CELL DIVISION ............MITOSIS ( PART 2)

What is mitosis .............?

Mitosis is a process of cell duplication, in which one cell divides into two genetically identical daughter cells. 

 Image :- Cell division. 

- In the various stages of mitosis, the cell's chromosomes are copied and then distributed equally between the two new nuclei of the daughter cells.

There are 5 phases in mitosis -

1) Prophase 

2) Metaphase 

3) Anaphase 

4) Telophase 

5) Cytokinesis 

 Fig :- phases of mitosis. 

1) prophase :- 

The first and longest phase of mitosis is prophase. During prophase, chromatin condenses into chromosomes, and the nuclear envelope (the membrane surrounding the nucleus) breaks down. 

- In animal cells, the centrioles near the nucleus begin to separate and move to opposite poles of the cell. 

 Fig :- Prophase. 

- Centrioles are small organelles found only in eukaryotic cells that help ensure the new cells that form after cell division each contain a complete set of chromosomes.

- As the centrioles move apart, a spindle starts to form between them. 

- Cell at the end of prophase, when viewed under the microscope , do not show golgi complex , ER , nuclear membrane and nucleolus .

2) Metaphase :- 

Meta means second phase .In metaphase the chromosome contain two chromatid attached to each other through the centromere .

- Mitotic spindle formation is complete . the phenomenon of bringing the chromosome on the equator of spindle is called congression .

- All the chromosome align themselves at the equator . The plane alignment of centromeres of all the chromosome of the at metaphase is referred as metaphasic plate. 

 Fig :- Metaphase. 

- The centromere join the two sister chromatid together. The surface of the centromere which hold the two sister chromatid of a chromosome is surrounded by a small disc shaped structure called kinetochore. 

- The spindle fibres are made up of microtubules . The kinetochore present around the centromere forms the site of attachment of these microtubule. 

- The microtubule of the spindle fibres attach to the kinetochore during metaphase .

3) Anaphase :- 

Ana mean up and phase mean stage . It is the phase where chromatid moves towards the pole.

- In anaphase, the sister chromatids separate from each other and are pulled towards opposite ends of the cell. 

- The protein “glue” that holds the sister chromatids together is broken down, allowing them to separate. Each is now its own chromosome. The chromosomes of each pair are pulled towards opposite ends of the cell. 

 Fig :- Anaphase. 

- Microtubules not attached to chromosomes elongate and push apart, separating the poles and making the cell longer.

- All of these processes are driven by motor proteins, molecular machines that can “walk” along microtubule tracks and carry a cargo. In mitosis, motor proteins carry chromosomes or other microtubules as they walk.

- It is the best stage to study shape of chromosome .

4) Telophase :- 

Telo means end phase , hence telophase is the end phase of mitosis .

- In telophase, the cell is nearly done dividing, and it starts to re-establish its normal structures as cytokinesis (division of the cell contents) takes place. 

- The mitotic spindle is broken down into its building blocks.

 Fig :- Telophase 

- Two new nuclei form, one for each set of chromosomes. Nuclear membranes and nucleoli reappear.

- The chromosomes begin to de condense and return to their “stringy” form.

- The nuclear envelop develops around the chromatin cluster at each pole forming two daughter nuclei . 

5) Cytokinesis :- 

Cytokinesis, the division of the cytoplasm to form two new cells, overlaps with the final stages of mitosis.

- It may start in either anaphase or telophase, depending on the cell, and finishes shortly after telophase.

 Fig :- Cytokinesis. 

-  In animal cells, cytokinesis is contractile, pinching the cell in two like a coin purse with a drawstring. The “drawstring” is a band of filaments made of a protein called actin, and the pinch crease is known as the cleavage furrow.

 Fig :- cytokinesis in animal cell. 

- Plant cells can’t be divided like this because they have a cell wall and are too stiff. Instead, a structure called the cell plate forms down the middle of the cell, splitting it into two daughter cells separated by a new wall.

 Fig :- Cytokinesis in plant cell. 

- When cytokinesis finishes, we end up with two new cells, each with a complete set of chromosomes identical to those of the mother cell.

- The daughter cells can now begin their own cellular “lives,” and – depending on what they decide to be when they grow up – may undergo mitosis themselves, repeating the cycle.