CELL CYCLE AND CELL DIVISION

POINTS TO REMEMBER :

• Cell cycle : The sequence of events by which a cell duplicates its genome, synthesis the other constituents of the cell and eventually divides into two daughter cells.

Phases of cell cycle :

Interphase :

• G₁ Phase: Cell metabolically active and grows continuously.
• S Phase: DNA synthesis occurs, DNA content increases from 2C to 4C.but the number of chromosomes remains some (2N).
• G₂ Phase: Proteins are synthesized in preparation for mitosis while cell growth continues.
• M Phase (Mitosis Phase): Starts with nuclear division, corresponding to separation of daughter chromosomes (karyokinesis) and usually ends with division of cytoplasm (cytokinesis).
• Quiescent stage (G₀): Cells that do not divide and exit G1 phase to enter an inactive stage called G₀. Cells at this stage remain metabolically active but do not proliferate.

MITOSIS :

• Prophase :
  • Replicated chromosomes, each consisting of 2 chromatids, condense and become visible.
  • Microtubules are assembled into mitotic spindle.
  • Nucleolus and nuclear envelope disappear.
  • Centriole moves to opposite poles.
  
  
• Metaphase :
  • Spindle fibres attached to kinetochores (small disc-shaped structures at the surface of centromers) of chromosomes.
  • Chromosomes line up at the equator of the spindle to form metaphase plate.
  
  
• Anaphase :       
  • Centromeres split and chromatids separate.
  • Chromatids move to opposite poles.
  
  
• Telophase :        
  • Chromosomes cluster at opposite poles.
  • Nuclear envelope assembles around chromosome cluster.
  • Nucleolus, Golgi complex, ER reform.
  
  
• Cytokinesis : Is the division of protoplast of a cell into two daughter cells after Karyokinesis (nuclear division).


• Animal cytokinesis : Appearance of furrow in plasma membrane which deepens and joins in the centre dividing cell cytoplasm into two.


• Plant cytokinesis : Formation of new cell wall begins with the formation of a simple precursor − cell plate which represents the middle lamella between the walls of two adjacent cells.

Significance of Mitosis :

• Growth − addition of cells.
• Maintenance of surface/volume ratio.
• Maintenance of chromosome number.
• Regeneration.
• Reproduction in unicellular organism.
• Repair and wound healing.

CBSE Biology (Chapter Wise) Class XI ( By Mr. Hare Krushna Giri ) 
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