A change in the sequence of bases in DNA is called a mutation. Your DNA is built from four chemical bases, often abbreviated A, T, C, and G, and their specific order spells out instructions for building proteins. When even a single base is swapped, added, or removed, the meaning of those instructions can change, sometimes with no effect at all and sometimes with serious consequences for how a cell functions.
How DNA’s Base Sequence Works
Cells read DNA in groups of three bases at a time. Each three-letter group, called a codon, corresponds to one of 20 amino acids, the building blocks of proteins. A protein might be hundreds of amino acids long, so the reading order matters enormously. Think of it like a sentence made entirely of three-letter words: if you change a letter, add one, or remove one, the meaning of every word that follows can shift.
Types of Base Sequence Changes
Not all mutations work the same way. The simplest is a substitution, where one base is swapped for another. For example, an A might be replaced by a G. At the chemical level, substitutions come in two flavors. Transitions swap bases that have a similar molecular shape (A with G, or C with T). Transversions swap bases with different shapes, exchanging a larger two-ring structure for a smaller one-ring structure or vice versa. Transitions are more common because the similar shapes make them easier errors for the cell’s copying machinery to make.
A single-base substitution can have three very different outcomes depending on where it lands:
- Missense mutation: The new codon codes for a different amino acid. The protein still gets built to full length, but with one wrong component. This may slightly alter the protein’s function or, in some cases, break it entirely.
- Silent mutation: The new codon happens to code for the same amino acid as the original. The protein comes out identical, and the organism is unaffected. This is possible because multiple codons can specify the same amino acid.
- Nonsense mutation: The new codon becomes a “stop” signal, telling the cell to quit building the protein early. The resulting incomplete protein usually can’t do its job.
Insertions, Deletions, and Frameshifts
Beyond single-base swaps, bases can also be inserted into or deleted from the sequence. If the number of bases added or removed is not a multiple of three, the entire reading frame shifts. Every codon downstream of the change gets misread, producing a completely garbled protein. This is called a frameshift mutation, and it’s typically far more damaging than a single substitution because it doesn’t just affect one amino acid; it corrupts the entire remaining sequence. The result is often a shortened, nonfunctional protein.
If exactly three bases (or a multiple of three) are inserted or deleted, the reading frame stays intact. The protein gains or loses one or more amino acids, but the rest of the sequence is still read correctly. These mutations can still cause problems, but they tend to be less catastrophic than a full frameshift.
What Causes These Changes
Mutations arise from both internal and external sources. Internally, the most common cause is simple copying errors. Every time a cell divides, it duplicates all 3 billion base pairs of your DNA, and mistakes happen. The human mutation rate is roughly one error per 100 million base pairs per generation, which works out to somewhere between 0.1 and 1 new mutation per genome each time a cell copies itself. Reactive oxygen molecules produced during normal metabolism can also damage bases directly, as can spontaneous chemical reactions like the loss of a chemical group from a base.
External sources, called mutagens, include ultraviolet radiation from sunlight (which causes neighboring bases to bond abnormally), ionizing radiation from X-rays or radioactive materials (which can break DNA strands outright), and certain chemicals found in cigarette smoke, industrial compounds, or even some chemotherapy drugs. These agents physically alter the structure of DNA bases, making them more likely to be misread during the next round of copying.
How Cells Fix Mistakes
Your cells have a sophisticated proofreading and repair system that catches and corrects the vast majority of errors. One key system, called mismatch repair, scans newly copied DNA for bases that don’t pair correctly. It recognizes the error, cuts out the section of the new strand containing the wrong base, and resynthesizes it using the original strand as a template. This system alone improves copying accuracy by 100 to 1,000 times. Other repair pathways handle damage from UV light and chemical mutagens by recognizing the distorted shape of damaged DNA and replacing the affected section.
Mutations that survive all these checkpoints become permanent parts of that cell’s DNA. Whether they matter depends largely on where in the genome they occur. A mutation in a stretch of DNA that doesn’t code for a critical protein may have no noticeable effect. A mutation in a gene essential for controlling cell growth, on the other hand, can contribute to cancer.
Germline vs. Somatic Mutations
Where a mutation occurs in the body determines whether it can be inherited. Germline mutations happen in egg or sperm cells. Because these are the cells that combine to form an embryo, a germline mutation gets copied into every cell of the resulting child and can be passed on to future generations. Many inherited genetic conditions, from sickle cell disease to cystic fibrosis, trace back to germline mutations.
Somatic mutations happen in any other cell in the body after conception. They affect only the cell where they occur and its descendants, not the whole organism. You can’t pass a somatic mutation to your children. Most cancers, for instance, arise from somatic mutations that accumulate in a specific tissue over a person’s lifetime. These mutations weren’t inherited and won’t appear in the next generation.
When Mutations Matter and When They Don’t
The word “mutation” often sounds alarming, but most base-sequence changes are harmless. Silent mutations produce no change in protein structure. Mutations in non-coding regions of DNA (which make up the vast majority of the genome) frequently have no functional impact. Even missense mutations sometimes swap in an amino acid similar enough in size and chemistry that the protein works fine.
Harmful mutations are the ones that disrupt proteins critical to cell survival, growth control, or organ function. Nonsense mutations and frameshifts tend to be the most damaging because they produce truncated or entirely garbled proteins. Over long timescales, though, the slow accumulation of mutations that aren’t harmful is the raw material for evolution, gradually introducing genetic variation that natural selection can act on.