What is Mitosis?
Mitosis is a process whereby a cell that is preparing to divide separates the duplicated pairs of chromosomes from each other so that each of the new daughter cells ends up with a complete set of chromosomes. The overall strategy is to produce two daughter cells that are identical to each other as well as to the original parent cell. The process can be thought of as a series of choreographed movements involving:
1.) the removal of the nuclear membrane and nucleolus to clear the path for transport,
2.) the "packing" and condensing of long molecules of DNA to protect them from ripping and tearing during moving,
3.) the building of "drawstrings" to physically separate duplicated DNA molecules, and
4.) the formation of new daughter nuclei.
Some cells, such as nerve and muscle cells, never undergo mitosis, but remain permanently in interphase. Other cells, such as embryonic cells, may divide every 20 minutes.
Mitosis is usually separated into stages, shown in the accompanying figure. Keep in mind, however, that the events take place on a continuum, and grade into each other so that a differentiation can be made between "early" and "late" phases of each stage.
Prophase
During the first stage, prophase, the nuclear membrane and nucleolus break down and disappear. DNA becomes condensed and coiled around proteins called histones, and chromosomes are formed. At this stage the chromosomes consist of identical paired chromatids, called sister chromatids, attached at a point called the centromere.
In cells other than plants, two pairs of small cylindrical structures called Centrioles near the center of the cell separate and begin to move, one pair toward each end of the elongating cell, along a spindle-shaped assembly of microtubules called spindle fibers. Each Centrioles pair consists of a mature Centrioles and a half-formed daughter. Near the end of prophase, the chromatids become attached to the spindle fibers at structures called kinetochores on the centromeres.
Metaphase
In the next stage, metaphase, the sister chromatids are pulled into position by spindle fibers, and align themselves at the equator of the cell to form the metaphase plate.
Spindle fibers called polar fibers extend from one end (pole) of the spindle to the other. Other fibers, called kinetochore fibers, join the ends of the spindle or spindle poles to an area of the chromosome centromere called a kinetochore. The kinetochore is specialized for attachment to spindle fibers.
In cells other than plants, at each pole, the centriole pair is embedded in a dense area called an aster, from which microtubules extend in all directions.
Anaphase
Anaphase follows metaphase, and the centromere of each pair of chromatids divides. Once separated, the chromatids are now called chromosomes, and they are drawn toward the poles by the spindle fibers. Each chromosome is derived from one member of the sets of paired chromatids.
Telophase
In the final stage, telophase, the chromosomes have become completely separated, with each chromosome from each pair positioned at opposite poles. Nuclear envelopes form around the chromosomes, or, in plants, cell walls are generated between the daughter cells. Nucleoli reform, chromatin disperses in the nucleus, and a nucleolus appears.
To review: the process of mitosis can therefore be thought of as a mechanism that accomplishes the following:
Avoids chromosome breakage by condensing and shortening the chromosome length so that they can move through the cytoplasm without getting torn.
Removes organelles and structures in order to clear a path for chromosomes to move through in their migration to opposite poles of the cell.
Organizes and orchestrates an orderly separation of duplicate DNA molecules from each other in order to ensure that each daughter cell receives a correct and complete set of chromosomes.
Builds the mechanical apparatus to physically separate the duplicated DNA molecules.
Restores the organelles and cellular structures that were previously disassembled.