Phases of mitosis

We know that the unit of life is cell and there are two types of cells prokaryotic and eukaryotic, but how do these cells reproduce on its own ? There has to be some process in order to do so. In this article we are going to be discussing about the process of mitosis which helps in reproducing the eukaryotic cells. 

What is mitosis ? 

Mitosis is a process where a cell divides in two equal sizes to form two daughter cells which contains the same number of chromosomes as of parent cell. Hence, this process is also called as equational cell division. It makes sure that the genetic material is distributed equally. This process can take place in haploid cells also. 

Mitosis in plant cell was first studied by Strassburger and in animal cell it was studied by Flemming. The word "Mitosis" was first coined by Flemming. This process can complete in 30 minutes or might even take over 3 hours. It takes place in the somatic cells hence this process is also known as somatic cell division or indirect cell division

It is a common type of cell division which is responsible for the healing of the cells which were/are damaged (wear and tear) in multicellular organisms. 

Phases of mitosis:

This process of mitosis is divided into 2 phases - Karyokinesis (Division of nucleus) and Cytokinesis (Division of cytoplasm). The nucleus and cytoplasm are going to divide only once during the whole process.


Karyokinesis:

In the process of karyokinesis the nucleus divides and goes through series of changes to form two daughter nuclei. This process is also called as nuclear division. It is further divided into 4 stages - prophase, metaphase, anaphase and telophase

Prophase:

Prophase is the first and longest process of karyokinesis. There is formation of thread-like, thin and denser chromosomes. The cells now are spheroidal, more refractile and viscous. This stage can be completed in few minutes or might even take several hours


Prophase is divided into 3 substages - Early prophase, mid prophase and late prophase.  

Early prophase: 

The early prophase starts by coiling and condensation (Formation of denser substance) of chromosomes. At first, each chromosome is single stranded, thin and long. The nuclear chromatin network starts to coil which forms tiny thread-like chromosomes. 

These chromosomes start revealing two identical sister chromatids which are held together by the centromere. The centromere contains kinetochores (Disc shaped protein structure) where spindle fibres are attached. 

There is formation of asters which are ultra-microscopic microtubules extending from the centrioles which helps in pulling away the centrioles from one another and pulls it towards opposite ends. These microtubules are made of up proteins and later become astral rays

Mid prophase:

 The the chromatins undergo dehydration and coiling which gets more denser resulting in shorter and thicker chromosomes. The microtubules keep getting longer and moves the asters away from each other. 

The nucleus decreases in size and slowly disappears. There is breakdown of nuclear envelope which disappears in the cytoplasm.

Late prophase (Prometaphase):

In late prophase, majority of the cell organelles starts to disappear. Chromosomes keep getting denser (condensation) and they start to move away from the centre clearing the central area of the cell. The central part is now the equator and the end are the poles where the daughter centrioles occupy position. There is degeneration and disappearance of nuclear envelope. 

Spindle formation can now be seen clearly. There are fibres which extend from pole to pole (asters to asters) and from pole to centromere of the chromosome. The fibres which extend from pole to pole are called interpolar fibres and the ones which extend from pole to centromere are called chromosomal fibers. Hence, two types of fibres are seen. This whole set up of microtubules and centrioles is known as mitotic apparatus

Metaphase:

Metaphase is the middle phase of mitosis where condensation of chromosomes is completed and the chromosomes are shorter and thicker and are organised along the equitorial plane of the cell where they are perpendicular to longitudinal axis. This process is completed in 2-10 minutes

Metaphase is characterized by formation of kinetic spindles which are made up of contractile proteinaceous chromosomal fibres and inter-polar fibres. The chromosomes are held in position by the spindle fibres. The centromere of the chromosome is at the equator and their arms are towards the pole. Kinetochore (Disc shaped protein structure) is attached to the spindle fibre. 


In animal cells, it is called astral mitosis because the spindle fibres emit asters from centrioles called the astral rays whereas in plant cell these asters are absent and so it is called anastral mitosis

Anaphase:

Anaphase starts with division of centromeres into two which results in separation of sister chromatids and these separated chromatids are called the daughter chromosomes. This phase is the shortest phase which is completed in 2-3 minutes

The splitting of chromatids is because of increase in Ca2+ ions in cytosloic (fluid present in cytoplasmic matrix). The membrane containing Ca2+ get collected at spindle poles and release calcium ions to start the process of anaphase. 

The chromosomes now start moving towards the poles due to the shortening of chromosomal fibres. This way, the centromeres are now towards opposite poles and their arms are towards the equator. The chromosomes appear in U, V or J-shaped


Since the chromosomes are now facing each other they are connected by inter-zonal fibres. In this process, there is equal distribution of genetic material in two sets of chromosomes. 

Telophase:

Telophase takes 1 hour to complete where chromosomes (daughter chromosomes) which were separated during anaphase comes to the end of the poles uncoil and undergo hydration forming chromatin network. They form two different sets of chromosomes near the opposite ends. 

These chromosomes develop new nuclear membrane, nucleolus, endoplasmic reticulum, golgi body, etc.. The nuclear membrane forms around daughter chromosomes giving rise to daughter nuclei. Nucleolus is formed from nucleolar organising region of the nucleolar chromosome. 


Chromatin is formed by decondensation of chromatin material. There is formation of centromeres from the centrioles at the poles and the spindle fibres break into small pieces and gets absorbed in the cytoplasm. It can be said that telophase goes in a reverse manner. 

Cytokinesis:

Cytokinesis is division of cytoplasm of the mother cell. It usually starts during Metaphase so you can say that the process of karyokinesis and cytokinesis happens simultaneously. It takes place differently in animal cell and in plant cell. 

In animal cell, it occurs by furrowing of the plasma membrane. Many cell organelles arrange themselves on opposite sides of the equator. The mid-body starts to develop during metaphase and it has a ring like appearance. The furrowing makes the membrane narrower and it keeps deepening and the cytoplasm is divided into two. This is called as cleavage

In plant cell, a thin plate with many holes develops near the equator which is mainly made up of cellulose. This thin plate is extended from the centre to the periphery (outer side of the cell) which gets denser resulting in division of cytoplasm. 

Significance of mitosis:

The importance of mitosis are as follows:
  1. Mitosis helps in growth and development of an organism. 
  2. There is equal distribution of genetic material in the chromosomes. Hence, the amount of RNA and DNA contents in maintained. 
  3. Regeneration capacity is because of mitosis. 
  4. Mitosis makes the surface/volume ratio of the cell like it was before. 
  5. The damaged tissues are repaired and wounds are healed because of mitosis. 
  6. Mitosis increases the number of sex cells
  7. Asexual methods of reproduction in animals involve mitotic cell division. 
  8. Nucleocytoplasmic ratio is maintained. 

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