7. Cell Division - part 04 - Meiosis I

 


7. Cell Division - part 04 - Meiosis I



Meiosis :  

  • The term meiosis was  coined by J. B. Farmer  in  1905. It  takes  place  only in reproductive cells during the formation of gametes.  
  • By this  division, the  number  of chromosomes is reduced to half, hence it is also called  reductional division. 
  • The cells in which meiosis take place are termed as meiocytes. 
  • Meiosis produces four haploid daughter cells from  a diploid parent  cell. 
  • Meiosis is of two subtypes :
  1. First meiotic division or  Heterotypic division – (Meiosis I) 
  2. Second  meiotic division or  Homotypic division (Meiosis II) A. 
Meiosis I /First meiotic division/ Heterotypic division :
  • During 1st  meiotic  division, diploid cell is divided into two haploid cells.  
  • The daughter cells resulting  from this division are different from the parent cell in chromosome number.
  •  Hence this division is also called  heterotypic division. 
  • It consists of the phases like -  
  1. prophase-I
  2. metaphase-I
  3. anaphase-I
  4. telophase-I and 
  5. cytokinesis-I 
Prophase-I :  

  • This phase has longer duration. 
  • Significant features which are peculiar to meiosis occurs in this phase.  
  • This phase can be sub-divided into five sub-stages  as 
  1. Leptotene
  2. Zygotene
  3. Pachytene
  4. Diplotene and 
  5. Diakinesis. 
Leptotene : 
  • The volume of  nucleus increases. 
  • The chromosomes  become distinct, long thread-like and coiled.  
  • They take up a specific orientation-  the 'bouquet stage' inside the nucleus.  
  • This is characterised with the ends of chromosomes converged towards that side of the nucleus where the centrosome  lies. 
  • The centriole  divides into  two and migrate  to opposite poles. 
Zygotene :  
  • Intimate pairing of non-sister chromatids  of homologous chromosomes takes place by formation of synaptonemal complex.  
  • This pairing  is called  synapsis. 
  • Each pair consists of a maternal  chromosome and a paternal chromosome. 
  • Chromosomal pairs are called bivalents or tetrads. 
Pachytene :  

  • Each individual chromosome begins  to split  longitudinally  into  two similar chromatids.  
  • At this stage, tetrads become more clear in appearance because of presence of four visible chromatids.  
  • The homologous chromosomes of each pair begin to separate from  each  other. 
  • However, they  do not completely  separate but remain attached together at one or more points.
  • These points appear like a cross (X) known as chiasmata.  
  • Chromatids break at these points and broken segments are exchanged between non-sister chromatids of homologous chromosomes.  This is called as crossing-over or recombination. 
Diplotene :  

  • Though chiasmata are formed in pachytene, they become clearly visible in diplotene  due to the beginning  of repulsion between  synapsed homologous chromosomes. This is called desynapsis.  
  • It involves disappearence of synaptonemal complex.

Diakinesis :  
  • In this phase, the chiasmata  beings to move along the length of chromosomes from the centromere towards  the ends of chromosomes.  
  • The displacement  of chiasmata is termed  as terminalization.  
  • The terminal chiasmata exist till the metaphase. 
  • The nucleolus  disappears and the nuclear  membrane also begins  to  disappear. 
  • Spindle fibres starts to appear in the cytoplasm.

Metaphase-I :  

  • The spindle fibres become  well developed. 
  • The tetrads move towards the equator and they orient themselves on the equator in such a way that centromeres of homologous tetrads lie towards  the poles  and arms towards the  equator.
  • Due to increasing repulsive  forces between homologous chromosomes, they are ready to separate from each other.

Anaphase-I :  

  • In this  phase, homologous chromosomes are pulled away  from each other and carried towards opposite poles by spindle  apparatus. This is disjunction.  
  • The  two sister chromatids of each  chromosome  do not separate in meiosis-I.
  • This is reductional division.  
  • The sister chromatids of each chromosome  are connected  by a common  centromere. 
  •  Both sister  chromatids  of each  chromosome  are  now different in terms of  genetic content as  one of them has undergone the recombination
Telophase-I :  
  • The haploid number of chromosomes after reaching their respective poles, become uncoiled  and elongated.  
  • The nuclear  membrane  and the nucleolus  reappear and thus two daughter nuclei are formed.

Cytokinesis-I :  

  • After the karyokinesis, cytokinesis occurs and two haploid cells are formed.
  • In many cases, these daughter  cells pass through a short resting phase or interphase / interkinesis. 
  • In some cases, the  changes of the telophase  may not occur.  
  • The anaphase directly leads to the prophase of meiosis II.

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