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The human cell cycle consists of two main phases: interphase and mitotic phase. For most cells, the mitotic phase involves mitosis, but reproductive cells also undergo meiosis. The interphase part of the human cell cycle is when cells both engage in their normal cell function and prepare for division. Mitosis is where the cells divide, creating two separate but identical daughter cells. In meiosis, the product is four daughter sex cells, or gametes, with half the amount of chromosomes as its mother cell.
Most cells undergo mitosis as the final part of the human cell cycle. As cells grow during interphase, they may become too large to be efficient in waste disposal and nutrient absorption. That and other signals, such as hormones, may tell the cell when it is ready to divide. Before it divides, a cell will replicate its DNA so that each daughter cell will receive a copy. Once mitosis is complete, the result is two daughter cells with identical DNA.
There are many controls and stops that regulate cell division and growth. At several points along the human cell cycle, the cell will have to meet certain criteria before it can move on to another phase. In addition, cells will often stop dividing once they touch another cell. Cancer cells are an exception to the rule. These cells no longer have the ability to stop dividing and so can produce dangerous tumors.
In order to produce reproductive cells — that is, sperm or eggs — a cell must undergo meiosis as a part of the human cell cycle. Meiosis is a process in which a cell divides into four cells, each with half the chromosomes as the mother cell. During meiosis, the cells divide twice. Before the first division, chromosomes are copied and undergo two unique processes called synapsis and crossing-over. These processes provide the four resulting daughter cells with greater genetic variety after the second meiotic division.
The interphase portion of the human cell cycle is the phase in which the cell performs most of its cell activity. There are three main parts of interphase — G1, S, and G2. The “S” refers to “synthesis” and is the part of the cell cycle where DNA is replicated for mitosis. The two “G” phases refer to the time before the S phase begins and the time between the end of the S phase and the beginning of mitosis. Though the cell may prepare for cell division during the G and S phases, it is also constantly performing its cell duty throughout interphase.
Once it is created, a cell enters the G1 phase. The amount of time a cell spends in this phase depends on the type of cell it is. In some cells, it can last for day and in others for years. A cell in the G2 phase is undergoing final preparations for cell division. Another phase, G0 refers to a cell that will temporarily or permanently not divide.