Mitosis: Definition, Purpose, Stages, Cytokinesis and Applications

What is Mitosis?

Mitosis is the process of cell division in which one cell gives rise to two genetically identical daughter cells, resulting in cell duplication and reproduction.

The number of chromosomes is preserved in both the daughter cells.

Mitosis is a short period of chromosome condensation, segregation, and cytoplasmic division.

The mitosis occurs in the somatic cells, and it is meant for multiplication of cell numbers during embryogenesis and blastogenesis of plants and animals.

As a process, mitosis is remarkably similar in all animals and plants.

Purpose of Mitosis

The process of mitosis is significant in both cell division and cell reproduction:

• Growth of organism - Continuous mitosis increases cell number, enabling growth from a single cell to a complex living organism.
• Replacement of cells - Different cells like skin cells and red blood cells are continuously replaced by mitosis. About 5×10⁹ cells are formed per day in humans via mitosis.
• Repair and regeneration - Mitosis is involved in repair and regeneration of body structures like in the starfish.
• Asexual reproduction - In multiple organisms, mitosis is the method of asexual reproduction.

Overview of Cell Cycle and Mitosis

Mitosis is a part of the cell cycle and is preceded by the S phase of interphase.

It is usually followed or accompanied by cytokinesis.

Replication of chromosomes and synthesis of proteins required for spindle fiber formation are completed prior to onset of mitosis.

Mitosis is divided into phases based on completion of one set of activities and onset of the other.

Stages of Mitosis

Interphase

Interphase is a part of the cell cycle where the cell copies its DNA as preparation for the M phase.

Metabolism of the cell increases in interphase. It is often termed the most active phase of the cell cycle.

A series of metabolic changes occur during this phase, divided into three subgroups.

G1-phase or Pre-DNA Synthesis Phase

• Longest phase of cell cycle, followed by M phase of previous cycle.
• Termed resting phase as no DNA synthesis takes place.
• Several cell organelles increase in size and cell rapidly synthesizes different types of RNA and proteins.
• Important events include:
  • Transcription of three types of RNAs.
  • Synthesis of regulatory proteins.
  • Synthesis of enzymes required for DNA synthesis.
  • Synthesis of tubulin proteins and other mitotic apparatus.

S-phase or DNA Synthesis Phase

• Involves replication of nuclear DNA and synthesis of histone proteins.
• Replication of cytoplasmic DNA can take place at any phase in the cell cycle.
• At end of S phase, each chromosome has two DNA molecules and a duplicate set of genes.
• This phase lasts for about 6-10 hours.

G2-phase or Post DNA Synthesis Phase

• Termed second gap phase or resting phase of interphase.
• Synthesis of RNA and proteins required for cell continues.
• Cell division involves enormous expenditure of energy, thus cell stores ATP in the G2 phase.
• By end of this phase, cell enters division or M-phase of cell cycle.

Prophase

Prophase is the first stage of mitosis characterized by appearance of thin-thread like condensing chromosomes:

• Cell becomes spheroid while cytoplasm becomes more refractile, viscous and pale.
• Each chromosome is composed of two coiled filaments, the chromatids, result of DNA replication during S phase.
• As prophase progresses, chromatids become shorter and thicker.
• Two sister chromatids of each chromosome are held together by special DNA-containing region called centromere.
• Chromosomes approach nuclear envelope, causing central space of nucleus to become empty.
• Two pairs of centrioles surrounded by microtubules radiating in all directions migrate to opposite poles.
• Nucleolus gradually disintegrates, marking end of prophase.
• In some primitive classes of plants and animals, nuclear envelope does not dissolve during mitosis.

Prometaphase

Prometaphase is initiated with breakdown of nuclear envelope, enabling interaction of spindle fibers with chromosomes:

• Chromosomes are violently rotated and oscillated back and forth between spindle poles.
• Centromeres capture ends of microtubules and are pulled by captured microtubules.
• By end of prometaphase, sister chromatids are attached to spindle fibers on opposite ends.
• Chromatids are held on the metaphase plate.

Metaphase

• During metaphase, chromosomes are shortest and thickest.
• Centromeres of sister chromatids occupy plane of equator forming metaphase plate.
• Chromosome arms remain directed towards poles.
• Two chromatids of a chromosome repulse each other.
• Microtubules remain stationary and under tension.

Anaphase

Anaphase begins abruptly with synchronous splitting of each chromosome into sister chromatids:

• Split chromatids are called daughter chromosomes, separating from the centromere.
• Original article notes splitting during prophase is caused by increase in cytosolic Ca2+. Scientifically, separation is driven by separase cleavage of cohesin after activation of anaphase-promoting complex.
• Movement of chromatids towards pole due to shortening of microtubules.
• During poleward migration, centromeres remain forward.
• Chromosomes characteristically appear U, V or J-shaped.
• Interzonal fibers expand and support movement towards pole.
• Total of 30 ATPs are required to carry chromosomes to poles, used by motor proteins.

