7. Cell Division - part 01 - Cell cycle

 

7. Cell Division - part 01 - Cell cycle


  • Life of all multicellular  organisms starts from single  cell  i.e. zygote.  
  • Growth of every  living  organism depends on cell  division. 
  • As stated  in the  cell  theory, every  cell  arises from the pre-existing cell.
Cell cycle :

  • Sequential  events occurring in the life of a cell is called cell cycle.  
  • There are two phases of  cell  cycle  as -
  1. Interphase  and 
  2. M-phase. 
  • During interphase, cell undergoes growth or rest  as per  the  need.  
  • During M-phase, the  cell undergoes division. 
  • Interphase alternates  with the period of division.  
Interphase : 
  • Interphase is the stage between two successive cell  divisions. 
  • It is the longest phase of cell cycle during which the cell is highly active and  prepares  itself for  cell division.  
  • The interphase is  divisible into  three sub-phases  as G-phase, S-phase and  G-phase. 
G1-phase :
  • This is also known as first gap period or first growth period. 
  • It starts immediately after cell division. 
  • Cell performs RNA synthesis (mRNA, rRNA  and t-RNA), protein  synthesis and synthesis of membranes during this phase. 
S-phase :
  • It is synthesis phase in which DNA is  synthesized  or  replicated,  so that  amount  of DNA per cell doubles.
  • Histone proteins are also synthesized during this phase. 
G2  phase : 
  • G2  is the second growth phase, during  which  nucleus  increases in  volume. 
  • Metabolic activities essential for  cell division occur during this phase.  
  • Various proteins necessary for cell  division are synthesized during this phase. 
  • Besides,  RNA  synthesis also occur during this phase. 
  • In animal  cells, a daughter pair of centrioles appear near the pre-existing pair.
M-phase or  period of division :
  • 'M'  stands for mitosis or meiosis. 
  • M-phase involves -
  1. karyokinesis and 
  2. cytokinesis. 
  • Karyokinesis is the division of nucleus into two daughter nuclei.
  • Cytokinesis  is  division  of cytoplasm resulting in two daughter cells. 

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