Mitosis :
Mitosis is a type of cell division in which a cell divides to form two daughter cells retaining same chromosome complement. It ensures equal distribution of genetic material. In this process the division of nucleus and cytoplasm occurs only once.
Mitosis is a common type of cell division, which is responsible for growth and compensation of wear and tear in multiplication in unicellular organism and multiplication in unicellular organisms. Since this cell division results in the formation of somatic cells equal to that of mother cell so that it is also called equal to that of mother cell so that it is also called equational division. In animal cells there are distinct centrosomes and the kinetic spindle is organized with reference to the centrioles. Such a division is called 'centric' while in plant cells it is 'acentric'. The term 'mitosis' was coined by Walther Flemming, a German biologist who for the first time observed dividing cell in1878.
The cell division involves several continuous and / or simultaneous events, which for the sake of convenience are divided into two stages - karyokinesis and cytokinesis.
Karyokinesis :
Karyokinesis or nuclear division is the stage in which nucleus undergoes a series of changes to form two daughter nuclei.
Karyokinesis may be further distiguished into prophase, metaphase, anaphase and telophase.
Prophase:
During early prophase the chromatin network condenses and resolves into definite number of chromosomes. Initially each chromosome appears as single stranded (monad chromosome) thin and long. As the condensation progresses, each of them reveals identical chromatids held together b centromere (died chromosome). Throughout the prophase, chromosomes undergo dehydration and coiling to become thick and short. In late prophase the nuclear membrane and nucleolus begin to disappear. The daughter centrioles (in animal cells) start moving away from each other, til they occupy polar position. Prophase is the longest phase.
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| A) Early Prophase, B) Late Prophase |
Metaphase :
During metaphase, the condensation of chromosomes is completed and the thick chromosomes get organised along equatorial plane of the cell. Hence this stage is called metaphase (meta = middle; phase = stage). This phase is characterized by formation of kinetic spindle. The spindle is made up of contractile proteinaceous chromosomal fibres and inter-polar fibres. It is called mitotic apparatus.
In animal cells the spindle fibres radiate as an aster from centriole and called astral rays. Hence it is called astral mitosis.
In plant cells asters are absent and so it is called anastral mitosis.
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| Metaphase |
Anaphase :
It is the shortest phase of karyokinesis in which centromeres divide into two, resulting in the separation of chromatids. Each separated chromatid is now called daughter chromosome.
The kinetic spindle now has four types of fibres made up of microtubules. They are -
i) Astral rays - radiating away from centriolar pairs towards the periphery of cell wall. (Absent in plant cells).
ii) Inter-polar fibres - extending between the opposite pairs of centrioles (poles).
iii) Chromosomal fibres - connecting centromeres to respective poles.
iv) Inter chromosomal fibres - that appear between the centromeres of daughter chromatids, also called inter-conal fibres.
The chromosomal spindle fibres undergo contraction and pull the daughter chromosomes to the respective poles while inter-chromosomal fibres elongate pushing the daughter chromosomes till they reach the respective poles.
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| Anaphase |
Telophase :
At the poles, daughter chromosomes uncoil and undergo hydration tp form chromatin network. The nuclear membrane begins to reappear around each group of daughter chromosomes and the daughter nuclei are formed. The spindle fibres disappear while nucleolus reappears in each newly formed nucleus.
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| Telophase |
Cytokinesis:
The division of cytoplasm of the mother cell is called cytokinesis.
In animal cells it occurs by furrowing of plasma membrane that deepens and the daughter cells are formed. This process is called cleavage. In plant cells it takes place formation and extension of cell plate, from centre to the periphery.
Mitosis plays an important role in growth, development, healing and repair of multicellular organisms by forming new somatic cells. It maintains chromosome number of parent cell in the daughter cells. It ensures equal distribution of genetic material and maintains characteristics of species. It plays a significant role in asexual reproduction of unicellular organisms.
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| Cytokinesis in an Animal cell and a Plant cell |