Cell Cycle M phase, or mitotic phase, is an important stage in the cell cycle responsible for cell division. Mitosis is a process where a single cell divides into two identical daughter cells. The process of mitosis consists of two main steps: karyokinesis, which involves the division of the cell nucleus, and cytokinesis, which is the separation of the cytoplasm and other organelles. The four stages of mitosis are prophase, metaphase, anaphase, and telophase. Cytokinesis completes the cell cycle and is responsible for the physical division of a parent cell into two daughter cells. It helps in the survival and growth of organisms, maintenance of chromosome number, and repair of damaged cells.
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Mitosis – Cell Division
Mitosis is a cell division that involves the duplication and equal distribution of chromosomes from a parent cell to two identical daughter cells. The term “mitosis” is derived from the Greek word “mitos,” meaning fibril or thread, reflecting the thread-like appearance of chromosomes during the process. Flemming coined the term “mitosis” in 1882. Mitosis is also called as equational division because the distribution of genetic material is equal in the daughter nuclei. In eukaryotic cells (having a cell nucleus) including animal, plant, fungal, and protist cells, the cell cycle is divided into two main stages: interphase, and mitosis in the M phase that also includes cytokinesis.
Mitosis occurs in somatic cells and represents a small percentage (about 1-5%) of the total cell cycle duration. The duration of mitosis can vary based on cell types and species, ranging from 30 minutes to 3 hours. Mitosis is especially prominent in meristematic regions, such as the root apex, shoot apex, intercalary meristem, lateral meristem, leaves, flowers, fruits, embryos, and seeds.
Diagram of Mitosis
Diagram below showing different phases of Mitosis.
Stages of Mitosis
The process of mitosis consists of two main steps: karyokinesis or mitosis which involves the division of the cell’s nucleus into two identical nuclei, and cytokinesis which completes the cell division by separating the cytoplasm and other organelles, resulting in the formation of two distinct daughter cells. The M phase is important for cell division, development, growth, and cellular repair.
Mitosis initiates with the nuclear division process known as karyokinesis, where “karyon” refers to the nucleus and “kinesis” denotes movement. The four stages of Mitosis are prophase, metaphase, anaphase, and telophase that are described below.
Prophase
The term “pro” signifies the first stage. Prophase, often subdivided into early, mid, and late stages, is the initial and longest phase of karyokinesis. Its various stages are discussed as follows:
Early prophase
- In this substage the nucleus and cell become spheroid and the nucleus appears as a ball of wool. Chromatin fiber condenses to make an elongated chromosome, which increases the refractivity and viscosity of the cytoplasm.
- The nuclear envelope begins to dissolve, and spindle fibers emerge.
- In animal cells, duplicated centrioles. (S stage of interphase) start to go towards opposite poles of the cell. Each centriole radiates out fine microtubular fibrils called astral rays. In animal cells and cells of lower flowers, fibrils seem like spokes of a wheel around each centriole to make an aster.
Mid-prophase
In mid-prophase, chromosomes shift towards the periphery and then leave a clear central area. It becomes shorter and thicker. Each chromosome consists of two threads that are longitudinal chromatids. Both chromatids are attached by the centromere, and so are known as sister chromatids.
Late prophase
In this substage, spindle fibers start showing up around the nucleus. The size of chromosomes is much reduced as compared to the prophase. In plant cells, spindle poles are formed without asters, while in animal cells, asters are involved in the formation of spindle poles.
Nucleolus and other cell organelles (like mitochondria, Golgi complex, ER, vacuoles, etc.) disappear. The presence of the spindle is essential for mitosis. If cells are treated with colchicines, which inhibit spindle formation, the anaphasic movement of the two groups of chromosomes toward the poles doesn’t take place.
Prometaphase
Prometaphase is indeed the intermediate stage between prophase and metaphase in mitosis. The term “pro” signifies before, “meta” refers to the second, and “phase” denotes stage.
- The nuclear membrane completely degenerates in this stage. So the mixing of cytoplasm with nucleoplasm occurs. It is understood as extranuclear mitosis or mitosis.
- In many protozoa, fungi, and some animal cells, the nuclear membrane does perhaps not degenerate throughout cell division known as intranuclear or premitosis.
- A spindle fiber consists of 4-20 microtubules formed of the protein tubulin. Spindle fibers extend from the centrosomes, which are structures located at opposite ends or poles of the cell. It has the maximum diameter in the middle, known as the equator.
