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Cell Division, Mitosis and Meiosis

There are many reasons for the division of cells as cells divide to replace old, dead or damaged cells. Cells divide so that living things can grow. Organisms grow not because cells are getting larger but because cells are dividing to produce more and more cells. In human body, about two trillion cells divide every day. In cell division, the cell that is dividing is called the ‘parent’ cell. Parent cell divides into daughter cells and this process is repeated known as cell cycle. There are three ways for cell to divide depending upon its type of cell- Amitosis, Mitosis and Meiosis. Each of these methods of cell division has special characteristics.
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According to the theory old cells split into new cells and the formation of new cells is known as cell division or cell production. This was firstly observed by Flemming in 1882 but an extensive in details was given by Belar in 1920.

Usually cell division is of three types:

(i) Amitosis (ii) Mitosis (iii) Meiosis

Amitosis: This cell division takes place in less developed cell of unicellular organism and firstly nucleus of the cell is divided and then later cytoplasm, ultimately two new cells are formed. In bacteria, blue-green algae, yeast, amoeba, protozoa etc this type of cell division takes place.

Mitosis: This cell division is also known as Somatic cell division in which two identical cells are produced. Although cell is divided but number of chromosomes remains the same and mitosis is a continuous process. Various phases like prophase, metaphase, anaphase, telophase, cytokinesis occur and cell divides itself stage by stage.

Prophase: First stage of mitosis.

- In all animal cells and in some plants like fungi and some algae.

- The centriole duplicates itself and divides two new centrioles (centrosomes) move to opposite ends of the cell (poles).

- The spindle fibres or series of fibres radiates from vicinity of each centriole towards the nucleus.

- Except fungi and some algae the spindle fibres develop without the presence of centrioles.

- The chromosomes which are already duplicated become shorten and thicken.

- Chromatids are the duplicated halves of each chromosome which are held together by the centromere.

- The nucleus and the nuclear membrane begin to disintegrate in the late prophase.

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Metaphase:

- The pairs of chromosomes align themselves in such a way that the centre of the cell and each centromere becomes attached to one spindle fibre from each pole.

- The centromere divides and the separated chromatids become independent daughter chromosomes.

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Anaphase:

- Spindle Fibres begin to shorten.

- This exerts a force on the sister chromatids that pulls them apart.

- Spindle fibres continue to shorten, pulling chromatids to opposite poles.

- This ensures that each daughter cell gets identical sets of chromosomes.

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Telophase:

- The chromosomes decondense.

- The nuclear envelope forms i.e nuclear membrane forms around each new group of chromosomes.

- Daughter chromosomes reach the poles.

- Spindle fibres totally disappear.

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Cytokinesis:

- After the division of nucleus, cytoplasm starts to divide.

- The original large cell becomes two smaller identical cells and each daughter cells take food, grows, being divided and the process continues.

- It maintains the continuity of metabolism by transmitting to the daughter cells.

- Plays significant role in wound healing, regeneration of damaged parts (like tail of lizard), the replacement of cells (the skin surface), and it may give rise to tumours or cancerous growth if uncontrolled process be occur.

*In mitosis it is also ensured that two daughter cells inherit the same number of chromosomes and hence have the same characteristics as the parent cell.

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Meiosis: It is a special type of cell division that occurs in Sexually Reproducing organisms and thus gametes (sex cells) are produced. It consists two successive cell division that resembles like mitosis but chromosomes are duplicated only once. Thus, gametes have half the number of chromosomes normally found in the body cells. Two sub stages – meiosis I and meiosis II occur.

• Meiosis I: Can be broken down into four sub stages: Prophase I, Metaphase I, Anaphase I and Telophase I.

• Meiosis II: Can be broken down into four substages: Prophase II, Metaphase II, Anaphase II and Telophase II.

Meiosis I:

Prophase I: Most of the significant processes of Meiosis occur during Prophase I

- The chromosomes condense and become visible.

- The centrioles form and move toward the poles.

- The nuclear membrane begins to dissolve

- The homologous pair up, forming a tetrad

- Each tetrad is comprised of four chromotids.

- Homologous chromosomes will swap genetic material in a process known as crossing over which increase genetic diversity by creating four unique chromatids.

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Metaphase I:

- Microtubules grow from the centrioles and attach to the centromeres where tetrads line up along the cell equator.

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Anaphase I:

- The centromeres break, cytokinesis begins and homologous chromosomes separated but the sister chromatids are still attached.

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Telophase I:

- Depend upon species the chromosomes may decondense and cytokinesis reches to its completion by creating two haploid daughter cells.

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Meiosis II:

Prophase II:

- The nuclear membrane dissolves, centrioles form and move toward the poles.

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Metaphase II:

-  Microtubules attach to the centromeres and grow from the centrioles and the sister chromatids line up along the cell equator.

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Anaphase II:

- Cytokinesis begins, centromeres break and sister chromatids separate.

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Telophase II:

- Depends upon species chromosomes may decondense, Cytokinesis reaches completion creating four haploid daughter cells.

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Comparison between Mitosis and Meiosis

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Image courtesy: www.uic.edu

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