Mitosis is a process of cell division that results in two genetically identical daughter cells from a single parent cell. It’s critical for growth, repair, and asexual reproduction. Mitosis is classically divided into either four or five stages: prophase, prometaphase (sometimes included in prophase), metaphase, anaphase, and telophase. Each phases features unique events concerning chromosomal alignment, spindle formation, and the division of cellular contents.
The discovery of mitosis traces back to the 18th and 19th centuries, when scientists began using dyes and microscopes for observing cell division. The term “mitosis” was coined by Walther Flemming in 1882 while documenting the process of chromosomal division in salamander larvae. The term comes from the Greek word ‘mitos’ meaning ‘thread,’ referring to the thread-like appearance of chromosomes during mitosis. Other names for the process are ‘karyokinesis’ (Schleicher, 1878) and ‘equatorial division’ (August Weismann, 1887). The discovery of mitosis was pivotal for cytology and later for genetics, as it revealed the mechanisms by which cells replicate and inherit genetic information.
The cell prepares for mitosis in the part of the cell cycle called interphase. During interphase, the cell prepares for mitosis by undergoing critical growth and replication processes. It increases in size (G1 phase), duplicates its DNA (S phase), and produces additional proteins and organelles while also beginning to reorganize its contents to facilitate eventual division (G2 phase).
There are either four or five mitosis phases: prophase (sometimes separated in prophase and prometaphase), metaphase, anaphase, and telophase. Cytokinesis follows telophase (some texts classify it as the final stage of telophase).
Prophase: During prophase, the chromatin condenses into visible chromosomes. Since DNA replicated in interphase, each chromosome consists of two sister chromatids joined at the centromere. The nucleolus fades and the nuclear envelope begins to disintegrate. Outside the nucleus, the mitotic spindle, comprised of microtubules and other proteins, starts forming between the two centrosomes. The centrosomes begin moving toward opposite poles of the cell.
Prometaphase: In prometaphase, the nuclear envelope completely breaks down and the spindle microtubules interact with the chromosomes. The kinetochores, protein structures on the chromatids at the centromeres, become attachment points for the spindle microtubules. This is crucial for chromosome movement. The microtubules begin moving the chromosomes toward the center of the cell, an area known as the metaphase plate.
Metaphase: The hallmark of metaphase is the alignment of chromosomes along the metaphase plate. Each sister chromatid is attaches to spindle fibers coming from opposite poles. The kinetochores are under tension, which is a signal of proper bipolar attachment. This alignment ensures that each new cell receives one copy of each chromosome.
Anaphase: Anaphase starts when the proteins holding the sister chromatids together break apart, allowing them to separate. The microtubules attached to kinetochores shorten and the cell elongates due to the pushing forces exerted by overlapping non-kinetochore microtubules. The sister chromatids are now individual chromosomes that are pulled toward opposite poles of the cell.
Telophase: Telophase is the reversal of prophase and prometaphase events. The chromosomes arrive at the poles and begin decondensing back into chromatin. Nuclear envelopes re-form around each set of chromatids, resulting in two separate nuclei within the cell. The spindle apparatus disassembles and the nucleolus reappears within each nucleus.
Cytokinesis: Cytokinesis follows telophase. It is often considered a separate process from mitosis. In cytokinesis, the cytoplasm divides and forms two daughter cells, each with one nucleus. For animal cells, this involves a contractile ring that pinches the cell in two. In plant cells, a cell plate forms along the line of the metaphase plate, eventually leading to the formation of two separate cell walls.
Open vs Closed Mitosis
There are variation in these phases. Open and closed mitosis refer to whether the nuclear envelope remains intact during the process of cell division.
Closed Mitosis: In closed mitosis, the nuclear envelope does not break down. Chromosomes divide within an intact nucleus. This is common in some fungi and algae. The mitotic spindle forms within the nucleus, and division of the nuclear contents occurs without the dispersal of nuclear components into the cytoplasm.
Open Mitosis: In contrast, open mitosis involves the breakdown of the nuclear envelope early in mitosis. Open mitosis is typical of most animals and plants. This allows the chromosomes to condense and become accessible to the mitotic spindle in the cytoplasm. After the chromosomes separate into daughter nuclei, the nuclear envelope reassembles around each set of chromosomes.
The choice between open and closed mitosis likely reflects different evolutionary solutions to the problem of segregating chromosomes into daughter cells while maintaining critical nuclear functions during cell division.
