Mitosis

Mitosis is a fundamental process in biology where a single eukaryotic cell divides to produce two genetically identical daughter cells. It's an equational division because the chromosome number in the daughter cells is the same as in the parent cell. This process is crucial for the growth, repair, and maintenance of multicellular organisms, and it also serves as a method of asexual reproduction in many single-celled organisms.

Cause of Mitosis

Kern-Plasm-Theorie: 

• Hertwig entwickelte die Kern-Plasm-Theorie. According to this theory, mitosis occurs due to a disturbance in the Karyoplasmic Index (KI) or the Nucleocytoplasmic ratio of the cell. 
• Karyoplasmic Index:

$KI=\frac{V_n}{V_c-V_n}$

• Vn = Volume of nucleus
• Vc = Volume of cell
• Vc - Vn = Volume of cytoplasm
• The Karyoplasmic Index of small cells is high as they have less cytoplasm. The nucleus efficiently controls cytoplasmic activity in small cells.
• In a large cell, the nucleus fails to control cytoplasmic activity. To maintain the nucleus's metabolism, a large cell divides into two cells.

Surface-Volume Ratio

• Surface-to-volume ratio is also considered a factor in cell division. 
• When a cell grows in size, its volume increases faster than its surface area. 
• A stage will be reached when the cell's surface area becomes insufficient to draw materials from the environment. 
• At such a critical stage, the cell increases its surface-to-volume ratio by division.

1.0Phases of Mitosis

The process of mitosis is a continuous sequence of events but is divided into four distinct stages for easier understanding.

1. Prophase In this first stage, the cell begins the process of division.

• The chromatin (thread-like DNA) condenses into compact, visible chromosomes. Each replicated chromosome consists of two identical sister chromatids joined at the centromere.
• The mitotic spindle, a structure composed of microtubules, begins to assemble between the two centrosomes, which have moved to opposite poles of the cell.
• The nuclear envelope and nucleolus begin to disappear.

2. Metaphase This is the stage where the chromosomes align.

• The chromosomes are completely condensed and migrate to the centre of the cell, forming the metaphase plate or equatorial plate.
• The spindle fibres attach to the kinetochore, a protein structure at the centromere of each sister chromatid. This ensures that each chromosome is correctly positioned for separation.

3. Anaphase This is the shortest and most dynamic phase of mitosis.

• The centromeres of each chromosome split, and the sister chromatids separate.
• The now-separated chromatids (called daughter chromosomes) are pulled toward opposite poles of the cell by the shortening spindle fibres. This ensures that each new nucleus will receive a complete and identical set of chromosomes.

4. Telophase This is the final stage of nuclear division, essentially a reversal of prophase.

• The daughter chromosomes arrive at the opposite poles of the cell.
• The chromosomes decondense and return to their thread-like chromatin state.
• New nuclear envelopes form around each set of chromosomes, and the nucleoli reappear.
• The spindle fibres disassemble.

Cytokinesis 

• Following mitosis, cytokinesis is the division of the cytoplasm, which results in the formation of two distinct daughter cells. 
• In animal cells, a cleavage furrow pinches the cell membrane inwards to divide the cell. 
• In plant cells, a cell plate forms in the middle and eventually develops into a new cell wall.

Significance of Mitosis

Mitosis is critical for several biological functions in living organisms.

• Growth and Development: Mitosis is the primary way that multicellular organisms increase in size. A single-celled zygote develops into a complex organism through numerous rounds of mitosis.
• Cell Replacement and Repair: Old, worn-out, or damaged cells are continuously replaced through mitosis. This process is essential for healing wounds and maintaining the integrity of tissues, such as the skin and the lining of the digestive tract.
• Genetic Stability: Mitosis ensures that each daughter cell receives a precise and complete copy of the parent cell's genetic material, thus maintaining the chromosome number and genetic stability of the organism.
• Asexual Reproduction: In many single-celled organisms, such as Amoeba, mitosis is the sole mechanism of reproduction, creating offspring that are genetically identical to the parent.

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