7. Cell Division - part 05 - Meiosis II

 


7. Cell Division - part 05 - Meiosis II


Meiosis II / Second meiotic  division / Homotypic Division :

  • During this division,  two haploid  cells formed during first meiotic  division divide further into four  haploid cells.  
  • This division is similar to mitosis.  
  • The daughter cells formed in second meiotic  division are similar  to their parent cells  with respect  to the chromosome number formed in meiosis-I. Hence this division is called homotypic division.
  • It consists of the following  phases : 
  1. prophase-II
  2. metaphase-II
  3. anaphase-II
  4. telophase-II and 
  5. cytokinesis-II. 
Prophase-II :  
  • The  chromosomes  are distinct with two chromatids.  
  • Each  centriole  divides into two resulting  in the formation  of two centrioles  which  migrate  to opposite  poles and form asters. 
  • Spindle fibres are formed between the centrioles.  
  • The nuclear membrane and nucleolus disappear. 
Metaphase-II : 
  • Chromosomes gets arranged at the equator.  
  • The two chromatids of each chromosome are separated by the division of the centromere. 
  • Some spindle fibres are attached to the  centromeres and some are arranged end to end between two opposite centrioles. 
Anaphase-II : 
  • The separated chromatids become  daughter chromosomes and move to opposite poles due to the contraction  of the spindle fibres attached to centromeres. 
Telophase-II : 
  • During this stage the daughter chromosomes uncoil.  
  • The nuclear membrane surrounds each  group of  chromosomes  and the nucleolus reappears.

Cytokinesis-II : 
  • Cytokinesis occurs after nuclear division.  
  • Two  haploid cells are formed from each haploid cell. Thus, in all, four haploid daughter cells  are formed.  
  • These cells undergo further changes to develop into gametes. 
Significance of Meiosis :  
  • Meiotic division produces gametes.  
  • If it  is absent, the  number of chromosome would double or quadruple resulting in the formation of monstrosities (abnormal forms).  
  • The constant number of  chromosomes in a  given species across generations is maintained  by meiosis. 
  • Because of crossing over, exchange  of genetic  material takes place leading to genetic variations, which are the raw materials for evolution.



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