Functions and Importance of Mitosis
Mitosis is a critical process for eukaryotic organisms. It serves several essential functions:
- Growth and Development:
- Multicellular organisms require mitosis for growth from a fertilized egg into a fully developed organism. Repeated rounds of mitosis give rise to the vast number of cells that make up the tissues and organs of a body.
- Tissue Repair and Regeneration:
- Mitosis replaces the lost or damaged cells when tissues are damaged due to injury or wear and tear. This helps in healing wounds and regenerating tissues. For example, the human liver has a remarkable capacity to regenerate through mitotic cell division.
- Cell Replacement:
- Some cells have a very short lifespan and need constant replacement. For instance, human skin cells, blood cells, and the cells lining the gut have high turnover rates. Mitosis is the process that continuously replenishes these cells to maintain tissue integrity and function.
- Asexual Reproduction:
- In some organisms, mitosis is a form of asexual reproduction called vegetative reproduction. Single-celled organisms, such protozoa and yeasts, as well as some multicellular organisms like hydras and plants, reproduce asexually through mitosis. Here, mitosis creates clones of the original organism.
- Maintenance of Chromosome Number:
- Mitosis ensures that each daughter cell receives an exact copy of the parent cell’s genetic material. This is crucial for maintaining the species-specific chromosome number in all body cells, which is important for normal functioning.
- Genetic Consistency:
- By precisely duplicating the genetic material and segregating it equally into two daughter cells, mitosis ensures genetic consistency. This means that all body cells of an organism (except for the gametes, which form via meiosis) contain the same DNA.
- Developmental Plasticity and Cell Differentiation:
- Mitosis allows a single fertilized egg to become a complex organism with diverse cell types. As cells divide, they differentiate into various cell types with specialized functions. While the regulation of gene expression controls this process, mitotic cell division initiates it.
- Immune System Function:
- Mitosis is essential for the proliferation of lymphocytes, which are white blood cells that play a critical role in the immune response. When activated by antigens, lymphocytes rapidly divide by mitosis to build up a force capable of fighting infection.
- Cancer Prevention:
- Normally, mitosis is a highly regulated process. However, when these regulatory mechanisms fail, it leads to uncontrolled cell division and cancer. Understanding mitosis is crucial for developing treatments and prevention strategies for cancer.
Animal vs Plant Cell Mitosis
Mitosis in plant and animal cells follows the same fundamental process, but with some differences that stem from their unique cellular structures. Here are the key distinctions:
Centrosomes and Spindle Formation:
- In animal cells, centrosomes containing a pair of centrioles are the organizing centers for microtubules and thus spindle formation. The centrosomes migrate to opposite poles of the cell during prophase.
- Plant cells lack centrioles. Instead, spindle microtubules form around nucleating sites in the cytoplasm called microtubule organizing centers (MTOCs).
- Animal cells undergo cytokinesis through the formation of a cleavage furrow. Actin and myosin microfilaments constrict the middle of the cell, pinching it into two daughter cells.
- Plant cells are surrounded by a rigid cell wall, so they cannot be pinched. Instead, they form a cell plate during cytokinesis. Vesicles from the Golgi apparatus coalesce at the cell’s equator, forming a new cell wall that expands outward until it fuses with the existing cell wall.
Presence of Cell Wall:
- The rigid cell wall in plant cells restricts the movement of the cell during mitosis. For example, plant cells do not form asters (star-shaped microtubule structures) as seen in animal cells.
- Animal cells change shape during mitosis, which aids in the division process.
- Animal cells utilize centrosomes and astral microtubules for spatial orientation during mitosis.
- Plant cells rely more on the spatial structure provided by the cell wall and vacuoles for the organization of their mitotic spindle.
Formation of Mitotic Structures:
- In animal cells, the mitotic spindle forms from the centrosomes and extends across the cell to organize and separate the chromosomes.
- In plant cells, the spindle forms without centrosomes and establishes a bipolar structure without the aid of astral microtubules.
Despite these differences, the end goal of mitosis in both plant and animal cells is the same: to produce two genetically identical daughter cells from a single parent cell. The variations in the process are adaptations to the structural and material constraints inherent in the different types of cells.
Does Mitosis Occur in Prokaryotes?
Mitosis does not occur in prokaryotes. Prokaryotic organisms, such as bacteria and archaea, have a simpler cell structure without a nucleus and lack the complex chromosome structures found in eukaryotes. Instead of mitosis, prokaryotes undergo a different process called binary fission to replicate and divide.
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