Genetic mutations are changes in the DNA sequence that makes up a person’s genetic material. They can happen in a single DNA letter, affect a larger stretch of a chromosome, or involve changes to the number or structure of chromosomes.
Mutations are a normal part of biology. Most have little or no noticeable effect, some can be harmful, and a smaller number can be beneficial. Mutations also provide the genetic variation on which evolution depends. Understanding what causes them requires looking at how DNA is copied, repaired, and exposed to environmental influences.
DNA copying errors are a major source of mutations
Every time a cell divides, it must copy its DNA so that the new cells receive genetic instructions. This copying process is remarkably accurate, but it is not perfect.
DNA is made from four chemical bases—adenine (A), thymine (T), cytosine (C), and guanine (G). During replication, enzymes build a new DNA strand by matching bases with the appropriate partners. Occasionally, the replication machinery inserts the wrong base, skips a base, or adds an extra one.
Cells have proofreading and DNA-repair systems that detect and correct many of these mistakes. When an error escapes those systems, it can become a permanent mutation after the DNA is copied again.
Some DNA sequences are particularly difficult for the replication machinery to copy accurately. Repeated stretches of DNA, for example, can cause the copying machinery to slip, producing insertions or deletions—changes in which DNA letters are added or removed.
Mutations can also result from DNA damage
DNA is chemically stable enough to store genetic information but can still be damaged by normal cellular activity and outside influences.
One important internal source is the chemical activity of metabolism. Cells use oxygen and other molecules in countless reactions, and some of these reactions can produce chemically reactive molecules. These substances can alter DNA bases or damage the DNA molecule itself.
DNA can also be damaged by spontaneous chemical changes. For example, a DNA base can occasionally lose or change part of its chemical structure. If the resulting damage is not correctly repaired before DNA replication, it can produce a mutation.
Cells continually monitor DNA for this kind of damage. Specialized repair mechanisms can remove damaged sections, replace incorrect bases, or repair breaks in the DNA molecule. Mutations are more likely to persist when damage is repaired incorrectly or escapes repair altogether.
Environmental factors can increase DNA damage
Certain environmental exposures can damage DNA and increase the likelihood of mutations. These are sometimes called mutagens.
Ultraviolet radiation from sunlight can damage DNA in skin cells. Some forms of radiation have enough energy to damage DNA directly or generate molecular changes that interfere with it. Certain chemicals can also react with DNA or disrupt the processes that copy and repair it.
Tobacco smoke is an important example because it contains chemicals capable of damaging DNA. Some of these substances can form chemical attachments to DNA, creating lesions that may lead to mutations if they are not repaired correctly.
Not every exposure to a mutagen produces a mutation, however. The outcome depends on factors such as the amount and duration of exposure, the type of DNA damage, and how effectively the affected cell repairs that damage.
Mutations can occur during chromosome division
Not all genetic mutations involve individual DNA bases. Errors can also occur when chromosomes are copied or separated during cell division.
Chromosomes are long structures containing DNA and associated proteins. Before a cell divides, its chromosomes are duplicated, and the copies must be distributed accurately to the daughter cells. Errors in this process can produce cells with missing or extra chromosomes.
A well-known example is trisomy, in which a cell has three copies of a particular chromosome instead of the usual two. Down syndrome, for instance, is most commonly caused by an extra copy of chromosome 21.
Chromosomes can also undergo larger structural changes. A piece of a chromosome may be deleted, duplicated, inverted, or moved to another chromosome. Such changes can alter genes or their regulation, sometimes affecting health.
Mutations can arise in different kinds of cells
Where a mutation occurs matters because it determines whether the change can be passed to future generations.
A somatic mutation occurs in a body cell other than an egg or sperm cell. It remains within the cell lineage descended from that cell. Somatic mutations are not ordinarily passed from a parent to a child, but they can accumulate during a person’s lifetime.
