cell nucleus is dissolved, and the chromosomes line up. The chromosomes are pulled apart - the cell membrane retracts. Now, two new cell nuclei are formed around the chromosomes, and the cell division is complete.

From one parent cell, two daughter cells are created, both of which have exactly the same number of, and identical chromosomes to the parent cell. This is an ordinary cell division. It's called mitosis. But there's another kind of cell division. The first stage looks similar to ordinary cell division -- -- but there is an important difference.

These chromosomes are not exact copies of those of the mother cell. These are brand new, unique chromosomes. And watch! A second division appears! Without the chromosomes becoming more numerous!

They are shared out among the new daughter cells. Four cells with half the number of chromosomes. And every set of chromosomes is totally unique. This kind of cell division, from one cell to two... to four cells with unique genes, is called meiosis.

Meiosis is what makes reproductive cells. They're called eggs, in the female, and sperm, in the male. When an egg and a sperm fuse together during conception, they form a completely new individual. Since each reproductive cell has its unique set of genes, the new individual also has a unique set of genes. That is why you are -- unique.

Mitosis goes on in your whole body, in the lungs, the heart, and the skin... ... Whereas meiosis takes place only in our reproductive organs: in women's ovaries, and in men's testicles. In ordinary cell division, mitosis, the mother cell divides into two daughter cells with exactly identical chromosomes. In meiosis, the mother cell divides into four sex cells, each of them with a unique set of chromosomes. That's how it is, Leon.

Your knee heals, and you grow, thanks to mitosis. You are unique, thanks to meiosis. Yes! You're unique alright.