Mitosis is a fundamental biological process through which a single parent cell divides into two genetically identical daughter cells. It enables organisms to grow from a single cell into complex multicellular beings. It also continuously maintains body tissues and replaces old or damaged cells throughout an organism’s lifespan.
The Purpose of Mitosis in the Body
Mitosis serves several functions within the body. One primary purpose is the growth of an organism. A human, for example, begins as a single fertilized egg cell that undergoes countless mitotic divisions to form trillions of cells, leading to the development of all body parts. This process continuously increases the total number of cells, allowing for overall body growth.
Mitosis also repairs and regenerates damaged tissues. When skin is cut, new skin cells are generated through mitotic division to close the wound. Similarly, the liver can regenerate a significant portion of its tissue after injury or surgical removal due to mitotic activity.
Beyond growth and repair, mitosis constantly replaces cells that have reached the end of their lifespan. Many cells in the body, such as those lining the digestive tract, have a rapid turnover rate. Red blood cells, responsible for oxygen transport, are replaced approximately every 100 to 120 days through the division of precursor cells in the bone marrow.
The Stages of Mitosis
Mitosis is part of the cell cycle, which begins with a preparatory phase. This preparatory phase, known as Interphase, is where the cell grows, performs its normal functions, and duplicates its DNA. Each chromosome is copied, resulting in two identical sister chromatids that remain joined, preparing the cell for division.
Following Interphase, mitosis itself begins with Prophase. During Prophase, the duplicated chromosomes coil tightly and condense, becoming visible as distinct structures under a microscope. The nuclear envelope, the membrane surrounding the cell’s genetic material, starts to break down into small fragments. Specialized structures called centrosomes move towards opposite poles of the cell, organizing the cellular machinery for division.
Next is Metaphase, where the highly condensed chromosomes align precisely along the cell’s central plane, often referred to as the metaphase plate. Microtubules, which are protein filaments forming the mitotic spindle, extend from the centrosomes and attach to specific regions on each sister chromatid called kinetochores. This meticulous alignment ensures that each new cell will receive an accurate and complete set of chromosomes.
Anaphase follows with a separation of the sister chromatids. The connections holding the sister chromatids together break, and the microtubules shorten, pulling the individual chromosomes towards opposite ends of the cell. This synchronized movement ensures that an identical set of chromosomes migrates to each pole, leading to the formation of two new nuclei.
Telophase marks the formation of two new nuclei. Once the separated chromosomes arrive at the opposite poles of the cell, they begin to decondense and uncoil. New nuclear envelopes form around each set of chromosomes, creating two distinct nuclei within the original cell. The mitotic spindle disassembles as nuclear structures are re-established.
The final step, often overlapping with Telophase, is Cytokinesis, which involves the division of the cell’s cytoplasm. In animal cells, a contractile ring of actin and myosin proteins forms around the cell’s equator and pinches inward, forming a cleavage furrow that deepens until the cell is divided into two separate, genetically identical daughter cells. Plant cells, with their rigid cell walls, form a new cell wall plate between the two nuclei to accomplish this separation.
Regulating the Process
The cell cycle, including mitosis, is tightly controlled by a system of internal checkpoints. These checkpoints ensure that each step of the cell division process is completed accurately and only proceeds when conditions are favorable. They verify that the cell’s genetic material is intact and duplicated before division continues.
One significant checkpoint occurs in the G1 phase, prior to DNA replication. At this point, the cell assesses its environment, checking for resources and signs of DNA damage. If any damage is detected or conditions are unfavorable, the cell can pause its progression to allow for repair or to enter a quiescent state, preventing the replication of flawed genetic material.
A second checkpoint is found at the end of the G2 phase, just before the cell enters mitosis. This checkpoint confirms that DNA replication has been completed without errors and that the cell has grown to a sufficient size. It serves as a safeguard to ensure that the cell’s genetic information is complete and ready for distribution to daughter cells.
A third checkpoint operates during Metaphase of mitosis. This “spindle assembly checkpoint” ensures that all chromosomes are correctly aligned at the metaphase plate and that their kinetochores are properly attached to the mitotic spindle microtubules. This prevents premature separation of chromosomes and ensures that each daughter cell receives the correct number of chromosomes.
Consequences of Mitotic Errors
When the control systems governing mitosis fail, issues can arise, impacting cellular and organismal health. Uncontrolled proliferation of cells is a defining characteristic of cancer. Cancer cells often bypass the normal checkpoints, leading to unchecked division and the formation of tumors. This unregulated growth can allow cells to accumulate additional genetic mutations, potentially increasing their ability to invade surrounding tissues or spread.
Errors during the separation of chromosomes, known as non-disjunction, can result in daughter cells having an incorrect number of chromosomes, termed aneuploidy. For example, Down syndrome is caused by an extra copy of chromosome 21, resulting from a mitotic error during development.
Aneuploidy can also lead to developmental disorders or trigger cell death. Many cells with an abnormal chromosome count are eliminated by the body’s quality control mechanisms. However, those that persist can contribute to health problems depending on which chromosomes are affected and the extent of the chromosomal imbalance.