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Types of Cell Division

Cell cycle of Class 11

1. Mitosis takes place when new cells are added to multicellular organisms as they grow and when tissues are repaired or replaced. Root tips (root meristem) of onion (Allium cepa, 2n = 16) are best plant  material for the study of mitosis in labs. Root tips of Vicia faba (broad bean) are also used. In animals, cells at base of nail, bone marrow cells and skin cells (statum germinativum) are taken to study mitosis.

2. Meiosis occurs in the production of gametes by organisms which reproduce sexually. Best material to study meiosis in class room is anthers from young unopened buds (buds before anthesis) of Tradescantia and onion and testes of Grasshopper.

3. Aceocarmine is a nuclear basic stain used to study the cell division in plant material.

Factors Controlling Cell Divison

1. Cell Size When cells grow in size, its nucleo-cytoplasmic ratio and surface area - volume ratio decreases. To maintain these ratios cells divide as smaller cells have high ratios, therefore more active.

2. Mitogens are polypeptide growth factors that control cell proliferation. Common plant mitogen is hormone cytokinin. Mitogens in human beings include lymphokines, Epidermal growth factor (EGF) or platelet derived growth factor. (PDGF)

Generation time : Period between two successive divisions.

Cell Cycle : The entire sequence of events which takes place in a cell between one cell division and the next.

Three major events of cell cycle are

(i) Replication of DNA

(ii) Karyokinesis (Nuclear division)

(iii) Cytokinesis (Cytoplasmic division)

Cell Cycle consists of

(a) Long nondividing – I-phase

(b) Short dividing – M-phase

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