Mitosis is a biological process where a single parent cell divides into two genetically identical daughter cells, a division essential for various biological functions in living organisms. Mitosis supports the growth and development of multicellular organisms, allowing a fertilized egg to develop into a complex adult. It also plays a role in repairing damaged tissues and replacing old or worn-out cells, such as skin cells and red blood cells. For single-celled organisms, mitosis serves as a primary method of asexual reproduction, generating new individuals. The entire process is part of the larger cell cycle, specifically the M phase.
Prophase
Prophase, the initial stage of mitosis, involves transformations within the cell. During this phase, diffuse genetic material, chromatin, undergoes condensation, forming compact, visible structures called chromosomes. Each of these chromosomes consists of two identical sister chromatids, joined together at a central region called the centromere.
As chromosome condensation progresses, the nuclear envelope begins to break down into small vesicles. The nucleolus disappears. Outside the nucleus, the mitotic spindle starts to form from centrosomes. These centrosomes, duplicated in the preceding interphase, move to opposite poles of the cell, preparing for chromosome segregation.
Metaphase
Following prophase, the cell enters metaphase, a stage characterized by the precise alignment of chromosomes. During metaphase, the mitotic spindle is fully developed, with centrosomes positioned at opposite ends of the cell. The spindle fibers, composed of microtubules, extend from these centrosomes and attach to protein structures called kinetochores, located at the centromere of each sister chromatid.
The attached spindle fibers exert pulling forces, guiding each chromosome to align along the cell’s equatorial plane, known as the metaphase plate. This alignment ensures that when the sister chromatids separate, each new daughter cell receives a complete set of genetic information. The cell employs checkpoint mechanisms to ensure proper alignment before proceeding to the next stage.
Anaphase
Anaphase marks the separation of sister chromatids. This stage begins with the cleavage of proteins holding the sister chromatids at their centromeres. Once separated, each chromatid is now considered an individual chromosome.
The newly independent chromosomes are pulled towards opposite poles of the cell. This movement is driven by the shortening of spindle fibers attached to the kinetochores. Other spindle fibers, not attached to chromosomes, lengthen, contributing to the elongation of the entire cell. This coordinated movement ensures that an identical set of chromosomes moves to each developing pole.
Telophase
Telophase represents the final stage of nuclear division, effectively reversing many events observed in prophase. As the separated chromosomes arrive at their poles, they begin to decondense into a more diffuse chromatin state. The mitotic spindle starts to disassemble and disappear.
New nuclear envelopes begin to reform around each complete set of chromosomes at the poles. These new membranes are formed from vesicles derived from the endoplasmic reticulum. The nucleoli also reappear within these newly formed nuclei. By the end of telophase, the cell contains two distinct, genetically identical nuclei, each ready to become part of a new daughter cell.
Cytokinesis
Cytokinesis is the final step in cell division, distinct from the nuclear division of mitosis. This process involves the physical division of the cytoplasm and its contents, resulting in the formation of two separate daughter cells. Cytokinesis typically begins during anaphase or telophase and concludes shortly after telophase.
The mechanism of cytoplasmic division differs between animal and plant cells. In animal cells, a cleavage furrow forms, an indentation in the cell membrane that deepens. This furrow is created by a contractile ring that constricts the cell, pinching it into two. Plant cells, with their rigid cell walls, cannot form a cleavage furrow. Instead, a new cell wall, called a cell plate, forms in the center of the cell and grows outward, dividing the parent cell into two daughter cells.