• Cell divisionmitosis image by caironbohemio from Fotolia.com
  
  In nature cells are constantly growing, reproducing, and dying. Each cell in your body is continually being replaced as old cells die. The cycle has four stages: growth, synthesis, more growth, and mitosis. The activities conducted by the cell in each stage of the cycle are controlled by proteins in the cell cytoplasm. Chemical checkpoint mechanisms ensure DNA integrity during the cell cycle.
  
  

Gap 1

• Preparing to Replicatemolecule image by Oleg Verbitsky from Fotolia.com
  
  Gap 1 is the first part of the cell cycle. In this stage of the cycle the cell grows and chromosomes develop. During this time the cell grows larger in preparation for cell division. The term "cell division" is actually a misnomer in that the cell does not divide. Instead, the DNA of the cell is copied and each copy then has the capacity to become a new cell.
  
  During gap 1 the first checkpoint of the cell cycle occurs. Special mechanisms inside the cell assess the DNA for damage and repair it if possible. This checkpoint is called the restriction point because cells with damaged DNA that cannot be repaired are shut down by the sensor mechanism, and the cycle ceases for that cell. Cells that have intact DNA continue to the next phase in the cycle.
  
  

Synthesis

• Chromatids form in the nucleus.atom image by Oleg Verbitsky from Fotolia.com
  
  During the synthesis stage of the cell cycle each chromosome of the cell is copied to produce two identical sets of DNA. The two strands of DNA material are joined together, forming a chromatid. The point where the DNA strands connect is called the centromere. After the strands separate they are called sister chromatids.
  
  After the cellular DNA has been copied the cell has twice as many chromosome pairs as before. For example, in a human cell with 23 chromosome pairs the genetic material becomes two strands of 23 chromosomes joined at the centromere. The DNA material is all inside the cell nucleus at this point in the cycle. The DNA material is not yet distinguishable as individual chromosomes because they are in the form of loosely packed fibers.
  
  

Gap 2

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  Gap 2 is a period of rapid growth in the cell. During this stage of the cell cycle the protein checkpoint mechanism again assesses the DNA material for damage, and if the DNA is undamaged, the cell is ready for the next step in the cycle.
  
  Understanding how the checkpoint mechanism in this stage of the cell cycle operates is helping in the treatment of cancer. According to the American Association for Cancer Research publication "Molecular Cancer Therapeutics" by Takumi Kawabe, G2 checkpoint inhibitors are being used to develop genetic treatments for cancer that would be less harmful to patients than traditional cancer treatments.
  
  

Mitosis

• Two new cellsdna 3 image by chrisharvey from Fotolia.com
  
  Mitosis is the final stage of the cell cycle. It consists of four distinct activities called prophase, metaphase, anaphase, and telophase. During mitosis the chromatid strands separate and are pushed by molecular motors to opposite ends of the cell.
  
  This effectively forms two nuclei each containing a complete copy of the cells genome. The materials that had formerly connected the two sets of genetic material now form into a cell wall separating the new cells. Now the cell has replicated itself and the cycle can begin again.