Metaphase
The term “meta” refers to the second, and “phase” denotes stage. The various changes that take place in this stage are as follows:
- The discontinuous fibers radiate out from two poles and get connected to the disc-shaped framework at the top of the centromere called kinetochores. A kinetochore is a complex protein structure that is analogous to the ring for the microtubule hook; it’s the point where microtubules connect on their own to the chromosome. Chromosomes or kinetochore fibers contract and bring chromosomes over the equator, this occurrence is called congression.
- Smaller chromosomes are directed toward the center while bigger ones are peripheral in position on the equator. The centromeres of all of the chromosomes lie on the equator forming a plate that is an apparent metaphasic or equatorial serve while hands are directed towards the poles.
- The kinetochores play an important role in connecting chromosomal spindle fibers to microtubules. They may contribute to the formation of chromosomal spindle fibers during metaphase and prometaphase by acting as centers for the polymerization of microtubule proteins.
- Metaphase is the optimal phase for determining the total number of chromosomes in any species, providing a detailed examination of chromosome morphology. Idiograms, representing the arrangement of chromosomes in decreasing sizes, are constructed during this stage.
Anaphase
The term “ana” refers to up, and “phase” denotes stage. The various changes that take place in this stage are as follows:
- Chromosomes undergo a brief period of alignment on the equatorial plate. During this phase, the centromeres of the chromosomes undergo division, giving rise to daughter chromosomes, each possessing its centromere.
- Due to their repulsive nature, the daughter chromosomes move toward opposite poles.
- Spindle fibers connected to the centromeres contract, facilitating the movement of chromosomes towards the poles.
- The speed of anaphasic motion remains independent of the chromosome size.
- In the movement of chromosomes during anaphase, the centromeres take the lead, with the limbs trailing behind.
- During anaphase, the centromeres guide the way while the chromosome limbs follow in their path.
At the end of anaphase, two groups of chromosomes are formed, one at each pole. The amount and kinds of chromosomes at each pole correspond to those in the parent nucleus.
Telophase
The term “Telos” refers to end, and “phase” denotes stage. The various changes that take place in this stage are as follows:
- Cytoplasm viscosity decreases.
- New nuclear membranes form around each set of chromosomes.
- Chromosomes overlap, and chromatin development occurs.
- The nuclear organizer generates nucleoli for each daughter nucleus.
- Two daughter nuclei are established at the spindle poles, and cellular organelles like the Golgi complex and endoplasmic reticulum are reformed.
Cytokinesis
The term “cytokinesis” originates from the Greek words “cyto,” referring to hollow or cell, and “kinesis,” signifying movement. Cytokinesis is the final stage of the cell cycle, responsible for the physical division of a parent cell into two daughter cells. This process ensures the equitable distribution of cellular components and organelles, ultimately resulting in the formation of two distinct and genetically identical daughter cells.
In animal cells, cytokinesis involves the formation of a cleavage furrow, where the cell membrane is drawn inward, leading to the separation of the cytoplasm. In plant cells, a cell plate forms, composed of new cell wall materials, dividing the cell into two. Cytokinesis completes the cell cycle and is essential for growth, development, and tissue repair in multicellular organisms.
Significance of Mitosis
The significance of Mitosis are as follows:
- Mitosis is responsible for the duplication and division of cells. It ensures the growth and development of multicellular organisms.
- It allow the repair of tissues as it replaces the damaged or dead cells.
- It ensures that each daughter cell receives an identical set of chromosomes from the parent cell.
- It preserves the genetic stability and integrity.
- Mitosis contributes to the formation and differentiation of various cell types in the growing organism.
- Mitosis allow organisms to maintain their structure and function by producing new cells.
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FAQs on Cell Cycle M Phase
1. What is Mitosis?
Mitosis is a cellular division where a single cell divides into two identical daughter cells, each with the same number of chromosomes as the parent cell. It plays an important role in growth, development, and tissue repair in multicellular organisms.
2. What is the Longest Phase of Mitosis?
The longest phase of mitosis is prophase, during which chromatin condenses into visible chromosomes, the nuclear envelope breaks down, and spindle fibers begin to form, preparing for the subsequent stages of cell division. It marks the initiation of the orderly separation of genetic material.
3. What is the Importance of Cell Division?
Cell division is important for cell growth, tissue repair, and reproduction in organisms. It ensures the maintenance of cell numbers and genetic stability across generations and plays an important role in development and the renewal of damaged or aging cells.
4. What is the Shortest Period of Mitosis?
The shortest period of mitosis is anaphase, characterized by the rapid separation and movement of sister chromatids towards opposite poles of the cell, ensuring equal distribution of genetic material to daughter cells.
5. What are the Four Major Stages of Mitosis?
The four major stages of mitosis are prophase, metaphase, anaphase, and telophase. These stages collectively ensure the accurate distribution of genetic material during cell division.