MITOSIS

• Flemming (1882) coined the term Mitosis.
• E . Strasburger (1875) observed mitosis in plant cell.
• The process of cell division where the chromosomes are duplicated and distributed equally to the daughter cells is called Mitosis.
• Mitosis occur in somatic cells so it is known as Somatic Cell Division. It is also known as the Equational Cell Division as the number of chromosome remain same in the daughter cells.
• The period between 2 mitotic cycle is called Interphase.
• Karyokinesis is followed by Cytokinesis.
• Karyokinesis is divided into Prophase, Metaphase, Anaphase and Telophase.

Cell Cycle :

• Interphase :
  • It is the longest phase of mitotic cycle, 95% of generation time.
  • It is the resting phase between 2 mitotic divisions called interphase.
  • Replication of DNA and synthesis of basic nuclear histone proteins takes place here.
  • The centriole divide to form a pair of new centrioles which are right angles to each other.
  • Synthesis of energy rich compounds and proteins takes place.
  • Interphase is divided into 3 phases
    • Post Mitotic gap phase (G₁)
    • Synthesis Phase (S)
    • Pre-Mitotic gap phase (G₂)
      • G₁ Phase :
        • Cell Metabolic rate is high.
        • Synthesis of cellular components like endoplasmic reticulum, chloroplast, mitochondria, lysosome etc occurs.
        • Structural and functional proteins are formed.
        • RNA and ribosomes are synthesized
      • S Phase :
        • Replication of DNA takes place.
        • Histone Proteins are synthesized.
      • G₂ Phase:
        • Cell volume increases.
        • Mitochondria and Chloroplast divide.
        • Centrioles get doubled
        • Cell has 2 complete diploid sets of chromosome.
      • M-Phase / Mitotic Phase :

• • • • • It is the final phase of cell cycle.
        • It starts with Karyokinesis and ends with cytokinesis.
        • The M–Phase is of 3 types
          • Amitosis
          • Mitosis
          • Meiosis
            • Mitosis :
              • Prophase :

• • • • • • • • • The Cell become spheroid, refractive and viscous.
                • Nuclear envelope starts disintegrating .
                • Each chromosome possess 2 chromatids.
                • Chromatids became thicken and shorten.
                • The nucleolus starts to disappear
                • Each Centriole separates and migrates towards the opposite pole of the cell.
              • Metaphase :

• • • • • • • • • Pro–Metaphase
                  • Disappearance of the nuclear membrance.
                  • No differentiation between cytoplasm and nucleoplasm.
                  • Such mitosis is called extra nuclear or eumitosis.
                • Metaphase
                  • Relational coiling between sister chromatids is absent.
                  • Nucleolus completely disappears.
                  • Spindle apparatus present.
                  • Each chromosome reaches the equator and arrange themselves radially to form the equatorial plate or metaphasic plate.
                  • Large chromosomes remain towards the periphery and smaller towards center.
                  • The fibers attached to the centromere of each chromosome is called chromosal fibres. The other fibers continuous from one pole to other pole are known as continuous fibers.
                • Anaphase:
                  • The centromere of each chromosome divides into two.
                  • The chromatids of each chromosome get separated.
                  • The fibers present between the chromosome are known as inter zonal fibres or inter chromosomal fibers.
                  • The chromosomes becomes shorter, thicker, and migrate towards opposite poles of the cell.
                  • As the spindle fibers attached to the centromeres contract and pull the chromosomes to opposite poles, they appear V , L or J shaped.
                • Telophase :

• The chromosomes elongate and became thread like.
• The nucleolus reappears.
• The nuclear membrane is recognized around each group of chromosomes.
• At the end of telophase phragmoplast appear at the equatorial plate of the cell.
• The micro tubules of mitotic spindle rearrange and disappear.

Cytokinesis :

• It is the division of cytoplasm which separates the two daughter cells.
• In plants, the cell plate formation occurs between the two groups of chromosomes.
• Vesicles from Golgi apparatus unite to form the cell plate.
• The cell plate grows from the middle towards the periphery to from the cell wall.
• The early cell plate is the middle lamella of the cell wall.
• Cell organelles like mitochondria, plastids, Golgi complex, lysosome and cytoplasmic matrix are distributed to two daughter cells.
• Cytokinesis in animals takes place by cleavage furrow method.
• Cytokinesis in plants takes place by cell plate formation method.

Control of cell cycle :

• The events of cell cycle are genetically controlled and highly conserved through evolutions.
• The control mechanism operates in the same manner in yeast, plants and animals.
• The cell cycle is controlled at three main check points G₁/S, G₂/M and spindle check point.
• The check points have stage where a cell cycle may stop if circumstances are not right for cell division.

• G₁/S Check Point :
  • It is the primary check point commonly known as start (in Yeast), restriction point or R-point in animals.
  • It access extra cellular growth factors or mitogens and intra cellular nutritional state.
  • Starvation or lack of mitogens halts the cell cycle at this point and the cell enters G₀ phase / quiescent phase.
• G₂/M Check Point:
  • It ensures the success of DNA replication.
  • DNA damaging agents can stop the cycle at this point.
• Spindle Check Point:
  • It operates at the beginning of the anaphase.
  • It access weather all chromosomes are attached to the spindle or not.
  • Passage to the above check point controlled by specific protein kinases that take part in phosphorylation and dephosphorylation.
  • For activity of protein kinases stimulators called cyclins are required. Hence protein kinases are called cyclin dependent kinases (CDKs) or mitosis promoting factors (MPF).

Significance of Mitosis :

• Maintains genetic stability by equal chromosome distribution between daughter cells.
• Surface volume ratio is restored.
• Provide new cell for growth, repair, regeneration and wound healing.
• It is a method of asexual reproduction and vegetative propagation.
• Nucleoplasmic index is restored.

Abnormal Mitosis :

1. Intranuclear Mitosis
2. Endo Mitosis
3. Dino Mitosis
4. Free nuclear Mitosis
5. C – Mitosis

Presented By                                                                                                     Fanee Mohanty ( Lecturer in Botany)