Mitosis is a biological process where a single cell divides into two genetically identical daughter cells. This duplication of cellular components allows organisms to grow, develop, and repair damaged tissues. It provides new cells for growth and replaces worn-out cells, ensuring the continuation of life in multicellular organisms and serving as a form of reproduction for some single-celled organisms.
The Cell Cycle’s Preparatory Phase
Before cell division, a cell undergoes interphase, a preparatory period accounting for approximately 90% of its life. Interphase is divided into three stages: G1, S, and G2. During the G1 phase, the cell grows, synthesizes proteins, and accumulates building blocks for DNA and energy reserves.
Following G1, the cell enters the S phase, or “synthesis.” Here, the cell’s entire DNA is replicated, resulting in two identical copies of each chromosome, known as sister chromatids. These sister chromatids remain attached at a constricted region called the centromere. The final stage of interphase, G2, involves further growth, replenishment of energy stores, and synthesis of proteins needed for cell division.
The Core Stages of Nuclear Division
The mitotic phase, or M phase, involves the steps of nuclear division, ensuring replicated chromosomes are accurately separated into two new nuclei. This process is divided into four main stages: prophase, metaphase, anaphase, and telophase. Each stage involves coordinated actions of chromosomes, microtubules, and associated proteins.
Prophase marks the beginning of visible changes within the cell’s nucleus. During this stage, chromatin condenses to form distinct, visible chromosomes. The nuclear envelope begins to break down. Centrosomes, duplicated during interphase, migrate to opposite ends of the cell, initiating the formation of the mitotic spindle.
As the cell transitions into prometaphase, chromosomes continue to condense. Specialized protein structures called kinetochores assemble at the centromere of each sister chromatid. These kinetochores serve as attachment points for kinetochore microtubules, which extend from the centrosomes at opposite poles of the cell.
Metaphase is characterized by the alignment of all chromosomes along a central plane of the cell, known as the metaphase plate. The mitotic spindle is fully formed, with centrosomes positioned at opposite poles. Each sister chromatid is attached to spindle fibers originating from opposite poles. This alignment is monitored by a spindle checkpoint, which delays progression until all chromosomes are correctly attached and aligned.
Anaphase begins with the separation of sister chromatids. The proteins holding the sister chromatids together at their centromeres break down, allowing them to separate. Once separated, each chromatid is considered an individual chromosome. These newly independent chromosomes are pulled by shortening kinetochore microtubules towards opposite poles of the cell. Non-kinetochore microtubules also lengthen, contributing to the elongation of the entire cell.
Telophase represents the final stage of nuclear division, where the separated chromosomes arrive at their respective poles. At each pole, the chromosomes begin to decondense. A new nuclear envelope reassembles around each set of chromosomes, creating two distinct daughter nuclei. The mitotic spindle disassembles, and nucleoli reappear within the newly formed nuclei.
Dividing the Cell
Following nuclear division, the cell undergoes cytokinesis, the physical division of the cytoplasm to form two separate daughter cells. This process overlaps with telophase.
In animal cells, cytokinesis involves the formation of a contractile ring composed of actin filaments. This ring assembles at the former metaphase plate and contracts, creating a cleavage furrow. The furrow deepens until the cell pinches into two.
Plant cells, with their rigid cell walls, employ a different mechanism. Golgi vesicles are transported to the metaphase plate region. These vesicles fuse to form a structure called a phragmoplast, which then develops into a cell plate. The cell plate grows outwards, eventually fusing with the existing cell walls and plasma membranes, dividing the plant cell into two.
The Outcomes of Accurate Mitosis
The completion of mitosis results in the creation of two daughter cells that are genetically identical to the original parent cell. This genetic fidelity ensures each new cell receives a complete set of genetic instructions. Errors in this process can lead to cells with an incorrect number of chromosomes, potentially contributing to diseases like cancer.
Mitosis plays a role in multicellular organisms, supporting their growth and development by increasing cell number. It is also important for tissue repair, replacing damaged or old cells. For many single-celled eukaryotes, mitotic divisions serve as their primary means of asexual reproduction.