When a human sperm and an egg come together, they combine all of their genes to form the first cell of a new human being. This first cell then must become all of the cells that make up a human: nose, hair, skin, teeth, heart, bones, and everything else! Mitosis is the process where one body cell becomes two body cells. The two cells that result are called daughter cells. Even though the two daughter cells have exactly the same genetic information as the mother cell, they may be slightly different from each other.
Every cell in the human body has 46 chromosomes of DNA in the nucleus. Those 46 chromosomes are all paired up, so we often say that humans have 23 pairs of chromosomes. You get one set of chromosomes from your mother and one set from your father. In the diagram, we can say that the dark chromosome came from your father and the light chromosome from your mother. This is called a pair of homologous chromosomes because they contain similar genetic information. What’s interesting is that every cell in your body (except one special place which we’ll see in a minute) has the same exact chromosomes!
When we talk about mitosis, we’re concerned with what the chromosomes do in each cell. The trick is that each of those 46 chromosomes needs to make a copy of itself so the two daughter cells can have the same chromosomes. The first thing that the cell needs to do is to grow in order to prepare for splitting into two cells. This first phase is called interphase, or the growth phase.
While the cell is growing, the chromosomes are not organized: they are like strands of spaghetti spread throughout the nucleus. During the next phase of mitosis, prophase, the chromosomes get organized and ready for mitosis. Then, all of the chromosomes line up in the middle of the cell in metaphase. In anaphase, the chromosomes split apart at the centromere, which is the button-like structure in the middle, into their sister chromatids. Those sister chromatids go to opposite sides of the cell, and then the cell begins to split into two separate cells. This final phase is called telophase and results in two identical cells. During the next interphase, each sister chromatid now makes a copy of itself in order to be complete. After that, the whole process happens again!
Normally, mitosis stops in parts of the body where cells are crowded together. In fact, certain genes in your cells tell them when they should stop reproducing. Sometimes, these genes get damaged and the cells keep reproducing; often, these cells that are growing out of control form cancers. There are many causes of cancer, because there are many reasons that the genes can become damaged: age, viruses, tobacco smoke, diet and disorders that you inherit from your parents can all cause cancer.