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Mitosis

Cell division is a process with sequence of steps that enables organisms to grow and reproduce. Genetic material is replicated in parent cells and is distributed equally to the two daughter cells. Cells undergo a period of growth called interpahse before entering mitosis. During the interphase, the genetic material replicates and the organelles prepare for division. In the process of mitosis, the parent's cell genome is transferred into the two daughter cells. The daughter cells are similar to each other and to their parent cell.The cell's genome is composed of chromosomes that are complexes of tightly coiled DNA that contain the genetic material which is vital for the proper functioning of the cell. 

The process of mitosis begins when the chromosomes condense. In most eukaryotic cells, the nuclear membrane segregates the DNA from the cytoplasm into membrane vesicles. The ribosomes also dissolve, the chromosomes align themselves. Microtubules pull apart the sister chromatids of each chromosome. The daughter chromosomes are pulled towards opposite ends. Nuclear membrane forms around the separate daughter chromosomes. In animal cells, the area of cell membrane pinches inwards, to form the two daughter cells, the imaginary line is called the cleavage furrow which separates the developing nuclei. In plant cells, the new dividing cell wall is constructed in between the daughter cells. The parent cell will thus split in half and give rise to two daughter cells. 

Events in Mitosis

 

Mitosis Definition

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The process by which a cell which has previously replicated chromosomes in the nucleus of the cell is separated into two identical sets of chromosomes is known as mitosis. Mitosis is the division of the mother cell into two daughter cells, these daughter cells are genetically identical to each other and to the parent cell. It is  a form of nuclear division. Mitosis is generally followed by cytokinesis, this process divides the nuclei, cytoplasm, cellular organelles and cell membrane into two cells of roughly equal shares of these cellular constituents. The M phase of the cell cycle is of mitosis and cytokinesis together.

Illustration of Mitosis Stages

Mitosis is divided into the following stages 
  • Prophase
  • Metaphase
  • Anaphase 
  • Telophase 

Phases of Mitosis

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Mitosis is a fast and highly complex process. The events of mitosis is divided into the following stages prophase, prometaphase, metaphase, anaphase and telophase. 

Below figure shows the stages of mitosis:

Detailed View of Different Stages of Mitosis
Interphase
  • Mitosis alternates with interphase. In interphase the cell prepares itself for division. 
  • The interphase is divided into three phases G1, S and G2
  • During these phases the cells grow by producing proteins and cytoplasmic organelles. 
  • In the S phase the chromosomes replicate. In G1 phase the cell grows, in S phase the chromosomes duplicate and in G phase the cell grows more and prepares for mitosis and finally divides in the Mitotic cycle. 
  • The cell cycle is regulated by proteins.
  • The phases of the interphase follow strict order and have checkpoints. 
  • There is another phase in the interphase G0 where the cell has the option to enter this stage.
  • Interphase takes about 90% of the cell's life span. 
Prophase 
  • In the nucleus the genetic material is loosely bundled in coil called chromatin. 
  • At the onset of prophase the chromatin fibres become tightly coiled and condense into discrete chromosomes. 
  • Inside the nucleus, the nucleolus also disappears from view. 
  • The centrioles begin to move to opposite ends of the cell and the spindle fibres extend from the centromere.
  • Some fibres cross the cell to form the mitotic spindle fibres. 
Prometaphase
  • Prometaphase is sometimes considered as the end of prophase and early metaphase.
  • During the early stage of prometaphase the nuclear membrane disintegrates and the microtubules enter the nuclear space. 
  • This is known as "open mitosis" and it occurs in most multicellular organisms.
  • Organisms like fungi, some portists like algar or trichomonads undergo "closed mitosis" where the spindle formation happens inside the nucleus. 
  • The nuclear membrane stays intact and the microtubules are not able to penetrate the intact nuclear membrane. 
  • During the late prometaphase, at the centromere of each chromosome forms two kinetochores.
  • Kinetochore is a complex protein structure, it is the point where the microtubules attach themselves to the chromosome. 
Metaphase
  • The term metaphase is derived from Greek word 'meta' which means 'after'.
  • In the prometaphase after the microtubules are attached to the prometaphase the chromosomes start pulling the chromosomes towards the ends of the cell. 
  • The centromeres of the chromosomes assemble along the metaphase plate also known as the equatorial plane.
  • It is an imaginary line that is in between the centrosome poles and is called the spindle equator.
  • This helps to ensure that when the chromosomes are separated the new nucleus will receive one copy of each chromosome. 
    Pictorial Illustration of Mitosis Process
     
