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Human cell division is the process by which cells within the human body divide their chromosomes and split in to two daughter cells to sustain the organisms in which they reside. Cell division is also how humans, and the organs within them, grow. The process by which somatic, or body, cells divide is called mitosis. The steps that humans and animals undergo during mitosis are identical. There are some differences in the processes of human cell division and cell division in plants, although the major steps are the same.
Mitosis consists of four phases. During prophase, chromosomes condense, winding tightly around a mitotic spindle. The chromosomes, once invisible, can be seen under a microscope during most of mitosis. The mitotic spindle forms on the outside of the cell during human cell division, and around the nucleus in plants undergoing division. In plants, a cytoskeletal band also forms during prophase.
The second phase of mitosis is metaphase. During this phase, the membrane around the nucleus disappears and chromosomes move into the middle of the cell. The chromosomes split in half, giving the cell twice as many chromosomes as it started with. The steps of metaphase in human cell division are almost identical to that in plant cell division. The one exception is that the cytoskeletal band that formed during plant prophase disappears.
During anaphase, the chromosomes in the mother cell spread apart. Each group of chromosomes moves in an opposite direction toward the outside edges in preparation for cell division. There are no major differences in how anaphase is carried out in plant and animal cells.
Telophase is the last stage of mitosis. At the beginning of the phase, two sets of chromosomes appear at polar ends of the cell. Two nuclei begin to appear around each set of chromosomes and the chromosomes begin to unwind from the mitotic spindle and disappear. In human cell division, rings that will serve as the cell membrane in the daughter cells form around the middle of the cell. In plants, cytoskeletal proteins that will serve as the new cell wall appear.
Cytokinesis, which occurs during anaphase and telophase, is critical to cell division. The cytoplasm and organelles are divided during cytokinesis. In humans, the original cell is split in the center during cytokinesis, the new membrane is fused, and the nuclei separate. In plants, a new cell wall grows to form the two daughter cells.
The one thing that all cancers have in common is that they involve unregulated cell growth. This is why it usually doesn't make much sense when someone says they want to find a "cure for cancer" because the growth can be caused in different ways, and the different cells involved require different treatments as well.
I actually read a science fiction book once where the author speculated that they found a cure for cancer which eventually made it so that the whole population didn't live for longer than a handful of decades, because of course, if you interfere with cell division processes in the human body, you could end up with terrible, unintended consequences.
That's why they have to be
so careful. All the normal cells around the cancer still need to function or the person will die anyway. Targeting cancer cells that are dividing out of control (when, often they hardly look any different from the cells around them) is the difficult task.
@Iluviaporos - Actually, that estimate is something of a myth. Most cells in your body which are replaced are being replaced constantly. The only one I know of which usually has a seven year cycle is hair, but it can be anything from two to seven years. Most of them are much shorter, like days, weeks or months. I believe ed blood cells live for around four months before being replaced.
And as you say, certain brain cells don't usually get replaced (although they have found that in some instances they can replicate).
Plus, think about things like tooth enamel. If it was being constantly replaced every seven years we'd all have fewer cavities. Not to mention things like smokers' lungs and so forth.
It's a simple way to interpret the fact that our bodies are constantly renewing themselves. But it is a bit too simple for a real understanding of the body, and unfortunately, it invites misinterpretation.
I've heard that every seven years approximately, you become a whole new person because of cell division.
It doesn't happen just like that, of course, it happens gradually over the course of the seven years, with new cells replacing old cells in almost all parts of your body.
But if you were to take a person from seven years ago and a their current body, there would be almost no cells in common.
The only exception is the brain I think. Those cells pretty much stay with you all your life and don't replicate.