Mitosis

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Mitosis is the process by which a parent cell divides, distributing identical copies of DNA to two daughter cells. This ensures that each daughter cell has the same genetic material as the parent cell, essential for growth, repair, and asexual reproduction in multicellular organisms. The stages of mitosis are sequentially organized to allow accurate distribution of chromosomes.

The Role of Mitosis in Life

Mitosis is essential for organismal growth and development, enabling cells to multiply and replace old or damaged ones. It plays a key role in embryonic development, tissue repair, and asexual reproduction in certain organisms. In humans, for example, it is through mitosis that our skin cells regenerate and heal after injury.

Without mitosis, life as we know it would be unsustainable, as cells could not efficiently repair damage or enable growth in living organisms. Errors in mitosis, however, can lead to severe consequences, including developmental disorders and cancer.

DNA Synthesis and Preparation

Before mitosis, during the S phase of interphase, DNA replication occurs. This replication doubles the DNA content in diploid cells, creating tetraploid cells (cells with twice the DNA content), though chromosome count remains diploid. During mitosis, the DNA is equally divided, returning each daughter cell to the diploid chromosome number.

Phases of Mitosis

Mitosis is conventionally divided into four main stages:

  1. Prophase
    • Chromosome Condensation: Chromatin, which is extended and uncoiled during interphase, condenses into visible chromosomes. Each chromosome now consists of two sister chromatids (replicated DNA) joined at a central region called the centromere.
    • Spindle Formation: Outside the nucleus, the centrioles (in animal cells) separate and move towards opposite poles. Microtubules form between them, creating the mitotic spindle, and radiating microtubules (asters) establish the spindle poles.
    • Nuclear Envelope Disintegration: As prophase progresses, nucleoli disappear, and the nuclear envelope disintegrates, freeing the chromosomes within the cell.
  2. Prometaphase–Metaphase
    • Spindle Attachment: With the nuclear envelope gone, spindle microtubules connect to the chromosomes, specifically attaching to structures called kinetochores at each centromere.
    • Chromosome Alignment: Chromosomes move towards the equator of the spindle, forming the metaphase or equatorial plate. They align in a linear or ring formation, ensuring even distribution.
    • Cytoplasmic Organization: Late in metaphase, cytoplasmic movements help distribute organelles, such as mitochondria, around the cell.
  3. Anaphase
    • Separation of Chromatids: At the onset of anaphase, the enzyme separase cleaves cohesin proteins holding sister chromatids together. Each chromatid (now an individual chromosome) is pulled towards opposite poles by shortening spindle microtubules.
    • Equal Division: Chromatids, now grouped at each pole, ensure each new cell will have a diploid chromosome number. The cell membrane begins to fold at the center, forming the cleavage furrow.
  4. Telophase
    • Nuclear Reformation: Nuclear envelopes reform around each cluster of chromosomes, which then decondense back into chromatin. Nucleoli reappear, signifying the end of nuclear division.
    • Cytokinesis: The cell cytoplasm divides, and a contractile ring of actin and myosin filaments in animal cells tightens at the cleavage furrow, leading to the physical separation of the two daughter cells.
    • Spindle Breakdown: The mitotic spindle disassembles as division completes.

Cytokinesis: The Division of the Cytoplasm

Cytokinesis is the final stage of cell division, where the cytoplasm divides to form two separate daughter cells. This process occurs differently in animal and plant cells:  

  • Animal Cells: A cleavage furrow forms, pinching the cell membrane inward until the cell divides into two.  
  • Plant Cells: A cell plate forms, gradually expanding outward until it reaches the cell wall, dividing the cell into two.
A: The stages in mitosis, including the appearance and distribution of the chromosomes.
B: Immunofluorescence images of stages in mitosis in human carcinoma cells in culture. A, Metaphase, with spindle microtubules (green), the microtubule stabilizing protein (HURP; red) and chromosomal DNA (blue). B, Anaphase, with spindle microtubules (green), the central spindle (Aurora-B kinase, red) and segregated chromosomes (blue). C, Late anaphase, with spindle microtubules (green), the central spindle (Plk1 kinase, red, appearing yellow where co-localized with microtubule protein) and segregated chromosomes (blue). (Courtesy of Dr Herman Silljé, Max-PlanckInstitut für Biochemie, Martinsried, Germany.)

The Significance of Mitosis

Mitosis is crucial for various biological processes, including:

  • Growth: Mitosis enables organisms to grow by increasing the number of cells.  
  • Repair and Regeneration: It helps repair damaged tissues and regenerate lost cells.  
  • Asexual Reproduction: In many organisms, mitosis is the sole method of reproduction.

Potential Disruptions in Mitosis

  • Non-disjunction: Sometimes, sister chromatids may fail to separate properly, leading to one cell with extra chromosomes and another with fewer. This can result in genetic imbalances.
  • Environmental Effects: Exposure to ionizing radiation can prevent cells from undergoing mitosis properly, causing issues in rapidly dividing cells, such as skin and mucosal tissues. Chemical agents like vinblastine and taxol interfere with spindle microtubules, halting mitosis at metaphase.

References

  1. Walczak, C. E., & Heald, R. (2020). Mechanisms of mitotic spindle assembly and chromosome segregation. Annual Review of Biochemistry, 89(1), 821–848. https://doi.org/10.1146/annurev-biochem-071519-105336
  2. Lodish, H., Berk, A., Kaiser, C. A., Krieger, M., Bretscher, A., Ploegh, H., … & Amon, A. (2019). Molecular Cell Biology (8th ed.). W.H. Freeman.