Cell division is the foundational biological process by which a parent cell divides to form two or more daughter cells. Within the cells of organisms, there are two primary types of division: mitosis and meiosis. Mitosis produces genetically identical cells and is responsible for growth and tissue maintenance, while meiosis produces reproductive cells with a halved chromosome count.
Facilitating Growth and Development
Cell division, specifically through mitosis, drives the physical growth of a multicellular organism from its earliest stage. Life begins as a single fertilized egg, or zygote, which must rapidly divide to generate the trillions of cells that make up a mature body. The organism increases in size by dramatically increasing its cell number, not by cells simply getting larger. The controlled, sequential division of these genetically identical cells allows for organogenesis, the formation of distinct organs and complex tissue systems.
Controlled cell proliferation is necessary to build the specialized structures that give an organism its final form and function. For instance, the lengthening of bones and the expansion of the brain during childhood rely entirely on regulated mitotic events. This cell-number increase continues until an organism reaches maturity, at which point the rate of new cell production balances the rate of cell death.
Sustaining the Body Through Repair and Replacement
Beyond initial development, cell division maintains the body through continuous tissue maintenance and repair. This constant renewal, known as cellular turnover, is a dynamic process where old and worn-out cells are systematically replaced by new, healthy duplicates via mitosis. The human body produces approximately 330 billion new cells every day, with the majority of this activity concentrated in tissues facing high friction or environmental exposure.
For example, the epithelial cells lining the small intestine must be replaced every three to five days due to constant exposure to digestive enzymes and abrasive food particles. Blood cells also account for nearly 90% of the total daily cell turnover. Mitosis is also activated in response to acute damage, such as a cut or scrape, where cells near the injury site divide rapidly to fill the gap and restore the tissue’s protective barrier.
The Foundation of Sexual Reproduction
Meiosis enables sexual reproduction and generates genetic diversity. This process is restricted to the germline cells, producing specialized reproductive cells called gametes, such as sperm and egg cells. Unlike mitosis, meiosis involves two rounds of division, resulting in four daughter cells, each containing a haploid state (one set of chromosomes). This reduction is necessary for the stability of a species.
If chromosome numbers were not halved in the gametes, the fusion of two gametes during fertilization would result in an offspring with double the correct number of chromosomes. This doubling would lead to an exponential increase in chromosome number across generations, a state incompatible with life. Meiosis also introduces genetic variation through a precise event called crossing over, which occurs early in the process. During crossing over, homologous chromosomes—one inherited from each parent—physically exchange segments of DNA.
This exchange results in chromosomes that are a unique blend of maternal and paternal genetic material, creating novel combinations of alleles. The resulting gametes are genetically distinct from one another, ensuring that no two offspring are exactly alike, even from the same parents. This mechanism of genetic shuffling provides the variation that allows a species to adapt and survive in changing environments.
When Control is Lost: Cell Division and Disease
The tight regulation of cell division is essential, as losing control leads to disease, most notably cancer. Cancer is fundamentally a disease of uncontrolled mitotic cell division, where cells ignore the signals that normally halt proliferation. This failure leads to an abnormal and rapid increase in cell number, overwhelming the body’s normal functions. Errors during mitosis are a major source of genomic instability, often resulting in daughter cells with an incorrect number of chromosomes, a condition known as aneuploidy.
Faults in the meiotic process also lead to serious health conditions. If chromosomes fail to separate correctly during meiosis, a phenomenon called non-disjunction occurs. This leads to gametes that are missing a chromosome or have an extra one. When such an abnormal gamete is fertilized, the resulting embryo has an incorrect chromosome number, a common cause of birth defects, such as Down Syndrome, which is caused by the presence of an extra copy of chromosome 21.