Cancer is one important consequence of somatic mutations. When mutations affect genes that control cell growth, division, or survival, a cell can acquire characteristics that allow it to multiply abnormally. Cancer usually develops through the accumulation of multiple genetic changes rather than from a single mutation.
A germline mutation occurs in an egg, sperm, or the cells that give rise to them. If such a mutation is present in an egg or sperm involved in conception, it can become part of the child’s genetic makeup and potentially be inherited by later generations.
Some genetic conditions are caused by inherited mutations, while others result from mutations that arise in an egg, sperm, or very early embryo without being present in either parent’s other body cells.
Mutations can happen before birth as well as during adulthood
Mutation is not simply something that happens as people age. DNA changes can occur throughout development.
When a fertilized egg divides, its DNA must be copied repeatedly as the embryo develops. A mutation that occurs early can therefore be inherited by many descendant cells. A mutation that occurs later may be limited to a smaller group of cells.
This can produce mosaicism, a situation in which a person’s body contains genetically distinct populations of cells that originated from the same fertilized egg. The effects depend on which cells carry the mutation and what the altered gene does.
Mutations also continue to arise throughout life as cells divide and encounter DNA damage. The number and distribution of mutations therefore vary among tissues and between individuals.
What determines whether a mutation is harmful?
A mutation is not inherently harmful simply because the DNA sequence has changed. Its effect depends on where the change occurs and what it does.
A mutation in a region that has little functional importance may have no detectable effect. A change within a gene may alter the resulting protein, but even then, the effect can range from negligible to severe. Some changes alter a protein’s structure or amount; others prevent a functional protein from being produced.
Mutations can also affect gene regulation, changing when, where, or how strongly a gene is active without altering the protein-coding sequence itself.
The same type of mutation can have different consequences depending on the gene involved. A change that disrupts one gene may cause disease, while a similar change in another part of the genome may have little biological significance.
Some mutations are beneficial under particular conditions. Genetic variation can sometimes give an organism an advantage in its environment, allowing the associated genetic variant to become more common over generations.
Not every genetic difference is a new mutation
People naturally differ from one another at many positions in their DNA. These differences are called genetic variants. A variant may be inherited from a parent, arise as a new mutation, or have been passed through a population for many generations.
The word mutation is often used broadly to mean any change in DNA, but in medicine and genetics, researchers frequently use more specific terms such as variant when describing a DNA difference. A genetic variant is not automatically a disease-causing change.
This distinction matters because most genetic variation is not harmful. Determining whether a particular variant affects health requires considering its location, biological effect, inheritance pattern, and other evidence.
Why mutations persist despite DNA repair
Cells have several layers of protection against genetic errors. DNA polymerases can proofread newly copied DNA, specialized repair systems can correct different kinds of damage, and cells can sometimes eliminate severely damaged cells.
These defenses greatly reduce the number of mutations that persist. They cannot eliminate every error, though. Some damage is difficult to detect, some repair processes introduce their own errors, and some mutations arise in ways that escape correction.
Over time, mutations can therefore accumulate in individual cells. In cells that divide repeatedly, each new mutation can be inherited by descendant cells, creating a record of the cell’s history.
Mutations are essential to evolution
Although mutations are often discussed because of their role in genetic disease, they are also fundamental to life’s long-term diversity.
A mutation creates a new genetic variant. If that variant occurs in reproductive cells and is passed to offspring, it can enter a population’s gene pool. Natural selection, genetic drift, and other evolutionary processes can then influence how common the variant becomes.
Most new mutations do not produce a major advantage or disadvantage. Some are harmful and tend to become less common under natural selection; others can provide an advantage in a particular environment. Across many generations, this continual introduction of genetic variation helps populations change and adapt.
Genetic mutations, in other words, are not simply mistakes in DNA. They are an unavoidable consequence of maintaining and reproducing genetic information—and a fundamental source of both genetic disease and biological diversity.
