Mitosis

Cell division is the process by which one cell becomes two new cells. These cells may or may not be identical in nature. In multicellular organisms, cell division plays many roles and is the reason complex organisms such as humans are able to grow from a single, fertilized egg cell. Cell division continues throughout the lifetime of an organism, functioning in tissue growth, repair, and maintenance. Cell division is a major component of the cell cycle, the life of a cell, from its beginning to the time when it divides to produce a new cell. The phases of cell division can be superimposed over the phases of the cell cycle.

Most of the lifetime of a cell is spent growing. When it is time to divide, the cell replicates its genetic material, segregates the copies to opposite ends of the cell, and then pinches down the middle, forming two new cells. Each new cell is identical to the original cell, with a full complement of genetic material. The process of cell division that results in two cells that are genetic clones of the parent cell is called mitosis and occurs in most types of cells. The resulting daughter cells are diploid, which is the genetic state of a cell containing the full complement of chromosomes from its parent cells (2n). In sexually reproducing organisms, like humans, a different process, called meiosis, creates the sex cells (eggs and sperm). In meiosis, the cells resulting from division contain only half the person's genetic material. These cells are known as haploid. This allows daughter cells to combine with sex cells from another parent, forming a new organism with unique genetic material.

The cell cycle consists of four distinct phases: the G₁ phase, the first phase of the cell cycle, when a cell grows; the S phase, the second phase of the cell cycle, when DNA is replicated; the G₂ phase, the third phase of the cell cycle, when a cell grows more; and the M phase, the fourth phase of the cell cycle, when a cell divides. The G₁, S, and G₂ phases of the cell cycle are collectively known as interphase. Interphase is characterized by cell growth and DNA replication, while M is the phase of the cell cycle in which the cell divides to make two new cells. Some cells, such as mature nerve or muscle cells, actually leave the cell cycle and remain at rest until they die. The amount of time spent in any given phase of the cell cycle is dependent on the type of cell and the environmental conditions in which the cell lives. Mitosis is cellular division that occurs in somatic (nonreproductive) cells that are growing or repairing a multicellular organism. Mitosis begins after the G₂ phase. Mitosis consists of five distinct steps, followed by cytokinesis, the pinching off of the cytoplasm to form two new cells. DNA and other cellular structures are visible with a light microscope during mitosis, so the steps of mitosis have been understood since the late 19th century. These steps occur only in eukaryotic cells. Prophase—the first phase of mitosis, sister chromatids (strands that result from the duplication of DNA) condense, mitotic spindles (cytoskeletal structures that form during cell division to separate the sister chromatids) begin to form, and centrosomes (the structures that coordinate the formation of microtubules) segregate to opposite poles. The mitotic spindle helps align the sister chromatids correctly for proper cell division, ensuring each daughter cell gets one copy.

Prometaphase—the second phase of mitosis, in which the nuclear membrane breaks down and spindle fibers attach to the centromere (region of a chromosome where microtubules of the spindles attach). Specifically, the microtubules of the spindles attach to a group of proteins associated with the centromere called the kinetochore. The spindle fibers tug the chromosomes back and forth as they position them correctly for the next stage.

Metaphase—the third stage of mitosis, in which the sister chromatids line up along the cell equator. The chromosomes no longer move back and forth but stay aligned along the equator.

Anaphase—the fourth phase of mitosis, in which sister chromatids separate and are pulled to the opposite poles of the cell. This leads to separation of the sister chromatids into individual chromosomes. Each chromosome is still attached to a centrosome by spindle fibers and is pulled to opposite cell poles.

Telophase—the final stage of mitosis when the chromosomes arrive at their respective poles. Vesicles reassemble into a new nuclear membrane surrounding the DNA of each daughter cell. At the end of telophase, the cell has two distinct nuclei and mitosis is complete.

Mitosis is then followed by cytokinesis, or the pinching off of the cytoplasm to form two new cells. In human cells, a cleavage furrow, the indentation in a cell along which cytokinesis occurs, begins to form along the cell equator during telophase. This final cell division marks the end of M phase of the cell cycle. At this point, each cell resumes the G₁ phase or exits the cell cycle completely if the cell will no longer divide.