Cell division is one of the most carefully controlled processes in the human body. It allows a fertilized egg to develop into a body made of trillions of cells, replaces cells that wear out, and helps tissues repair themselves. To keep the body healthy, cells must divide at the right time, copy their DNA accurately, and distribute the copied chromosomes correctly.
When that control breaks down, the consequences can range from a single abnormal cell that is eliminated without causing harm to serious genetic disorders, tissue damage, or cancer. What happens depends on which part of cell division goes wrong, when the error occurs, and whether the cell can detect and correct the problem.
How normal cell division is controlled
Most human cells that divide go through the cell cycle, a sequence of stages that prepares a cell to make two daughter cells. Before division, the cell grows and copies its DNA so that each daughter cell can receive a complete set of genetic instructions.
During mitosis, the duplicated chromosomes are organized and separated. The cell then divides its cytoplasm, producing two daughter cells that normally contain the same number and essentially the same genetic information as the original cell.
This process is tightly regulated. Cells have molecular checkpoints that monitor whether DNA has been copied correctly and whether chromosomes are properly attached and positioned for separation. If a serious problem is detected, the cell may pause division while repairs are attempted. If the damage cannot be repaired, the cell may undergo apoptosis, a controlled form of cell death.
These safeguards are important because uncontrolled or inaccurate cell division can allow abnormal cells to survive and multiply.
What can go wrong during cell division?
Errors can occur at several points in the process. Some involve the DNA itself; others affect how chromosomes are copied, moved, or separated.
DNA can be copied incorrectly
Before a cell divides, it must duplicate its DNA. Although DNA replication is highly accurate, it is not perfect. A copying error can produce a mutation, meaning a change in the DNA sequence.
Many mutations have little or no effect. Some occur in DNA that does not substantially alter cell behavior, while others are corrected by cellular repair systems. But a mutation that affects a gene involved in cell growth, DNA repair, or cell death can have more serious consequences.
A particularly important problem occurs when mutations accumulate in genes that normally restrain cell division or help detect cellular damage. A cell can gradually lose the safeguards that keep its growth under control.
Chromosomes can fail to separate correctly
DNA is packaged into chromosomes. During mitosis, chromosomes must be distributed accurately between the two daughter cells.
If chromosomes do not separate properly, one daughter cell may receive an extra chromosome while another receives too few. This condition is called aneuploidy.
Chromosome-number errors are especially important during the formation of eggs and sperm, when cells undergo a different type of division called meiosis. If chromosomes fail to separate properly during meiosis, an egg or sperm can receive an abnormal number of chromosomes. If that cell contributes to fertilization, the resulting embryo may have an abnormal chromosome number in its cells.
One well-known example is Down syndrome, which usually results from having three copies of chromosome 21 rather than two.
Why some division errors are harmless
Not every mistake in cell division causes disease. The body has several layers of protection against abnormal cells.
A damaged cell may repair its DNA. A cell that cannot be repaired may stop dividing or undergo apoptosis. Cells with severe chromosome abnormalities may also fail to survive.
In addition, many mutations never affect the behavior of a cell in a meaningful way. A mutation matters most when it changes something important about how the cell functions, particularly its ability to grow, divide, repair damage, or respond to signals from neighboring cells.
The location of an error also matters. An abnormality in a single skin cell may have very different consequences from one affecting a developing embryo, a blood-forming cell, or a cell in an essential organ.
When cell division goes wrong during development
Cell division is especially important during embryonic development because a developing organism must repeatedly produce new cells while preserving the correct genetic information.
A chromosome-separation error early in development can affect many descendant cells. If it occurs after some normal divisions have already taken place, only a portion of the body’s cells may carry the abnormality. This is known as mosaicism.
The effects of a chromosome abnormality therefore depend partly on how early it occurs and which tissues inherit the affected cells. Some abnormalities are incompatible with continued development and can result in miscarriage. Others can produce congenital conditions that persist throughout life.
Cell division errors can also contribute to developmental abnormalities when mutations arise in genes that control how cells grow, specialize, or organize into tissues.
When abnormal cell division leads to cancer
Cancer is one of the most important consequences of disrupted cell-division control.
Normal cells respond to signals that tell them when to grow and divide. They also have mechanisms that prevent damaged cells from continuing to multiply. Cancer can develop when genetic changes disrupt these controls, allowing a cell to divide when it should not and survive when it should be eliminated.
Cancer usually does not result from one simple mistake. Instead, cancer cells typically acquire multiple changes over time. Some changes activate genes that promote growth, while others disable genes that normally suppress excessive growth or help repair damaged DNA.
As abnormal cells continue dividing, additional genetic changes can accumulate. A population of cells may eventually acquire characteristics that allow it to form a tumor, invade nearby tissues, or spread to distant parts of the body.
Not every tumor is cancerous. Benign tumors can grow because their cells divide excessively, but they generally do not invade surrounding tissues or spread to distant parts of the body in the way malignant tumors can.
What happens when cells divide too little?
Problems can also arise when cells fail to divide when they should.
Many tissues depend on a steady supply of new cells. The lining of the intestine, for example, is continually renewed, while bone marrow must continually produce blood cells. If cell division is severely impaired, damaged or aging cells may not be replaced adequately.
Some inherited disorders interfere with DNA replication or repair and can make it difficult for cells to divide normally. Treatments such as radiation and certain chemotherapy drugs can also interfere with cell division. This is one reason these treatments can affect healthy tissues as well as cancer cells: rapidly dividing normal cells may also be vulnerable.
Meiosis is different—and its errors have different effects
Not all cell division produces ordinary body cells.
Meiosis is the specialized form of cell division that produces eggs and sperm. It reduces the chromosome number by half so that fertilization can restore the usual chromosome number.
Meiosis involves two rounds of division and requires chromosomes to be separated in a carefully coordinated way. Errors can produce eggs or sperm with too many or too few chromosomes.
These errors can affect fertility, pregnancy, and the chromosome makeup of an embryo. Unlike a typical mutation that changes a DNA sequence, a meiotic error can alter the number or distribution of entire chromosomes.
How cells protect themselves from division errors
The body relies on several mechanisms to limit the consequences of faulty cell division.
DNA repair systems identify and correct many forms of DNA damage or copying errors. Cell-cycle checkpoints can stop a cell from proceeding when important problems are detected. Apoptosis can remove cells that are too damaged or abnormal to remain safely in the body.
There is also a broader tissue-level system of communication. Cells receive signals from neighboring cells and their surroundings that influence whether they grow and divide. A healthy tissue therefore depends not only on each cell’s internal machinery but also on communication between cells.
These safeguards are powerful but not infallible. DNA damage can escape repair, checkpoints can become defective, and abnormal cells can acquire changes that help them evade normal controls.
The consequences depend on the kind of error
A mistake in cell division is not automatically a disease. The outcome depends on what went wrong and what happens afterward.
A small DNA-copying error may have no noticeable effect. A severe chromosome-separation error may prevent a cell from surviving. A mistake during early development can affect many tissues. And a series of mutations that disables normal growth controls can contribute to cancer.
In other words, cell division goes wrong in several fundamentally different ways. The important question is not simply whether an error occurred, but whether the error changes the cell’s genetic information, its chromosome number, its ability to control growth, or its ability to survive and function normally.