Telophase

End of migration of daughter chromosomes to poles marks beginning of telophase:

• Events of prophase occur in reverse sequence.
• Nuclear envelope reassembles around each group of chromosomes to form two daughter nuclei.
• Disappearance of mitotic apparatus occurs.
• Reduction in viscosity of cytoplasm followed by synthesis of RNA takes place.
• Chromosomes resume long, slender, extended form.
• Nucleolus reappears at end of telophase.

Cytokinesis

Cytokinesis is division of cytoplasm followed by mitosis, resulting in formation of two separate daughter cells:

• Usually begins in anaphase and continues through telophase and into interphase.

Cytokinesis in Animal Cells

• Occurs through constriction and furrow formation.
• First sign of cleavage is constriction of plasma membrane during anaphase.
• Constriction invariably occurs in plane of metaphase plate, at right angles to long axis of mitotic spindle apparatus.
• Constriction grows more in-depth from outside to inside.
• Ultimately cell divides into two daughter cells.

Cytokinesis in Plant Cells

• Occurs by cell plate formation as constriction is not possible due to rigid cell wall.
• Golgi apparatus arrange themselves on equator to form phragmoplast.
• Phragmoplast later forms the cell plate.

Applications of Mitosis

Mitosis has been utilized for many lab-based techniques in molecular biology and biotechnology:

Cloning

• Cloning is technique employed in biotechnology to produce identical copies of cells or DNA fragments.
• Number of organisms is increased by process of mitosis.
• Cloned cells are used in wide array of biological experiments like fingerprinting.

Tissue Culture

• Growth of tissues or cells outside body in liquid, semi-solid, or solid growth medium is called tissue culture.
• Tissue culture is based on process of mitosis where cell undergoes division to form multiple tissues.
• Tissue culture may lead to organ culture in various organisms.

Stem Cell Regeneration

• Stem cells are group of cells that can be directed to form specialized cells in body.
• Stem cells can undergo mitosis to regenerate and repair diseased or damaged tissues in people.

Summary

Mitosis is somatic cell division producing two genetically identical daughter cells with preserved chromosome number. It is a short period of chromosome condensation, segregation and cytoplasmic division, similar in plants and animals. Its purposes include organismal growth, daily replacement of about 5×10⁹ cells in humans, repair and regeneration as in starfish, and asexual reproduction. Stages include interphase with G1 longest resting phase, S-phase of DNA replication lasting 6-10 hours, and G2 with ATP storage, followed by prophase with condensing chromatids held at centromere and centriole migration, prometaphase with nuclear envelope breakdown, metaphase with shortest thickest chromosomes at metaphase plate, anaphase with synchronous splitting into U, V, J shaped daughter chromosomes requiring 30 ATPs, telophase with reverse events and nuclear envelope reassembly, and cytokinesis by furrowing in animals and by phragmoplast and cell plate formation from Golgi apparatus in plants. Applications include cloning, tissue and organ culture, and stem cell regeneration.

References

1. Alberts B, Johnson A, Lewis J, Morgan D, Raff M, Roberts K, Walter P. Molecular Biology of the Cell. 6th Edition. Garland Science. Chapter 17, The Cell Cycle.
2. Lodish H, Berk A, Kaiser CA, Krieger M, Bretscher A, Ploegh H, Amon A. Molecular Cell Biology. 8th Edition. W.H. Freeman. Chapter 19, Cell Cycle and Mitosis.
3. Cooper GM, Hausman RE. The Cell: A Molecular Approach. 8th Edition. Oxford University Press. Chapter 14, Mitosis and Cytokinesis.
4. Karp G, Iwasa J, Marshall W. Karp's Cell Biology. 8th Edition. Wiley. Chapter 14, Mitosis and Cell Division.
5. Campbell NA, Urry LA, Cain ML, Wasserman SA, Minorsky PV, Reece JB. Biology. 12th Edition. Pearson. Chapter 12, The Cell Cycle.
6. NCERT. Biology Textbook for Class XI. Reprint 2023-24. National Council of Educational Research and Training. Chapter 10, Cell Cycle and Cell Division.
7. Morgan DO. The Cell Cycle: Principles of Control. 2nd Edition. New Science Press. 2007.
8. McIntosh JR. Mitosis. Cold Spring Harbor Perspectives in Biology. 2016. Volume 8.
9. Peters JM. The Anaphase Promoting Complex/Cyclosome: A Machine Designed to Destroy. Nature Reviews Molecular Cell Biology. 2006. Volume 7.
10. Pines J. Mitosis: A Matter of Getting Rid of the Right Protein at the Right Time. Trends in Cell Biology. 2006. Volume 16.
11. Guertin DA, Trautmann S, McCollum D. Cytokinesis in Eukaryotes. Microbiology and Molecular Biology Reviews. 2002. Volume 66.
12. Jürgens G. Cytokinesis in Higher Plants. Annual Review of Plant Biology. 2005. Volume 56.
13. Staehelin LA, Hepler PK. Cytokinesis in Higher Plants. Cell. 1996. Volume 84.
14. Samuels AL, Giddings TH, Staehelin LA. Cytokinesis in Tobacco BY-2 and Other Plant Cells. Journal of Cell Biology. 1995. Volume 129.
15. Hartwell LH, Weinert TA. Checkpoints: Controls That Ensure the Order of Cell Cycle Events. Science. 1989. Volume 246.