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Cell Cycle and Cell Division

Mitosis

Have you noticed how a cut on your finger is gradually repaired in a few days? This is enabled by a process called Mitosis. Mitosis is the process responsible for regeneration and repair. Mitosis helps in cell growth and development. Cells can grow old and wear off or they can get bruised and injured but eventually, they repair and regenerate.

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Mitosis

Mitosis constitutes a comparatively small portion of a complete cell cycle but it is one of the imperative parts of the cell cycle. German Physician and cell biologist “Walther Flemming” coined the term “mitosis” in the year 1882. He explained the process of how cells split and separate their chromosome.

The process of cell division that results in the formation of two new daughter cells is termed as Mitosis. The newly formed daughter cells are genetically identical to the parent cell and to each other. It plays a crucial role in a living organism’s life cycle. However, the level of significance may vary depending on the type of organism (multicellular or single-celled).

In unicellular organisms such as bacteria, mitosis helps in asexual reproduction as it produces an identical copy of the parent cell. Another example of the Eukaryotic unicellular organism is “Amoeba.” An amoeba uses cell division for the production of new individuals. In the case of multicellular organisms, mitosis helps in growth and repair by producing more number of identical cells. For example plants, animals depend on cell division for their growth by addition of new cells.  It is also used for repairing the injured tissues or replacing the worn-out tissue by regenerating new cells.

Mitosis refers to the splitting of chromosomes in the eukaryotic cells during the cell division process. The parent cell divides into two daughter cells that are identical to the parent cell during the process of cell division. During the mitosis process, the cell’s nucleus along with the chromosome is divided to form two new daughter cell nuclei. The daughter nuclei inherit the same number of chromosomes as that of the parent nucleus.

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Learn more about Cell Cycle here.

Importance of Mitosis in Living Process

  • Genetic stability- Mitosis helps in the splitting of chromosomes during cell division and generates two new daughter cells. Therefore the chromosomes form from the parent chromosomes by copying the exact DNA.  Therefore, the daughter cells formed as genetically uniform and identical to the parent as well as to each other. Thus mitosis helps in preserving and maintaining the genetic stability of a particular population.
  • Growth- Mitosis help in increasing the number of cells in a living organism thereby playing a significant role in the growth of a living organism.
  • Replacement and regeneration of new cells- Regeneration and replacement of worn-out and damaged tissues is a very important function of mitosis in living organisms. Mitosis helps in the production of identical copies of cells and thus helps in repairing the damaged tissue or replacing the worn-out cells. But the degree of regeneration and replacement in multicellular organisms vary from one another. For example, mitosis process is used in order to regrowth the legs of newts and crustaceans. However, the degree of regrowth may vary.
  • Asexual reproduction- Mitosis is used in the production of genetically similar offspring. For example budding of hydra and yeast, binary fission in amoeba, etc.

Five Basic Stages of Mitosis

Mitosis_cells_sequence

Prophase

In this phase the DNA supercoils, chromatin fibres become coiled and condense into chromosomes. The chromosomes consist of two chromatids connected at the centromere. The microtubule spindles fibres start forming at the opposite ends of the cell. The mitotic spindle is composed of microtubule proteins that slowly increase in length during the prophase which eventually initiate the cell division process by elongating it.

The pairs of centrioles (two pairs) move away from each other towards the poles during the microtubule lengthening process. Finally, the nuclear envelope disintegrates and the nucleus dissolves.

Late Prophase

Late prophase is also known as the prometaphase. The nuclear envelope dissolves. The microtubules composed of spindle fibres move from the pole to the centre of the cell (cells equator). Kinetochores attach themselves to specialized microtubules called kinetochore fibres.

Kinetochores are special protein structures that develop on a chromatid during the process of cell division. It helps in attaching the spindle fibre to the chromosome. The kinetochore fibres combine with spindle polar fibres. Finally, the chromosomes start migrating toward the centre of the cell.

Metaphase

In metaphase, the spindle entirely develops. The nuclear membrane dissolves completely. Polar fibres keep on extending from the poles to the centre of the cell. Chromosomes assemble and disassemble themselves and try to find the centromere of sister chromatids. The chromosomes arrange themselves in the metaphase plate at 90 degrees to the spindle poles. The polar fibres produce equal forces and push the centromere of chromosome thus holding the chromosomes together at the metaphase plate.

Anaphase

The pairs of centromeres present in the chromosome start to move away in this phase. The sister chromatids, paired chromosomes, separate and form a complete chromosome, also termed as daughter chromosomes. The daughter chromosomes start moving towards the poles in the opposite ends through the spindle apparatus.

The centromere move first and gradually the kinetochores decreases in length as the chromosomes move closer to the pole. The two cell poles migrate further away during the anaphase and prepare for telophase. At the end of this stage, both the pole contains a complete set of chromosomes. Cytokinesis starts at this stage and continues through the next stage.

Telophase

The polar fibres keep on lengthening. Nuclei forms at the opposite end. Nuclear envelopes start developing from the leftover pieces of the nuclear envelope of the parent cell and from the endomembrane system. Nucleoli start reappearing. Chromatin fibres of the chromosome unwind. At this point, the process of mitosis is almost complete and the genetic material of the parent cell is equally divided into two.

Cytokinesis

The division of the cell cytoplasm is termed as cytokinesis. It starts before the anaphase stage and ends just after telophase. Two genetically identical daughter cells are formed after the end of cytokinesis. The new daughter cells are identical diploid cells. Each cell contains a full set of chromosome.

Solved Example for You

Q: Name the structure responsible for moving the chromosomes during the mitosis process?

  1. Cytoplasm
  2. Nuclear membrane
  3. Nucleolus
  4. Spindle

Sol: The correct answer is option “d”. The microtubules of the spindle are responsible for moving and arranging the chromosomes during mitosis.

FAQ’s for You

Q1. The term mitosis was introduced by?

Answer: Walther fleming coined the term mitosis in the early 1880s. He introduced this term from Greek word for thread, reflecting the shape of mitotic chromosomes.
Mitosis is a type of cell division during which one cell gives rise to two genetically identical daughter cells.

Q2. Give significance of mitosis.

Answer: Significance of mitosis:
It helps in maintaining the same number of chromosomes in daughter cells after division.
It is responsible for growth and development of multicellular organisms.
It helps in repairing of damaged tissues.
It helps the cell to maintain proper size.

Q3. Mitosis occurs in?

Answer: In cell biology, mitosis is a part of the cell cycle when replicated chromosomes are separated into two new nuclei. Cell division gives rise to genetically identical cells in which the number of chromosomes is maintained. In general, mitosis (a division of the nucleus) is preceded by the S stage of interphase (during which the DNA is replicated) and is often accompanied or followed by cytokinesis, which divides the cytoplasm, organelles and cell membrane into two new cells containing roughly equal shares of these cellular components. Mitosis occurs only in eukaryotic cells. Animal cells undergo “open” mitosis, where the nuclear envelope breaks down before the chromosomes separate, whereas fungi undergo “closed” mitosis, where chromosomes divide within an intact cell nucleus.

Q4. Which of the following is not a stage of mitosis?
A. Anaphase
B. Metaphase
C. Interpahse
D. Prophase

Answer: After which the chromosomes are pulled at the poles of the dividing cell by the spindle fibers attached to their kinetochores which is the characteristic of anaphase.
The last phase is the telophase where the nuclear membrane starts to appear again and the cytokinesis and karyokinesis takes place.
Therefore the option ‘Interphase’ is the correct answer.

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