10. CELL CYCLE AND CELL DIVISION
MITOSIS
It is also called as equational division as the number of chromosomes in the parent and progeny cells is same.
Mitosis is generally seen in diploid cells. It also occurs in haploid cells of some lower plants and some social insects.
It involves major reorganization of all cell components.
The karyokinesis of mitosis has 4 stages: Prophase, Metaphase, Anaphase & Telophase.
1. Prophase
It is the longest phase in mitosis.
It follows the S and G2 phases of interphase.
In the S & G2 phases, DNA molecules are intertwined.
Characteristic events:
The nuclear envelope completely disintegrates. Hence the chromosomes spread through the cytoplasm of the cell.
Chromosome condensation is completed. They can be observed and studied easily under the microscope. They will have two sister chromatids.
Chromosomes come to lie at the equator. The plane of alignment of the chromosomes at metaphase is called the metaphase plate.
The spindle fibres from both poles are connected to chromatids by their kinetochores in the centromere.
It is the shortest phase in the mitosis.
Centromere of each chromosome divides longitudinally resulting in the formation of two daughter chromatids (chromosomes of the future daughter nuclei).
As the spindle fibres contract, the chromatids move from the equator to the opposite poles.
Chromosomes cluster at opposite poles and uncoil into chromatin fibres.
Nuclear envelope develops around the chromosome clusters at each pole. Thus 2 daughter nuclei are formed.
Nucleolus, Golgi complex and ER reappear.
The spindle fibres disappear.
It is the division of cytoplasm to form 2 daughter cells.
- Chromosomal materials (chromatin fibres) are untangled and condensed to form mitotic chromosomes. They are seen to be composed of two chromatids attached together at the centromere.
- Centrosomes begin to move towards opposite poles of the cell. Each centrosome radiates out microtubules called asters. The two asters together with spindle fibres forms mitotic apparatus.
2. Metaphase
The nuclear envelope completely disintegrates. Hence the chromosomes spread through the cytoplasm of the cell.
Chromosome condensation is completed. They can be observed and studied easily under the microscope. They will have two sister chromatids.
Chromosomes come to lie at the equator. The plane of alignment of the chromosomes at metaphase is called the metaphase plate.
The spindle fibres from both poles are connected to chromatids by their kinetochores in the centromere.
3. Anaphase
It is the shortest phase in the mitosis.
Centromere of each chromosome divides longitudinally resulting in the formation of two daughter chromatids (chromosomes of the future daughter nuclei).
As the spindle fibres contract, the chromatids move from the equator to the opposite poles.
4. Telophase
Chromosomes cluster at opposite poles and uncoil into chromatin fibres.
Nuclear envelope develops around the chromosome clusters at each pole. Thus 2 daughter nuclei are formed.
Nucleolus, Golgi complex and ER reappear.
The spindle fibres disappear.
Cytokinesis
It is the division of cytoplasm to form 2 daughter cells.
It starts when telophase is in progress.
Cytokinesis in animal cell:
Cytokinesis in animal cell:
- Here, a cleavage furrow is appeared in the plasma membrane.
- It gradually deepens and joins in the centre dividing the cytoplasm into two.
- It is different from the cytokinesis in animal cells due to the presence of cell wall.
- In plant cells, the vesicles formed from Golgi bodies accumulate at the equator. It grows outward and meets the lateral walls. They fuse together to form the cell-plate. It separates the 2 daughter cells. Later, the cell plate becomes the middle lamella.
In some organisms, karyokinesis is not followed by cytokinesis. As a result, multinucleate condition (syncytium) arises. E.g. liquid endosperm in coconut.
Significance of Mitosis
- It produces diploid daughter cells with identical genome.
- It helps to retain the same chromosome number in all somatic cells.
- It helps in the body growth of multicellular organisms. Mitosis in the meristematic tissues helps in a continuous growth of plants throughout the life.
- It restores the nucleo-cytoplasmic ratio that disturbed due to cell growth.
- It helps in cell repair & replacement. E.g. cells of the upper layer of the epidermis, lining of the gut & blood cells.
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