7. Cell Division - part 03 - Mitosis

 


7. Cell Division - part 03 - Mitosis


Mitosis :  

  • This is a type of cell division in which a  cell divides to form two similar daughter cells  which are identical  to the parent cell. 
  • It is completed in two steps as -
  1. Karyokinesis and 
  2. Cytokinesis. 
  • Karyokinesis is nuclear division which is sub-divided into -  
  1. prophase
  2. metaphase
  3. anaphase and 
  4. telephase.  
  • Although  for the sake of convenience  above mentioned  steps are used, it  must be  remembered  that  mitosis is a continuous process that starts with the disappearance of nuclear membrane  in prophase and ends with  separation  of two fully formed cells after cytokinesis. 
1. Prophase :

  • This  phase  involves -
  1. condensation of chromatin  material,
  2. migration  of centrosomes, 
  3. appearance of  mitotic apparatus and 
  4. disappearance of nuclear membrane.

  • Due  to  condensation,  each  chromosome becomes visible under light microscope which can be  seen with its sister-chromatids connected  by centromere.  
  • The nucleolus starts to disappear. 
  • Nuclear membrane disintegrates and disappeares gradually.
  • Centrosome  which had undergone duplication during interphase begins  to move  towards opposite  poles of the cell. 
  • Mitotic apparatus is almost completely formed.
 2. Metaphase :

  • In  this  phase,  chromosomes are completely  condensed so  that they appear very short. 
  • Sister-chromatids and centromere become very prominent.  
  • All the chromosomes lie at equatorial plane of  the cell.  This  is called metaphase plate. 
  • Mitotic spindle is fully formed. 
  • Centromere  of each chromosome divides into two, each being associated with a chromatid.


3. Anaphase  :

  • The chromatids of each chromosome separate and form two chromosomes called daughter chromosomes.
  • The formed chromosomes  are pulled away in opposite direction by  spindle apparatus. 
  • Chromosomes being pulled  away  appear like a bunch  of banana during  midway of anaphase. 
  • Each set of chromosomes reach at opposite poles of the cells marks the end of anaphase.
4. Telophase  :

  • The telophase  is the final stage of karyokinesis.  
  • The chromosomes  with their centromeres at the poles begin to uncoil, lengthen and loose their individuality.  
  • The nucleolus begins  to reappear. 
  •  The nuclear membrane begins to appear around the chromosomes.
  •  Spindle fibres break down and get absorbed in the cytoplasm.  Thus two daughter nuclei are formed in a cell.
Do you know ?
  • Pulling away of daughter chromosomes is achieved by elongation and shortening of two types of spindle fibres. 
  • Spindle  fibre present between centriole  and centromere,  called as kinetochore  fibres contract and  the spindle fibres  present between two opposite centrioles,  called  as polar fibres elongate.

Cytokinesis :
  • The  division of the  cytoplasm into two daughter cells is called cytokinesis. 
  • The  division starts with a constriction.  This constriction  gradually deepens and ultimately joins in the  centre  dividing into  two daughter cells.  
  • This process of division of cytoplasm is perpendicular  to the spindle.  
  • This mechanism  of cytokinesis is characteristic of animal cells. However, plant cells are covered by  a relatively  non-flexible  cell wall.
  •  Due to this, furrow can not be formed. Instead, cell  wall/ partition starts to appear at the centre of the cell and grows outward to meet the existing lateral  walls.  
  • The formation of the new cell wall begins with the formation of a simple precursor, called the 'cell-plate'  that represents the middle lamella between the walls  of  two adjacent cells. 
  • At the time  of cytoplasmic  division, organelles  like mitochondria and plastids get distributed between the two daughter cells. 
Significance of mitosis :
  • As  mitosis  is equational division, the chromosome number is maintained  constant. 
  • It ensures equal distribution of the nuclear and  the cytoplasmic content between the daughter cells, both quantitatively  and qualitatively.  
  • The hereditary material  (DNA)  is also equally distributed. 
  • It helps in the growth and development  of organisms.
  • Old and worn-out cells  are replaced through mitosis.  
  • It helps  in the asexual reproduction of organisms and vegetative propagation  in plants. 
  • The process of mitosis also maintains the  nucleo-cytoplasmic  ratio. Although mitosis is a very reliable  process for  preserving the genetic make-up of  cells or organisms, it cannot introduce variation or new combination of existing genes.
Death of cell  :
  • You may think of it as a bad for cells in your body to die. In many cases, that's true:  it's not  good for cells  to  die  because of an injury (for example,  due to scrape or a harmful chemical),  which is called necrosis. 
  • However, some  cells  of our body die;  not randomly but in a carefully controlled  way. 
  • For example,  during the embryonic development, the cells between the embryonic fingers died in a process called apoptosis to give a definite shape to the fingers.  
  • This is a common  form of programmed cell death where cells undergo "cellular  suicide" when they receive  certain signals.  
  • Apoptosis  involves the cell death, but it benefits the organism as a whole (for instance, by letting fingers develop  or by eliminating potential cancer cells). 

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