. Explain the term Cytokinesis


Best Answer

During mitosis the organelles of the cell becomes distributed towards the two poles of the cell (The ER and Golgi apparatus break up into small fragments and become distributed towards the poles).

In animal cells cytokinesis occurs by the formation of cleavage furrow or invagination of the cell membrane at the region previously occupied by the equator of the spindle. A dense remnant of spindle material collects at the equator and is known as mid body as a bridge. The furrow is formed by the a contractile band of microfilaments of actin and myosin attached to the inner surface of plasma membrane of the furrow. The constriction deepens and divides the cytoplasm into two. The mid body then disappears. The cleavage cytokinesis is also observed in some protists and pollen forming meiocytes of angiosperms. Actin filaments attached to the cell membrane contract and the furrow deepens and its membranes meet in the centre of the cell and fuse forming two complete daughter cells by centripetal cleavage.

In plant cells the spindle fibres present at the equator move outwards and increase in number forming a barrel shaped structure called phragmoplast to trap Golgi vesicles to form a cell plate. The rest of fibres of the spindle disintegrated. The vesicles from the Golgi apparatus line up along the region right angles to spindle apparatus. These vesicles fuse to form a cell plate. The cell plate extends towards the periphery along the equatorial line and meets the parent plasma membrane and fuses with it. The contents of the vesicles form the middle lamella. The membrane of the cell plate forms the new plasma membranes of the opposing daughter cells at the region of their contact. Cellulose cell walls are laid down on these new membranes. Thus two separate daughter cells are formed. In some areas the vesicles of the cell plate do not fuse and maintain cytoplasmic contacts between daughter cells. These contacts are known as plasmodesmata. On either side of middle lamella the cytoplasm deposits a primary cell wall of cellulose, hemicellulose and pectin thus dividing the parent cell into two daughter cells. Thus in plant cells cytokinesis progresses centrifugally.

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