MITOSIS

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Write a short note on Mitosis 

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 The mitotic phase or M-phase of cell-division

                    

Mitosis:

For an organism to grow needs appropriate coordination of the process of cell division, DNA replication and cell growth. 

Together these processes make up the cell cycle which after completion forms progeny cells containing intact genomes.

 Cell division is the process through which transport of genetic material i. e. chromosomes and the genes they bear, take place from one generation to another.

Note: ' * ': know to understand the topic

* The cell gives rise to another cell by completing one cell cycle where all events are included

 1. The cell duplicates its genome ( complete set of genes in a cell), 

2. Synthesizes the other constituents of the cell ( cytoplasm and its constituents) 

3. At last divide the cell into two daughter cells. 

This whole process is a cell cycle. In this cell cycle, the M phase is the mitotic phase where actual division occurs. The rest is interphase made by G1(gap1), S ( synthesis), and G2 ( gap2) phases.

As the organism grows the number of cells increases through the process of cell division.

The cell division starts in the M- phase of the cell cycle.

M- phase (Mitotic Phase):

In the M-phase ( or the mitotic phase), the actual division of the cell occurs and interphase is the period between two subsequent M-phases when the cell is not dividing but preparing to divide.

* ( In the case of humans, the duration of cell division proper is only one hour on average and on average the cell cycle is 24 hours)

M phase is divided into two stages,

Karyokinesis and cytokinesis.

Karyokinesis: 

This is the stage of indirect nuclear division. 

M phase or mitosis starts with indirect nuclear division when the separation of daughter chromosomes occurs.

Mitosis is a process of asexual reproduction. It occurs only in somatic cells ( body cells), not the sex cells ( spermatocytes in males and oocytes in females. So it is also called somatic division.

During Mitosis there occurs reorganization of all components of the cell.

Four phases of Mitosis:

In the period of Mitosis occurs major reorganization of almost all components of the cell. Even though the process is continuously occurring for the convenience of understanding it is subdivided into 4 phases.

Mitosis diagram

1. Prophase:

Prophase is the first phase of mitosis, after the S and G2 phases of an interphase period of the cell cycle when the cell is not dividing.

Till G2, the new DNA molecules formed are not distinct but interwind. Now at this stage, the condensation of chromosomal material starts and forms compact mitotic chromosomes containing two chromatids attached at the centromere. The nucleus appears as a ball of wool.

Early Prophase:

1.  Chromatin fibres condense and form elongated chromosomes.
2.  The nucleus appears as a ball of wool.
3.  The centrosome has already been divided.
4.  The daughter chromosomes begin to move away from each other.

Mid Prophase:

1.  Chromosomes are condensed and distinct with each having two chromatids attached at the centromere.
2.  The centrosome develops astral rays and migrates further.

Late Prophase:

1.  Centrosomes reach the pole with asters.
2.  Spindle fibres begin to develop.
3.  The nucleolus degenerates and the nuclear envelope starts breaking.

*( In plant cells, centrosomes are absent, spindle fibres develop without them.)

Metaphase:

1. Complete disintegration of nuclear envelope resulting in spreading of chromosomes in the cytoplasm of the cell.
2. Condensation of chromosomes is completed.
3. A bipolar spindle is produced.
4. Chromosomes move towards the equatorial plane forming equatorial or metaphase plates with their centrosome attached to both poles by chromosomal fibres.

*( At this stage the metaphase chromosomes are made up of two sister chromatids which are held together by centromere. The disc-shaped structure at the centre of the centromere is called Kinetochores, where the spindle fibres get attached to the chromosomes which are made to move to the centre of the cell.

 *( It is anastral i.e. without esters, in plants and amphiaster i.e. two esters on both poles, in animals).

* Metaphase is characterized by all the chromosomes coming to lie at the equator with one chromatid of each chromosome connected by its kinetochore to the spindle fibres from one pole and its sister chromatid connected by its kinetochore to spindle fibre from the opposite pole.

* Metaphase plate: The plane of alignment of the chromosomes at metaphase is called a metaphase plate.

*(Metaphase is the best time to see the chromosomes as colchicine, a stain and antimitotic drug that arrests the cells in metaphase.

Anaphase:

1. The centromere of each chromosome divides.
2. The spindle fibres attached get shortened, pulling the chromatids apart
3. Chromatids move to the opposite poles. The centromere of each chromosome is towards the arms of the pool and the arms of chromosomes trail behind. Ultimately groups of chromosomes come to lie at the spindle poles.

*(These two daughter chromatids are going to be the chromosomes of future daughter nuclei)

Telophase:

1. The completion of the migration of two sets of chromosomes to the opposite poles.
2. Chromosomes again unfold elongate forming chromatin fibres.
3. The nucleus, nucleoplasm and nuclear membrane appear, forming the Endoplasmic reticulum so that two daughter nuclei are formed.

*( In Dino flagellates the type of nuclear division occurs where the nuclear membrane persists and the spindle is not formed so the movement of chromosomes occurs while attached to the inner membrane of the nuclear envelope.

Cytokinesis:

Cytokinesis or the division of the cytoplasm of the cell also occurs during the telophase of Mitosis.

So by mitosis not only does the segregation of duplicate chromosomes into daughter nuclei (karyokinesis) but the cell itself is divided into two daughter cells by cytokinesis.

Since the number of chromosomes in parent and progeny cells is the same, mitosis is also called equational division.

This process is different in plant and animal cells because of the presence of a relatively extensible membrane, the cell wall in plants which is not there in animals.

In animal cells:

It is cleavage cytokinesis. The cell membrane constricts and develops a centripetal furrow or cleavage in the middle which deepens, and divides the parent protoplast into two uninucleate protoplast (or two cells).

* Sometimes cleavage cytokinesis is also seen in some lower plants where the cell wall material is deposited in the furrow between the two daughter protoplast.

In plant cells:

In plant cells, it is cell plate cytokinesis. 

* It occurs in plant vesicles having peptic compounds. The other wall materials appear in the middle of the persisting spindle called phragmoplast. They fuse to form a simple precursor called the cell plate. With a membrane on either side, the cell plate grows centrifugally and functions as the middle lamella. The primary wall is deposited on either side by two daughter protoplasts. During endoplasmic division, organelles like mitochondria and plastids get distributed between the two daughter cells. This divides the parent binucleate cell into two daughter uninucleate cells.

*( In some organisms, karyokinesis is not followed by cytokinesis as a result of which multi-nucleate condition arises leading to the formation of syncytium (e.g. liquid endosperm of coconut)

Significance of mitosis:

Mitotic cell division has the following significance 

1. The process generates daughter cells with an equal number of chromosomes and hence the chromosome number is maintained.
2. It prevents the loss of cells and promotes growth.
3. The daughter cells have the same genetic features as the parent cells.
4. It maintains the balance between DNA and RNA.
5. It helps in the healing of wounds.