Mitosis occurs in four phases namely:
i. Prophase
ii. Metaphase
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iii. Anaphase
iv. Telophase
The various phases of mitosis are nearly similar in both animal and plant cells. For better understanding, the mitosis in animal cells is described here and wherever variation occurs, the mitosis in plant cell is also described.
(I) Prophase (Gk. Pro: Before, Phasis: Stage):
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1. This is the first and longest phase of mitosis.
2. The chromatin material undergoes condensation (becomes short and thick) and changes into thread-like structures called chromosomes.
3- Each chromosome has two thread-like structures called sister chromatids, which lie close to each other and remain attached at a point called centromere.
4. As prophase advances, the chromosomes become shorter and condensed (thickened).
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5. In animal cells, the centrosome initiates and regulate the cell division. The centrosome splits into two small round bodies called centrioles. The two centrioles of the centrosome migrate to the opposite sides (poles) of the cell.
6. Soon two star-like structures with radiating fibres known as asters are formed around the centriole at each pole. Between the separating centrioles, spindle fibres are formed by the aster.
7. Towards the end of the prophase, the nucleolus and the nuclear membrane disappear.
(II) Metaphase (Gk. Meta: Between, Phasis: Stage):
l. Chromosomes become still shorter and thicker due to condensation.
2. The chromosomes arrange themselves on, the equatorial plane in such a way that their centromeres lie on the equator and arms face towards the poles.
3. Centromere of each chromosome is attached by a spindle fibre.
(III) Anaphase (Gk. Ana: Back, Phasis: Stage):
1. This is the shortest phase of mitosis.
2. The centromere of each chromosome divides into two halves (sister chromatids) so that each chromatid has its own centromere.
3. The sister chromatids separate and begin to move towards the opposite poles due to the contraction of the spindle fibres, and due to the repelling force developed between them.
4. Depending on the position of the centromere, the chromosomes appear as U, V or J-shaped.
5. The anaphase ends when all the chromatids (now behaving like chromosomes) reach the opposite poles.
(IV) Telophase (Gk. Telo: End, Phasis. Stage):
1. This is the last phase in karyokinesis (nuclear division). The events of prophase occur in reverse sequence during telophase.
2. The chromatids (daughter chromosomes) uncoil, elongate and change into network of chromatin threads.
3. The nuclear membrane reappears around the chromatin network at each pole.
4. Nucleolus reappears in each daughter nucleus and spindle fibres disappear.
5. In animal cells, centrosome organizes itself above the nucleus, thus making the completion of karyokinesis.
The karyokinesis (mitosis proper) is followed by the division of the cytoplasm known as cytokinesis.
Cytokinesis (Gk. Cyto: Cell, Kinesis: Movement):
It is the division of cytoplasm to form two daughter cells. It begins during late anaphase and is completed soon after telophase. It is different in animal and plant cells.