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Cell Cycle

The life of a somatic (body) cell has 2 basic parts: interphase and mitosis. Interphase, which is the growth period of the cell, consists of the G1, the S, and the G2 phases. The G1 phase is the phase during which the cell goes about its normal specialized function. There is a point called the restriction point, which marks the transition from G1 phase to S phase. Once the cell passed this point, it has started the process leading up to cell division, and cannot go back. At this point, there is also a checkpoint called the G1 checkpoint, which makes sure that the cell is the right size and is in good condition for division. During S phase, the DNA is replicated, forming the two sister chromatids that are necessary for mitotic division. When replication is complete, the cell proceeds into G2 phase during which all the proteins needed for mitosis are translated and prepared.

Finally, mitosis begins. Mitosis consists of four phases: prophase, metaphase, anaphase, telophase. During prophase, the nuclear membrane dissolves, the chromosomes condense, and the spindle fibers form. Then metaphase starts with the chromosomes attaching to the spindle fibers by their kinetochores and beginning to seperate. The daughter chromatids, the two copies of the chromosome that used to be attached in an 'X' shape, finish moving to the opposite ends of the cell during anaphase. Finally, the nuclear membranes reform, the spindle fibers disassemble, and the chromosomes uncondense during telophase. At the end of mitosis, is cytokinesis, which is when the cell membrane pinches and divides into two seperate cells.

A cell will divide approximately fifty times before apoptosis, programmed cell death. This is called the Hay Flick Limit. This average exists because every time a cell divides, the telomeres on the chromosomes shorten. The telomeres are DNA caps on the ends of the chromosomes that pad the important DNA. When the telomeres disappear, vital DNA is damaged and the cell dies.

There is another phase that can occur during interphase in certain cells, such as mature brain cells. This phase is called the G0 phase. The cell never moves towards division in this phase. This phase can be induced by depriving the cell of nutrients.