Anaphase
  • After the metaphase stage the chromosomes proceed to the anaphase stage. 
  • The term anaphase is derived from the Greek word "ava" which means "up", or "against", or "back", or "re".
  •  First the proteins that bind the sister chromatids are cleaved making the sister chromatids as separate daughter chromosomes and are pulled apart towards the respective centrosomes to which they are attached. 
  • The microtubules at the poles pull the set of chromosome that are attached to it the opposite ends of the cell. At the end of anaphase the microtubules all degrade. 
Telophase
  • Telophase is derived from the Greek word "telos" meaning "end". 
  • It is a reversal of prophase and prometaphase events. In the telophase stage the polar microtubules continue to lengthen elongating the cell. 
  • The daughter chromosomes attach at opposite site ends of the cell.
  • New membranes are formed around the daughter nuclei.
  • The chromosomes spread and are no longer visible under the light microscope. 
  • The spindle fibers also disperse, cytokinesis may also begin during this stage. 
Cytokinesis 
  • Cytokinesis is a separate process that begins at the same time as the telophase. 
  • Cytokinesis is not a phase of mitosis, it is a separate process necessary for completing cell division.
  • In animal cells a pinch like cleavage furrow containing a contractile ring develops at the position of the metaphase plate separating the nuclei.
  • In the animal and plant cells the division of cell is driven by vesicles derived from the Golgi apparatus.
  • In plant cells, the rigid wall requires a cell plate be synthesized between the two daughter cells. 

Comparison of Mitosis in Plant and Animal Cell

Significance of Mitosis

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Significance of Mitosis:
  • Mitosis is an equational division.
  • It is the division through which daughter cells that are identical are produced.
  • The daughter cells have the same amount and type of genetic constitution as that of the parent cell. 
  • Mitosis division is responsible for growth and development of a single-celled zygote into a multicellular organism. 
  • The chromosome number remains the same in the cells produced by this division. 
  • The daughter cells have the same characters as those of the parent cell.
  • Mitosis division helps in maintaining the proper size.
  • Mitosis also helps in restoring wear and tear in body tissues, replacing damaged or lost part, healing wounds and regeneration of detached parts. 
  • This method of multiplication is seen in unicellualr organisms. 
  • Mitotic division of cell is unchecked and it may result in uncontrolled growth of cells leading to cancer or tumor.  

Plant Cell Mitosis

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Mitosis in plant cell is different from the animal cells in the following characteristics
  • There is a preprophase stage which occurs only in plant cell prior to the prophase stage.
  • Centiroles are absent.
  • Aster is not formed.
  • Cytokinesis in plant cell occurs by the cell plate formation.
  • This kind of division is seen in the meristems of the plant body. 
Plant Cell Mitosis

Preprophase stageThis occurs only in the plant cell. The prophase stage is preceded by the pre-prophase stge. In vacuolated plant cells, the nucleus migrates to the centre of the cell before the onset of mitosis. Preprophase is characterized by the formation of phragmosome, phragmosome is a sheet of cytoplasm that bisects the cell along the future plane of cell division. There is also formation of a ring of microtubules and actin filaments known as the preoprophase band under the plasma membrane around the equatorial plane of the mitotic spindle. This preprophase band is the position where the cell wall eventually divide. The cell of flowering plants lack centrioles, microtubules form a spindle  on the nuclear surface and are then organized into spindle by the chromosomes.

The cells of plant have cell wall and hence the cytokinesis cannot proceed with a cleavage furrow. During the telophase stage in plant cell, a cell plate is formed across the cell in the location of the old metaphase plate.
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