The process of somatic cell division called Mitosis is divided into 4 phases namely prophase, metaphase, anaphase and telophase. Each of these phases of mitosis could be clearly recognised microscopically by its characteristic features when appropriate cell preparations are made and observed.
Fig: Diagrammatic Representation of Different Stages of Mitosis in an Animal Cell
The prophase begins with the separation of centrioles and their migration along the edge of the nuclear membrane in opposite directions. As the centrioles reach the opposite poles they get surrounded by microtubules, the astral rays (representing spindle fibres). Centrioles at this stage appear as 'star' like bodies and are called asters. In plants lacking centrioles the microtubular system of spindle fibres is synthesised in the general cytoplasm.
By late prophase, the nucleolus disappears, the nuclear membrane breaks down, and the microtubular system gets organised into the mitotic spindle. It is with the help of this spindle that the separation of chromosomes will be effected. The chromosomes soon get attached to the spindle at their centromere regions.
Another conspicuous feature of late prophase is the shortening of chromosomes by coiling. Chromosomes become swollen and distinctly visible. With the help of a high resolution power microscope it can be observed that each chromosome is split lengthwise into identical parts (Chromatids) except at the centromeric region. Each chromatid is a fully functional chromosome. However, the term 'chromosome' is used for the longitudinally split halves of a late prophase chromosome having an undivided centromere. As soon as the centromere divides in late metaphase and the chromatids start moving towards opposite poles, each chromatid is called a chromosome.
The spindle apparatus becomes distinct as soon as the nuclear membrane and nucleolus disappear. Initially the chromosome may get attached to the fibres anywhere on the spindle apparatus. Eventually, all the chromosomes are brought to a position of equilibrium, at a point half way between the poles. The chromosomes are the shortest and thickest at this stage, and become oriented in such a way that all centromeres lie in one plane, forming a metaphase plate. At this point the centromere divides, and the chromatids of each chromosome are ready to be separated as complete chromosomes.
Division of the centromere marks the beginning of anaphase. The spindle fibres uniting the chromosomes with the poles, start contracting until the daughter chromosome are completely distinct from one another. Further contraction of fibres pulls the two groups of chromosomes further apart to their respective poles. As the chromosomes move to their respective poles through the cytoplasm they assume characteristic V, J or I shapes with the centromere proceeding towards the poles with chromosome arms trailing behind. Such variable shapes of chromosomes are due to variable position of centromere in different chromosomes. If the centromere is centrally located the chromosome assumes V-shape, if the centromere is subterminal in position the chromosomes have a J-shape and a terminal position of the centromere on the chromosome results in an I-shape of the chromosome.
At the end of anaphase, chromosomes which reach the poles of the spindle, start uncoiling, elongate and become thin and invisible. The nuclear membrane reappears and the nucleolus gets reconstituted at the nucleolar organizer region of the respective chromosome. Thus two distinct daughter nuclei are formed. Now these two daughter nuclei are separated into two daughter cells by the process of cytokinesis or cytoplasmic division.
In animal cells cytokinesis occurs by an infolding of the plasma membrane which invaginates in the middle of the parent cell, whereas in plant cells a cell plate forms in the center which extends laterally until it completely divides the cell into two. It is interesting that the coordination of various stages of the mitosis is so exact that each nucleus of each daughter cell has exactly equal amount of the chromosomal material and cytoplasm in most cases. However, karyokinesis unaccompanied by cytokinesis leads to multinucleate condition in plants like Rhizopus, in tissues like endosperm of seed plants and in the skeletal muscles of animals.
After completion of cell division the nuclei of daughter cells may again enter into interphase or the next round of division.
During telophase a cleavage furrow appears which deepens as the spindle breaks down. These furrows further deepen to form two daughter cells. The division of cytoplasm is called as cytokinesis and the division of nucleus is called as karyokinesis.