- 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.
