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.

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.