Mitosis is a biological process where a single parent cell divides into two genetically identical daughter cells. This process is central to growth, tissue repair, and asexual reproduction in many organisms. During mitosis, the cell duplicates and separates its chromosomes, ensuring each new cell receives a complete set of genetic instructions. This division allows multicellular organisms to grow and replace old or damaged cells. For single-celled organisms, mitosis is their method of reproduction.
Preparing for Division: Interphase
Before mitosis, a cell spends most of its existence in interphase. During interphase, the cell grows and carries out its normal metabolic functions. A crucial event is the S phase, where the cell’s DNA is replicated, resulting in two identical copies of each chromosome, known as sister chromatids. These sister chromatids remain attached at the centromere. The cell also produces organelles and proteins, accumulating resources for division.
Prophase
Prophase marks the initial stage of nuclear division. During this phase, chromatin coils and condenses into compact, visible chromosomes. Each condensed chromosome is composed of two identical sister chromatids joined at their centromeres. The nucleolus disappears, and the nuclear envelope breaks down. Concurrently, the mitotic spindle begins to form in the cytoplasm, composed of microtubules extending from centrosomes that moved to opposite poles.
Metaphase
Following prophase, the cell enters metaphase, characterized by the alignment of chromosomes. The nuclear envelope has completely disintegrated, allowing the mitotic spindle to interact with the chromosomes. Microtubules from the spindle attach to kinetochores at the centromere of each sister chromatid. These spindle fibers pull the chromosomes to align along a central plane, known as the metaphase plate. This meticulous arrangement ensures each daughter cell receives an identical and complete set of chromosomes.
Anaphase
Anaphase is a short stage where duplicated genetic material is separated. This phase begins with the splitting of centromeres that hold sister chromatids together; once separated, each chromatid is considered an individual chromosome. The mitotic spindle fibers attached to these newly independent chromosomes begin to shorten. This shortening action, along with cell elongation, pulls the chromosomes towards opposite poles. By the end of anaphase, an identical set of chromosomes has arrived at each pole.
Telophase and Cytokinesis
Telophase marks the completion of nuclear division, followed by cytokinesis, the division of the cytoplasm. As chromosomes arrive at opposite poles, they decondense, returning to their chromatin form. New nuclear envelopes reform around each set of chromosomes, creating two distinct nuclei. The nucleoli reappear, and the mitotic spindle disassembles.
Immediately following telophase, cytokinesis commences, dividing the cytoplasm and its contents. In animal cells, cytokinesis occurs through a cleavage furrow, an indentation formed by a contractile ring that pinches the cell membrane inward until the cell is cleaved into two daughter cells. In contrast, plant cells undergo cytokinesis differently. Golgi vesicles gather at the former metaphase plate and fuse to form a cell plate, which grows outward from the center towards the existing cell walls, eventually developing into a new cell wall and plasma membrane. The culmination of telophase and cytokinesis results in two genetically identical daughter cells.