Mitosis is a fundamental biological process where a single cell divides to produce two identical daughter cells. This process is essential for growth, allowing organisms to increase in size from a single fertilized egg to a fully developed organism. It's also crucial for repair, replacing damaged or dead cells throughout our lives. Some organisms even use mitosis for asexual reproduction. During mitosis, the parent cell's genetic material, organized into structures called chromosomes, is precisely duplicated and distributed equally to ensure each daughter cell receives an identical copy of the DNA.
The cell cycle is a highly regulated process that cells go through as they grow and divide. It consists of four main phases. G1 phase is the longest phase where the cell grows in size and carries out its normal functions. During S phase, the cell replicates its DNA, creating identical copies of each chromosome. G2 phase is a preparation period where the cell continues to grow and produces proteins necessary for chromosome condensation and mitosis. Finally, M phase is mitosis itself, where the cell actually divides. The entire cycle typically takes about 24 hours in human cells, with mitosis being the shortest phase at only about one hour.
Before mitosis can begin, the cell's genetic material must be properly organized and prepared. During most of the cell's life, DNA exists as chromatin - long, thin, thread-like structures that are loosely packed in the nucleus. However, as the cell prepares for division, this chromatin undergoes a dramatic transformation called condensation. The DNA coils and folds tightly to form visible structures called chromosomes. Each chromosome consists of two identical copies called sister chromatids, which are joined together at a specialized region called the centromere. This joining ensures that the genetic material stays together until it's time to separate. In human cells, this process creates 46 visible chromosomes, organized as 23 pairs, each containing the complete genetic blueprint needed for life.
The first phase of mitosis is prophase, where dramatic changes occur within the cell. The nuclear envelope, which normally protects the chromosomes, begins to break down and disappear. Simultaneously, the chromosomes continue condensing into their most compact form, making them visible under a microscope. Two important structures called centrosomes, which contain the cell's microtubule organizing centers, move to opposite sides of the cell and begin forming spindle fibers. These spindle fibers will be crucial for moving chromosomes. Next comes metaphase, where the chromosomes align perfectly at the cell's equator, forming what we call the metaphase plate. The spindle fibers attach to specialized protein structures called kinetochores at each chromosome's centromere. The cell has an important checkpoint here - it will not proceed until every single chromosome is properly attached to spindle fibers from both sides, ensuring accurate chromosome distribution.
Anaphase marks the point of no return in mitosis. The sister chromatids, which have been held together at their centromeres, suddenly separate and begin moving toward opposite poles of the cell. This movement is driven by the shortening of spindle fibers attached to the chromatids, while other spindle fibers elongate to push the poles apart, stretching the entire cell. This ensures that each future daughter cell will receive exactly one copy of every chromosome. Following anaphase comes telophase, where the cell begins to return to its interphase appearance. New nuclear envelopes form around each set of chromosomes at opposite ends of the cell, effectively creating two separate nuclei. The chromosomes start to decondense back into chromatin, and the spindle apparatus begins to disappear. Simultaneously, cytokinesis begins with the formation of a contractile ring made of actin and myosin proteins at the cell's equator, preparing to physically divide the cell into two separate daughter cells.