What is mitosis?

Mitosis—A Detailed Study of Cell Division

Introduction

All living organisms grow, develop, and maintain themselves through cell division. One of the most important types of cell division is mitosis, which ensures that new cells formed are genetically identical to the parent cell. This process is essential not only for the continuity of genetic material but also for the survival and evolution of life.

Mitosis was first described by the German biologist Walter Flemming in 1882 when he studied the thread-like structures (chromosomes) inside the nucleus. The word mitosis comes from the Greek word mitos, meaning “thread,” referring to the thread-like appearance of chromosomes during division.

Definition of Mitosis

Mitosis is the type of equational nuclear division in which a single parent cell divides to produce two daughter cells, each having the same number and type of chromosomes as the parent. It is followed by cytokinesis, the division of the cytoplasm.

• It occurs in somatic (body) cells of animals and plants.

• It helps maintain the chromosome number (diploid → diploid) from one generation of cells to the next.

• It ensures genetic stability.

Cell Cycle and Mitosis

Mitosis is part of the larger cell cycle, which includes:

1. Interphase – Preparation phase (90% of cell cycle).

   • G₁ phase (Gap 1): Cell grows, produces proteins and organelles.

   • S phase (Synthesis): DNA replicates, forming sister chromatids.

   • G₂ phase (Gap 2): Cell prepares for division by synthesizing enzymes and spindle proteins.

2. Mitotic phase (M phase): Division of the nucleus (mitosis) and cytoplasm (cytokinesis).

Stages of Mitosis in Detail

1. Prophase

• Longest phase of mitosis.

• Chromatin condenses into chromosomes, each made of two sister chromatids joined at a centromere.

• The nucleolus disappears, and the nuclear envelope starts disintegrating.

• Centrosomes move to opposite poles, and spindle fibers (microtubules) begin to form.

• Spindle fibers attach to chromosomes at special protein structures called kinetochores.

2. Metaphase

• Chromosomes align at the cell’s equatorial plane, known as the metaphase plate.

• Each centromere is connected to spindle fibers from opposite poles.

• This stage is very important clinically, because many drugs (e.g., colchicine) arrest cells at metaphase to study chromosome number and structure.

3. Anaphase

• Shortest but most dramatic stage.

• Centromeres split and the spindle fibers shorten, pulling sister chromatids apart toward opposite poles.

• Once separated, chromatids are called daughter chromosomes.

• This ensures that each pole receives an identical set of chromosomes.

4. Telophase

• Chromosomes reach opposite poles and begin to de-condense back into thin chromatin fibers.

• The nuclear envelope reforms around each set of chromosomes.

• The nucleolus reappears in each nucleus.

• The spindle fibers disintegrate.

• Now, the cell has two nuclei, each genetically identical.

5. Cytokinesis

• Cytoplasmic division that usually overlaps with telophase.

• In animal cells: A contractile ring of actin filaments forms a cleavage furrow, pinching the cell into two.

• In plant cells: Vesicles from the Golgi apparatus align at the equator to form a cell plate, which later develops into the new cell wall.

• Finally, two daughter cells are produced, each identical to the parent cell.

Molecular Mechanisms Controlling Mitosis

Mitosis is regulated by cell cycle checkpoints:

• G₁ checkpoint: Decides if the cell should divide or enter a resting phase (G₀).

• G₂ checkpoint: Ensures DNA replication is complete and correct.

• M checkpoint (spindle checkpoint): Ensures that chromosomes are properly attached to spindle fibers before anaphase.

The process is controlled by cyclins and cyclin-dependent kinases (CDKs), which act as molecular switches to move the cell from one stage to the next.

Biological Significance of Mitosis

1. Growth and Development

   • Allows organisms to increase cell number, leading to overall growth.

   • Example: Growth of a child into an adult.

2. Tissue Repair and Healing

   • Dead or damaged cells are replaced by new identical cells.

   • Example: Healing of skin wounds, replacement of red blood cells.

3. Asexual Reproduction

   • Unicellular organisms (e.g., Amoeba, Paramecium, yeast) reproduce through mitosis.

   • Ensures rapid multiplication without the need for gametes.

4. Genetic Stability

   • Daughter cells are identical to parent cells → no genetic variation.

   • Maintains species identity.

5. Maintenance of Chromosome Number

   • Ensures diploid (2n) condition is preserved in somatic cells.

Errors in Mitosis and Diseases

• Non-disjunction: Chromosomes fail to separate, leading to abnormal chromosome number (aneuploidy).

• Cancer: Uncontrolled mitosis due to mutations in cell cycle regulatory genes (oncogenes, tumor suppressor genes).

• Apoptosis link: If errors occur, the cell may undergo programmed cell death to prevent abnormal growth.

Differences Between Mitosis and Meiosis

Feature	Mitosis	Meiosis
Type of division	Equational	Reductional
Number of divisions	One	Two
Daughter cells	2	4
Chromosome number	Same as parent (2n → 2n)	Half of parent (2n → n)
Genetic variation	None (clones)	Present (due to crossing over & independent assortment)
Where it occurs	Somatic cells	Germ cells (gametes)
Function	Growth, repair, asexual reproduction	Sexual reproduction

Flowchart of Mitosis:

Cell → Interphase → Prophase → Metaphase → Anaphase → Telophase → Cytokinesis → 2 Daughter Cells

Frequently Asked Questions (FAQs)

Q1. What is the main purpose of mitosis?
The main purpose of mitosis is to produce two genetically identical daughter cells for growth, repair, and maintenance of tissues. It also helps in asexual reproduction of unicellular organisms.

Q2. In which cells does mitosis occur?
Mitosis occurs in somatic (body) cells of animals and in vegetative cells of plants. It does not occur in gametes (sex cells), which divide by meiosis.

Q3. How many daughter cells are produced in mitosis?
Mitosis produces two daughter cells, both genetically identical to the parent cell.

Q4. What is the difference between mitosis and cytokinesis?

• Mitosis refers to division of the nucleus.

• Cytokinesis is the division of the cytoplasm, which usually follows telophase.

Q5. Why is mitosis called “equational division”?
It is called equational division because the chromosome number remains the same in daughter cells as in the parent cell (diploid → diploid).

Q6. What is the shortest phase of mitosis?
Anaphase is the shortest phase, where sister chromatids separate and move to opposite poles.

Q7. What happens if mitosis goes wrong?
Errors in mitosis can lead to aneuploidy (abnormal number of chromosomes) or uncontrolled cell division, which may result in cancer.

Q8. What is the role of spindle fibers in mitosis?
Spindle fibers attach to chromosomes at kinetochores and pull sister chromatids apart during anaphase, ensuring equal distribution of genetic material.

References

1. Alberts, B., Johnson, A., Lewis, J., Morgan, D., Raff, M., Roberts, K., & Walter, P. (2015). Molecular Biology of the Cell (6th ed.). Garland Science.

2. Cooper, G. M., & Hausman, R. E. (2019). The Cell: A Molecular Approach (8th ed.). Oxford University Press.

3. Lodish, H., Berk, A., Zipursky, S. L., et al. (2016). Molecular Cell Biology (8th ed.). W. H. Freeman and Company.

4. Raven, P. H., Johnson, G. B., Mason, K. A., Losos, J. B., & Singer, S. R. (2020). Biology (12th ed.). McGraw-Hill Education.

5. Flemming, W. (1882). Zellsubstanz, Kern und Zelltheilung. Leipzig: Vogel.

6. National Center for Biotechnology Information (NCBI) – Mitosis overview