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Mitosis is the process for making new cells. Occurring in a very short period of of cell's life span, mitosis is essential to repairing cuts, making new blood cells, and replacing any other cell in your body.
As mentioned on the main cell page, most of the work that a cell does occurs during the G1 phase of its growth. But as soon as that cell reaches its upper size limit, its DNA clock sets of a series of events that literal cuts the cell in half. Yes, the DNA clock will tell the cell to divide into two new daughter cells. But what happens?
The first event that must take place is the copying the cell's DNA, DNA replication . This occurs during the S phase of the cell's life cycle. During the S phase, the DNA spread through the cell's nucleus begins to go through DNA replication. Remember that at the end of mitosis, two cells will be made, and they need their own complete set of DNA to work properly. The two new, identical strands of DNA are called chromatids. These chromatids stay close to each other. In fact, they are still attached to each other at an area called the centromere.
Next, the DNA begins to condense, to coil with the help of proteins into the well defined shape we know as chromosomes. Why must DNA condense? When lecturing, I ask my students what would be easier when going on a trip, to pack their cloths in a suitcase or overnight bag, or to just grab cloths in their arms. Of course it is easier to carrying cloths in a suitcase, just as it is easier to move DNA to its new home in the daughter cell by packing and coiling first. So this is what the DNA does. This condensation of DNA into its chromosome form occurs in the prophase. Now the stage is set, and mitosis can begin in earnest.
Mitosis, it's all about the phases
by Willey Chen
The dance of mitosis begins with prophase, with the condensing and coiling of DNA into chromosomes. This is a step that takes up 60% of the time that the cell is undergoing mitosis. But as soon as the chromosomes are formed, and the nuclear membrane breaks down, the next step, metaphase, begins. During metaphase, centrioles sends out their spindle fibers like fly fishermen casting their lines. The centrioles are good fishermen, for they catch their fish, the chromosomes, pulling them this way and that, until they are form a neat line in the middle of the cell. This is metaphase.
Why should the centrioles line up the chromosomes. What happens sometimes when you tie your shoes? Every now and then, when you attempt to untie them, the strings form a knot. Why? This happened because the string became tangled. But if you looked carefully at your shoe strings first to make sure that when you pull the ends, they slide neatly pass each other, you would not get a knot. The centrioles make sure that the chromosomes do not get tangled with each other, and form a knot, get broken. For if that happens, one new daughter cell would not have all of the DNA that it needs, and the other daughter cell would have far more DNA than it is good for it. Still metaphase happens quickly, then the cell is ready for the next phase.
Next, anaphase occurs. In anaphase, the centrioles pull the spindle fibers in, like that fisherman reeling in a fish. Just the centrioles are pulling the chromosomes, pulling it in half. Simply said, the centrioles are pulling the chromatids apart, pulling each to different halves of the cell lickety-split.
Now comes the action, telophase. In telophase, the chromatids have been pulled to different sides of the cell, so they can now be repackaged. A new nuclear envelope begins to form around each set of chromatids, making two new nucleus in the old cell. Once the chromatids are safely tucked away in their new nucleus, the cell is ready for the last step in mitosis.
Finally, cytokinesis begins. Cytokinesis is the last step in mitosis, where the old cell pinches itself in the middle, until it literally pinches itself in half. The cell pulls its membrane into the middle, keeping each of the new nucleus on different sides of the pinch. When the membrane meets in the middle, it fuses, then splits to finally form two new daughter cells. That is mitosis, the last step in a cell's life cycle.