A eukaryotic cell typically has a great deal of genetic material. For example, if the DNA of a human cell were uncoiled and lined end-to-end, it would extend approximately two meters! This genetic material is replicated before cell division and then must be divided equally between daughter cells. In mitosis, which is the division of the nucleus, the chromosomes condense into bodies small enough to travel efficiently in the cell, and then the chromosomes play out an intricate choreography. At the end of mitosis, the chromosomes have been partitioned into two new nuclei. These nuclei become part of daughter cells that are genetically identical to each other and to the parent cell.
The accompanying animation depicts the movements of chromosomes in mitosis, the dissolution and reformation of nuclear membranes, the formation of the mitotic spindle, and the division of the cytoplasm (called cytokinesis) that results in two new daughter cells.
In mitosis, the nucleus divides to produce two nuclei that are genetically identical to each other and to the parent nucleus. To prepare for division, the DNA replicates in the preceding interphase. Although the chromosomes are not yet compacted and visible as discrete bodies, we illustrate them as such to show the formation of a second chromatid per chromosome.
Also during interphase, the cell replicates its centrosome. Centrosomes are microtubule organizing centers, which are important for chromosome movement during mitosis. In many organisms, each centrosome contains a pair of centrioles.
In the first phase of mitosis, called prophase, chromatin coils and condenses, resulting in compacted chromosomes that are visible with a light microscope.
In prometaphase, the nuclear envelope breaks down and microtubules emanating from the centrosomes attach to the chromosomes. The microtubules attach to structures called kinetochores, which are found in the centromeres of the chromosomes.
In metaphase, the centromere regions connecting paired chromatids become aligned in a plane at the cell's equator.
In anaphase, the centromere pairs from each chromosome separate, and the single chromatids—which are now considered individual chromosomes—move toward the poles.
In telophase, the separating chromosomes reach the poles. The division of the cytoplasm, called cytokinesis, also begins during this phase. As the cells enter the next interphase, the nuclear envelopes and nucleoli re-form and the chromatin again becomes diffuse.
Although mitosis is a continuous event, biologists typically divide it into a series of phases. The phases of mitosis are the following:
· In prophase, the chromatin condenses, causing the
chromosomes to shorten. The nucleolus also breaks down and
the centrosomes move apart and begin to build the mitotic
spindle.
· In prometaphase, the nuclear membrane breaks down, allowing
the spindle to invade the nuclear region. The spindle fibers
capture the condensing chromosomes by the kinetochores
found at their centromeres.
· In metaphase, the spindle fibers pull the chromosomes to the
equatorial plate.
· In anaphase, the spindle fibers shorten and the opposite poles of
the cell move apart. These processes pull the chromosomes
apart, such that each chromatid of a chromosome is dragged to
the opposite pole of the cell.
· In telophase, the spindle breaks down. The chromatin also
begins to uncoil, causing the chromosomes to elongate. Nuclear
membranes form around the decondensing chromosomes, and
nucleoli again appear.
Cytokinesis, the division of the cytoplasm, follows mitosis. In this part of the cell division cycle, a contractile ring pinches the cell at its midplane. The two daughter cells that form are each genetically identical to the parent cell.