Mitosis is a fundamental process where a single cell divides to produce two genetically identical daughter cells. This type of cell division is essential for growth in multicellular organisms. It also repairs damaged tissues and replaces old cells. In some organisms, mitosis is a form of asexual reproduction. This process unfolds through sequential stages that ensure the accurate distribution of genetic material.
Prophase
Prophase begins with significant changes within the cell’s nucleus and cytoplasm. During this phase, the cell’s genetic material, which exists as diffuse chromatin, condenses into distinct, visible chromosomes. Each chromosome consists of two identical sister chromatids joined together at a central region called the centromere.
As chromosomes condense, the nuclear envelope breaks down and disappears. Meanwhile, the mitotic spindle forms in the cytoplasm. In animal cells, this spindle originates from centrosomes, duplicated during interphase, which now move to opposite sides of the cell, establishing the two poles of the developing spindle.
Metaphase
Metaphase is characterized by the precise alignment of condensed chromosomes. The mitotic spindle becomes fully developed, extending microtubules from the centrosomes at opposite poles of the cell. Kinetochore microtubules attach to kinetochores at the centromere of each sister chromatid.
Spindle fibers exert balanced forces, causing the chromosomes to align along the cell’s equatorial plane. This line, positioned midway between the two spindle poles, is the metaphase plate. The precise arrangement of chromosomes at this plate ensures that each daughter cell receives an identical set of genetic information. The cell checks that all chromosomes are correctly aligned and attached to the spindle, a checkpoint that helps prevent errors in chromosome segregation.
Anaphase
Anaphase is a stage where duplicated chromosomes are separated and distributed to opposite ends of the cell. This process begins with the breakdown of cohesin, the protein that holds sister chromatids together at their centromeres. Once separated, each chromatid is considered an individual chromosome.
The separated chromosomes are pulled towards the opposite poles of the cell. This movement is driven by the shortening of the kinetochore microtubules, which retract the chromosomes towards the centrosomes. Meanwhile, other spindle microtubules lengthen, contributing to the elongation of the entire cell. This movement ensures that an equal set of chromosomes reaches each pole, setting the stage for the formation of two distinct nuclei.
Telophase and Cytokinesis
Telophase and cytokinesis are the final stages of cell division. In telophase, the separated chromosomes arrive at the opposite poles and decondense into chromatin. New nuclear envelopes form around each set of chromosomes, creating two nuclei.
As telophase concludes, cytokinesis divides the parent cell into two daughter cells. In animal cells, a contractile ring of actin filaments forms inside the plasma membrane at the former metaphase plate. This ring contracts, pinching the cell inward to create a cleavage furrow that deepens until division. Plant cells, with their rigid cell walls, undergo cytokinesis differently. A cell plate forms in the middle of the cell and grows outward, fusing with existing cell walls to separate the daughter cells.
Why Mitosis Matters
Mitosis facilitates the increase in cell number necessary for an organism to grow from a single-celled zygote into a complex being. Beyond growth, mitosis is continuously active for essential maintenance functions. It enables the repair of damaged tissues and the constant renewal of old cells, such as skin or blood cells. In certain organisms, it is the basis for asexual reproduction, producing genetically identical offspring. This process transmits genetic information to each new cell, preserving genetic stability.