Mitosis is a fundamental process of cell division where one cell divides to produce two new, genetically identical daughter cells. This mechanism is central to the life of multicellular organisms. It is a form of division that occurs in body cells, also known as somatic cells, but not in sperm or egg cells. The process ensures each new cell receives a complete set of chromosomes.
The Cell Cycle Context
Cellular division is part of a larger, highly regulated series of events known as the cell cycle. Mitosis itself represents only one portion of this cycle, referred to as the M phase. The majority of a cell’s life is spent in a preparatory period called interphase, which precedes mitosis. This phase is subdivided into three distinct stages: Gap 1 (G1), Synthesis (S), and Gap 2 (G2).
The G1 phase is a period of significant activity where the cell grows physically larger and synthesizes the proteins and organelles needed for its functions. Following G1, the cell enters the S phase, where it duplicates its entire set of DNA. At the end of this phase, each chromosome consists of two identical sister chromatids. The final G2 phase involves continued growth and final preparations for division.
The Purpose of Mitosis
The primary function of mitosis is to facilitate growth and development. A multicellular organism begins as a single fertilized egg, and it is through countless rounds of mitosis that it develops into a complex being with specialized tissues and organs. This process of creating new cells allows an organism to increase in size and complexity throughout its life.
Another function is tissue repair and the replacement of old or damaged cells. Our bodies are in a constant state of renewal; for example, skin cells and blood cells are continuously replaced. When an injury occurs, such as a cut, mitosis is activated to produce new cells to patch the wound. This ongoing replacement is necessary for maintaining the health of tissues and organs.
Mitosis also serves as a method of asexual reproduction for many single-celled eukaryotic organisms like yeast. In this process, a single parent organism divides to produce offspring that are genetically identical to itself. This is the sole method of reproduction for these organisms and is responsible for sustaining their populations.
Phases of Mitosis
The process of mitosis is a continuous sequence, conventionally divided into four stages: prophase, metaphase, anaphase, and telophase. Prophase marks the beginning of mitosis, where chromatin condenses into distinct, visible chromosomes. The nuclear envelope begins to break down, and the mitotic spindle starts to form from centrosomes, which move toward opposite ends of the cell.
Following prophase, the cell enters metaphase. During this stage, the mitotic spindle is fully developed, and the chromosomes align along the cell’s equator, an imaginary line called the metaphase plate. Microtubules from the spindle fibers attach to a protein structure on each sister chromatid called the kinetochore. This precise alignment ensures the chromosomes are positioned for equal separation.
Anaphase is a dynamic stage characterized by the separation of the sister chromatids. The proteins holding the sister chromatids together are broken down, allowing them to be pulled apart by shortening spindle fibers. Now considered individual chromosomes, they are drawn toward opposite poles of the cell. The cell itself also begins to elongate, preparing it for final division.
The final stage of nuclear division is telophase, where the separated chromosomes arrive at the opposite poles of the elongated cell. Here, the chromosomes begin to decondense and return to their less compact form. A new nuclear envelope re-forms around each set of chromosomes, creating two distinct nuclei within the single cell as the mitotic spindle is disassembled.
Completing the cell division process is cytokinesis, the physical division of the cytoplasm. This process begins during late anaphase or telophase but concludes after mitosis is complete. In animal cells, a contractile ring pinches the cell membrane inward, forming a cleavage furrow that deepens until the cell splits. In plant cells, a rigid cell wall prevents this, so a structure called a cell plate forms down the middle to create a new separating wall.
Regulation and Control
The cell cycle is not an automatic process; it is carefully governed by a complex internal control system. This system relies on checkpoints that act as internal surveillance mechanisms, ensuring each phase is properly completed before the next one begins. There are three major checkpoints: in the G1 phase, at the G2/M transition, and during metaphase. These checkpoints monitor for issues like DNA damage and can halt the cycle to allow for repairs.
Progression through these checkpoints is driven by specific proteins. Two key groups of these regulatory molecules are cyclins and cyclin-dependent kinases (CDKs). CDKs are enzymes that, when activated, trigger the events of each phase. However, CDKs are inactive on their own and require a partner protein, a cyclin, to function.
The concentrations of different cyclins rise and fall in a predictable pattern throughout the cell cycle. For example, M cyclin levels build up during G2 and peak in mitosis, driving events like nuclear envelope breakdown. When a specific cyclin binds to its corresponding CDK, the resulting complex pushes the cell past a checkpoint. Once the cell moves to the next stage, the cyclins from the previous stage are degraded, ensuring the cycle proceeds in one direction.
Consequences of Errors
The precise regulation of mitosis is important for maintaining genomic stability. When control mechanisms and checkpoints fail, errors in chromosome segregation can occur. This leads to the production of daughter cells with an incorrect number of chromosomes, a condition known as aneuploidy. Most aneuploidies during embryonic development are lethal, but those later in life are linked to diseases.
The most significant consequence of uncontrolled cell division is cancer. Cancer is a disease characterized by the failure of cell cycle regulation, where cells divide without restraint. Errors during mitosis are a major source of the numerical and structural chromosome alterations seen in tumor cells. Mutations in genes that control the cell cycle can allow cells with DNA damage to bypass checkpoints and continue to proliferate, leading to tumor formation.
Beyond cancer, the missegregation of chromosomes can directly cause DNA damage, promoting further genomic instability. This ongoing instability can allow cells to evolve and adapt, which may contribute to tumor progression and resistance to therapy. The presence of aneuploid cells is a hallmark of nearly 70% of solid human tumors, highlighting the profound impact of mitotic errors on human health.