A Somatic cell or vegetative cell divides by mitosis, in which two identical daughter cells are produced. The chromosome number of each daughter cell remains same. As the mother cell equally divides to form two daughter cells, mitosis is also known as equational division.
During cell division, first the nucleus, more precisely chromosome divides and then there is the division of the cytoplasm. The former is known as karyokinesis or nuclear division and latter is cytokinesis or cytoplasmic division.
Karyokinesis:
Karyokinesis involves the division of nucleus, which follows G, phase of the cell cycle. Depending on the behavior of the chromosome in karyokinesis, the process is divided into four stages: prophase, metaphase, anaphase and telophase.
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Prophase:
Chromatin starts condensing to appear as individual threads. Further shortening and thickening of the chromatin thread continues to impart small rod like structures called chromosomes.
Each chromosome is with two distinct chromatids, known as sister chromatids, held at the centromere. The nucleolus along with the nuclear membrane disappears.
Towards late prophase, microtubules arrange themselves at two poles to form a bipolar spindle. In animal cell, centrioles divide to form two pairs of centrioles. Each pair migrates to a pole of the dividing cell.
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The microtubules radiate from each centriole pair, known as astral rays. The centrioles and the spindle fibers form a frame-work known as spindle apparatus. The centrioles along with the radiating spindle fibers, at each pole form an aster. Thus there are two asters, one at each pole of a dividing cell.
Metaphase:
During early metaphase, the condensed chromosomes move towards the equator of the bipolar spindle apparatus. The chromosomes are arranged at the equator forming the metaphase plate.
The chromosomes show maximum condensation at this stage. Due to this property, the morphology of chromosome is best studied during prometaphase. Each chromosome gets attached at the centromere to the spindle fiber originated from the bipolar spindle.
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Anaphase:
Sister chromatids of each chromosome get separated from each other due to the division of centromere and start moving towards opposite poles. The contraction of spindle fibers facilitates the movement of sister chromatids towards each pole as daughter chromosomes. At mid anaphase, one can see the characteristic shapes of chromosomes based on the position of the centromere.
Telophase:
In telophase, the daughter chromosomes reach the two poles. The chromosomes uncoil and decondense into chromatin fibers. Spindle fibers disappear. Nuclear envelope and nucleolus reappear. Thus two nuclei are formed, each of which contains the same number of chromosomes and the same amount of DNA as that of the mother cell.
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Cytokinesis:
Division of cytoplasm, cytokinesis starts in telophase or just after telophase. Cytokinesis in plants differs from that of animals. A cell plate is fomed at the middle of a plant cell which divides the cell into two halves,-each half having a nucleus and thus each half gives rise to a daughter cell.
However, in animal cell, cytokinesis starts with the formation of a cleavage furrow. The furrow deepens and ultimately the cell divides into two daughter cells.
Significance of Mitosis:
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1. In mitosis, each nucleus contains same number of chromosomes and the same amount of DNA as that of the parent nucleus. Thus each cell produced by mitosis is genetically an exact replica of the parent cell. Thus genetic equality among cells is produced by mitosis.
2. In multicellular organisms, the number of cells increases by mitosis. Such an increase enables the organism to grow. Thus growth is an outcome of mitosis.
3. Many organisms reproduce by asexual reproduction. It is mediated by mitotic division.
4. Injury or wound in higher organisms is healed up by addition of cells produced by mitosis.
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5. The lost cells in animals are replaced regularly by mitotically produced same type of cells from the respective parent cells. Examples are the replacement of blood cells, intestinal cells etc.