It is the type of cell division, which ensures the same number of chromosomes in the daughter cells as that in the parent cells. It can take place in haploid as well as in diploid cells in nearly all parts of the body if and when required. In spite of slight differences, major steps of mitoses are similar in plants as well as in animals. However, to avoid the confusion our statement will base on the animal cell. Mitosis is a continuous process, but conventionally it may be divided into two phases, i.e. karyokineses, which involves the division of nucleus and cytokinesis that refers to the division of the whole cell.
Karyokineses
Early in the mitosis the two pair of centrioles separate and migrate to opposite sides of the nucleus, establishing the bipolarity of the dividing cells. At the beginning of the process in an animal cell, the partition of the centriole takes place, which have been duplicated during interphase but present in the same centrosome.
Three sets of microtubules (fivers) originate from each pair of centrioles. one set the astral microtubules, radiate outward and form aster, other two sets of microtubules compose the spindle. The kinetochore microtubules attach to chromosomes at kinetochores and polar microtubules from the opposite pole. These microtubules are composed of a protein tubulin and traces of RNA.
This specialized microtubule structure including aster and spindle is called mitotic apparatus. This is larger than the nucleus, and is designed to attach and capture chromosomes, aligning them and finally separating them so that equal distribution of chromosomes is ensured. Karyokineses can further be divided into prophase, metaphase, anaphase and telophase for thorough understanding, though it is a continuous process.
Prophase
During interphase (non-dividing phase) of the cell cycle the chromosomes are not visible even with electron microscope, but using histologic stains for DNA, a network of very fine threads can be visualized. This network is called as chromatin.
Chromosomes become more and more thick ultimately each chromosome is visible having two sister chromatids, attached at centromere. The chromatin material gets condensed by folding and the chromosomes appear as thin threads at the beginning of prophase. Towards the end of prophase, nuclear envelope disappears and nuclear material is released in the cytoplasm, nucleoli disappear. Mitotic apparatus is organized (as described above). Cytoplasm becomes more viscous.
Metaphase
Each metaphase chromosome is a duplicated structure which consists of two sister chromatids, attached at a point called centromere or primary constriction. The centromere has special area, the kinetochore, with specific base arrangement and special proteins where kinetochore fibers of mitotic apparatus attach.
The kinetochore fibers of spindle attach to the kinetochore region (specialized area in centromere) of chromosome, and align them at the equator of the spindle forming equatorial plate or metaphase plate. Each kinetochore gets two fibers one from each pole.
Anaphase
The kinetochore fibers of spindle contract towards their respective poles, at the same time polar microtubules elongates exert force and sister chromatids are separated from centromere. As a result, half sister chromatids travel towards each pole. It is the most critical phase of the mitosis, which ensures equal distribution of chromatids in the daughter cells.
Cytokinesis
During late telophase the astral microtubules send signals to the equatorial region of the cell, where actin and myosin are activated which form contractile ring, followed by cleavage furrow, which deepens towards the center of the cell, dividing the parent cell into two daughter cells. Mitotic events in plant cells are generally similar to the events observed in animal cells but there are some major differences. Moreover, shape of the plant cell does not change greatly compared with an animal cell because it is surrounded by a rigid cell wall. Most higher plants lack visible centrioles, instead they have its analogous region from which the spindle microtubules radiate. At cytokinesis, in place of contractile ring a membrane structure, phragmoplast is formed from vesicle which originate from Golgi complex. The membrane of vesicles becomes the plasma membrane of daughter cells. These vesicles originate actually during metaphase, line up in the center of the dividing cell, where they fuse to form phragmoplast at the end of telophase. These vesicles also contain materials for future cell wall such as precursors of cellulose and pectin.
