Mitosis is the process of nuclear cell division. During division, the nucleus of the cell divides, resulting in two sets of identical chromosomes, or organized DNA proteins. This process is almost always accompanied by a process called cytokinesis, in which the rest of the cell divides, leading to two completely separate cells, called daughter cells. There are four phases in the process: prophase, metaphase, anaphase and telophase. There are a number of reasons for this process, including reproduction and replacement of cells, and problems with it can seriously damage or kill cells. It's often confused with meiosis, but the processes differ in several ways.
The DNA in the nucleus has already been duplicated in the previous stage of cell division, so by the time prophase starts, the nucleus contains two complete identical sets of DNA. As prophase begins, the chromatin, which are normally spread throughout the nucleus, begin to condense into an X shape, held together in the middle with a specific sequence of DNA called a centromere. Each half of the X is one replicated half of DNA. Once they coil together into the X, they're called mitotic chromosomes. Towards the end of prophase, the material enclosing the nucleus and the cytoskeleton disappear, except in the case of some fungi, algae, and similar organisms, in which the process happens entirely inside the nuclear membrane. This is called closed mitosis.
Once the material enclosing the nucleus dissolves, or, in the case of closed mitosis, after the DNA forms into Xs, structures called centrioles move to opposite ends of the cell and help make a spindle apparatus of microtubules, which is essentially like ropes running across the cell. The chromosomes also develop structures in the middle called kinectochores, which are later used to hook onto the microtubules.
As prophase finishes and metaphase begins, the rope-like microtubules connect to the kinectochores on each side of the chromosome, so that they can later pull them apart. The chromosomes align themselves with the spindle apparatus, which is spread around the cell like the vertical lines on a globe. The soon to be divided chromosomes are symmetrically positioned on the metaphase plate, which is essentially the equator of the parent cell. At the end of metaphase, each chromosome has microtubules connected to both of its halves, and they are lined up in a straight line along the equator of the cell.
Once the chromosomes get lined up properly, the spindle apparatus immediately pulls the two identical DNA halves apart from one another and moves them to opposite sides of the cell. These two sets of chromosomes will develop into the nuclei of two daughter cells which are perfectly identical to each other and the parent cell.
After the chromosomes arrive at the ends of the cell, they start to uncoil and spread out again, as they were before they formed into Xs. This is basically the opposite of the beginning of prophase. While this happens, the spindle apparatus is broken down. After that, the nuclear membrane, which encases the nucleus, forms again around the chromosomes, unless it never dissolved in the first place, as in closed mitosis. Although this is the last phase, cell division is not complete until cytokinesis happens.
Cytokinesis is the next stage of cellular development, and is similar to mitosis, except it involves the other parts of the cell instead of the nucleus. During this phase, the equator-like metaphasal plate of the cell pinches together, separating the cell into two new cells. Once this is completed, there are two functioning, identical cells.
One of the main purposes of this process is the natural growth of the parent organism. It's also done to replace cells that are worn out, damaged, or just at the end of their natural lifespan. For instance, a person continually sloughs of dead skin cells, so the body has to divide cells to make new ones. Some animals also use this process to regenerate parts of themselves, like lizards who can regrow their tails after losing them. Additionally, some animals undergo this process as part of asexual reproduction.
Problems with mitosis are devastating for cells, and can result in their death. Even if the cell doesn't die, the chromosomes can be damaged or altered, which can lead to genetic disorders; Down syndrome, for instance, is caused by a chromosomal problem that's connected to mitosis. Additionally, damage to the chromosomes or problems with the timing of how the cell divides can lead to growths and sometimes cancer. This can also happen if the chromosome isn't pulled apart properly.
Mitosis and meiosis are both means of cellular division, but they differ in a few key ways. First, meiosis only happens in specific types of reproductive cells called gametes — in humans, eggs and sperm — and spores. Also, in meiosis, the DNA from each contributing cell gets mixed up, with little pieces of DNA from different cells forming parts of the X. This is different from mitosis where the two halves of the Xs are identical. Also, meiosis ends up with four cells that are completely genetically unique, whereas in mitosis, the end result is two completely identical cells.