Mitosis is the process by which new cells can be produced in order to heal an injury. The chromosomes in a somatic cell (body cell) are replicated before mitotic cell division can occur. This will insure a complete set of chromosomes will go to each new cell that results after cell division occurs. During the cell cycle, cells spend 90 percent of their time in interphase with 10 percent engaged in mitotic cell division. During interphase, a cell grows and synthesizes organelles during the first gap or G1. Next, the chromosomes are replicated during S phase. Finally, a second gap or G2 occurs when another growth period occurs. Now the cell enters the mitosis stage of the cell cycle. In prophase, the replicated chromosomes are visible under a microscope and are called sister chromatids. Meanwhile, the nuclear envelope is disappearing and a spindle is forming between the centrioles that are migrating to opposite poles in animal cells. Plant cells lack the centrioles. By metaphase, the chromosome pairs line up in the middle of the cell attached to the spindle fibers. Once this important checkpoint is reached, the cell begins to elongate and the chromosome pairs separate and move to opposite poles during anaphase. Next, in telophase each set of chromosome arrives at opposite sides of the cell and the cell's nucleus re-forms around them. In animals, a cleavage furrow splits the cytoplasm and pinches the cell into two identical daughter cells. In plants, a new cell plate forms from vesicles resulting in two identical daughter cells. Growth factors stimulate mitosis when there is an injury. Also, cells have density dependent growth and if an area is injured and cells are missing, cell division will occur until the area is filled in.