Mitosis describes the process by which an eukaryotic cell divides and reproduces itself. At each phase of mitosis the cell completes another step toward dividing into two genetically identical daughter cells. Some scientists include a sixth or seventh phase in the process, although there is some disagreement as to whether the interphase is a part of mitosis, or if it is just a phase of preparation for the mitosis to occur.
At this point the cell has a lot of metabolic activity occurring as it prepares for the rest of mitosis. The nucleus or center of the cell activity may or may not be visible as a black spot. It is during this phase that the cell grows storing enough of the energy required to fuel cell division. The DNA has now replicated but not formed a chromosome, it is called chromatin.
Chromatin, the building blocks of chromosomes, condense and begin to become visible under the microscope. The black spot of the nucleus disappears as the centrioles or microtubule organisms begin to separate and move toward opposite ends of the cell. This is seen through the fibers that appear to cross the cell in the center where the division will later take place. The DNA strands shorten and condense to form the chromosomes.
The membrane of the cell dissolves as the proteins of the cell begin forming the kinetochores and the chromosomes begin moving. The fibers begin to align all the genetic material or chromosomes along the middle of the cell. The genetic material on each side of the cell now should be identical with each side receiving one copy of each chromosome.
The identical chromosomes separate at the kinetochores formed by the proteins and begin moving to opposite sides of the cell. This causes what is seen as movement under the microscope. The interaction between the polar microtubes also causes a movement of sorts. The fibers shorten as the division of the cell begins.
The chromosomes are not visible in the remaining phases as new membranes form around each new cell. The fibers disperse and seem to pull the two cells apart when viewed in a microscope. These fibers then disintegrate. Technically, all phases of mitosis are complete at this point, but it leaves the final step of the process out.
Cytokinesis occurs in animal cells when the fiber ring made of actin forms around the center of the cell, which completes the process of the one cell becoming two. Each new cell now has its own nucleus.