Cells divide for two reasons:. Interphase : Each chromosome undergoes replication, making an identical copy of itself. At this point, the chromosomes are still long and thin, and are not visible inside the nucleus.
Cells spend most of their life in this non-dividing phase. See the graphic below. The first stages of the cell cycle involve cell growth, then replication of DNA. The single strand of DNA that makes up each chromosome produces an exact copy of itself. All of the organelles inside the cell are also copied. These processes happen in a stage of the cell cycle called interphase. Once the cell has completed all the necessary processes during interphase, it is ready to enter into the next stage of cell cycle.
The cell now undergoes a type of cell division called mitosis. In mitosis, the chromosome copies separate, the nucleus divides and the cell divides. This produces two cells called daughter cells. By metaphase, the chromosomes are perfectly lined up in the middle of the cell. During anaphase, the chromosomes split in two and are pulled to opposite ends of the cell. Finally, in telophase, each set of chromosomes is surrounded by a new nucleus, and the cell begins to cleave in half.
Through a process known as cytokinesis , the single cell breaks apart into two identical cells, completing the reproduction. As plants and animals age, most also grow in size. Mitosis creates the cells necessary to add more mass to the body, as well as more cells to cope with the growth, such as new blood cells.
It should be noted that not all cells in the human body undergo mitosis or other forms of reproduction. Nerves and muscle cells do not. After the body has made enough nerve and muscle cells, it does not have a mechanism in place to make any more. When an organism is injured, mitosis occurs to replace the damaged cells. This repair is especially important for skin and blood vessels, which protect and oxygenate muscles and organs in the body.
Mitosis also helps replace blood lost through a wound.
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