Most cells grow, perform the activities needed to survive, and divide tocreate new cells. These basic processes, known collectively as the cell cycle, are repeated throughout thelife of a cell. Of the various parts of the cell cycle, the division portion isparticularly important, because this is the point at which a cell passes itsgenetic information to its offspring cells. In many situations, division alsoensures that new cells are available to replace the older cells within anorganism whenever those cells die.

Prokaryotic cells, which include bacteria, undergo a type of cell divisionknown as binary fission. This process involves replication of the cell"schromosomes, segregation of the copied DNA, and splitting of the parent cell"scytoplasm. The outcome of binary fission is two new cells that are identical tothe original cell.

In contrast to prokaryotic cells, eukaryotic cells may divide via either mitosisor meiosis. Of these two processes, mitosis is more common. In fact, whereasonly sexually reproducing eukaryotes can engage in meiosis, all eukaryotes —regardless of size or number of cells — can engage in mitosis. But how doesthis process proceed, and what sorts of cells does it produce?

During mitosis, a eukaryotic cell undergoes a carefullycoordinated nuclear division that results in the formation of two geneticallyidentical daughter cells. Mitosis itself consists of five active steps, or phases:prophase, prometaphase, metaphase, anaphase, and telophase. Before a cell canenter the active phases of mitosis, however, it must go through a period knownas interphase, during which it growsand produces the various proteins necessary for division. Then, at a criticalpoint during interphase (called the Sphase), the cell duplicates its chromosomes and ensures its systems are readyfor cell division. If all conditions are ideal, the cell is now ready to moveinto the first phase of mitosis.

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Figure 1:During prophase, the chromosomes in a cell"s nucleus condense to the point that they can be viewed using a light microscope.
Prophase is the first phase ofmitosis. During this phase, the chromosomes inside the cell"s nucleus condense andform tight structures. In fact, the chromosomes become so dense that theyappear as curvy, dark lines when viewed under a microscope (Figure 1). Becauseeach chromosome was duplicated during S phase, it now consists of two identicalcopies called sister chromatids that are attached at a common center point called the centromere.
Important changes also take place outside of the nucleus duringprophase. In particular, two structures called centrosomes move to opposite sides of the cell during this phaseand begin building the mitotic spindle.The mitotic spindle plays a critical role during the later phases of mitosis as it orchestrates the movement of sister chromatids to opposite poles of the cell (Figure 2).
After prophase is complete, the cell enters prometaphase. During prometaphase, thenuclear membrane disintegrates and the mitotic spindle gains access to the chromosomes. During this phase, a protein structure called the kinetochore is associated with the centromere on each sister chromatid.Stringlike structures called microtubulesgrow out from the spindle and connectto the sister chromatids at their kinetochores; one microtubule from one sideof the spindle attaches to one sister chromatid in each chromosome, and onemicrotubule from the other side of the spindle attaches to the other sisterchromatid (Figure 3a).
In metaphase (a), the microtubules of the spindle (white) have attached and the chromosomes have lined up on the metaphase plate. During anaphase (b), the sister chromatids are pulled apart and move toward opposite poles of the cell.
Following prometaphase, metaphase begins. At the start of metaphase, the microtubulesarrange the chromosomes in a line along the equator of the cell, known as the metaphase plate (Figure 3b). Thecentrosomes, on opposite poles of the cell, then prepare to separate the sisterchromatids.
After metaphase is complete, the cell enters anaphase.During anaphase, the microtubules attached to the kinetochorescontract, which pulls the sister chromatids apart and toward oppositepoles of the cell (Figure 3c). At this point, each chromatid isconsidered a separate chromosome.
Figure 4:During telophase, two nuclear membranes form around the chromosomes, and the cytoplasm divides.
Finally, once anaphase is complete, the cell enters the last stage of the division process — telophase.During telophase, the newly separated chromosomes reach the mitoticspindle and a nuclear membrane forms around each set of chromosomes,thus creating two separate nuclei inside the same cell. As Figure 4 illustrates, the cytoplasmthen divides to produce two identical cells.
As previously mentioned, most eukaryotic cellsthat are not involved in the production of gametes undergo mitosis. Thesecells, known as somatic cells, areimportant to the survival of eukaryotic organisms, and it is essential thatsomatic parent and daughter cells do not vary from one another. With few exceptions,the mitotic process ensures that this is the case. Therefore, mitosis ensures thateach successive cellular generation has the same genetic composition as theprevious generation, as well as an identical chromosome set.

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