UV radiation damages DNA by causing neighboring bases on the DNA strand to fuse together, creating structural distortions that interfere with normal cell replication. A single hour of midday sun exposure can generate an estimated 100,000 to 200,000 DNA lesions in each skin cell. These lesions, if left unrepaired, can lead to mutations that accumulate over time and drive the development of skin cancer.
How UV Light Alters DNA Structure
DNA is built from four chemical bases, two of which, cytosine and thymine, belong to a class called pyrimidines. When UVB radiation (the shorter, more energetic wavelengths of sunlight) hits DNA, it transfers enough energy to make two pyrimidine bases sitting next to each other on the same strand form a chemical bond that shouldn’t be there. This bond fuses them into a single, bulky unit that warps the shape of the DNA helix.
About 75% of these fused units are cyclobutane pyrimidine dimers, where a four-carbon ring locks the two bases together. The remaining 25% are a second type of lesion where the bases bond in a slightly different configuration. Both types block the cell’s machinery from reading DNA correctly, but they behave differently afterward. The less common type is repaired relatively quickly, with a half-life of about 2 hours. Pyrimidine dimers, the more frequent type, linger much longer, with a half-life around 33 hours, giving them more time to cause problems during DNA replication.
UVA vs. UVB: Two Paths to Damage
UVB radiation causes the most direct harm to DNA because the DNA molecule itself absorbs UVB photons. That absorbed energy is what triggers the fusion of neighboring bases. UVA radiation, which makes up the majority of the UV light reaching Earth’s surface, works differently. UVA photons are lower in energy and aren’t absorbed as efficiently by DNA. Instead, they interact with other light-absorbing molecules in the cell, which then generate reactive oxygen species, essentially unstable oxygen molecules that attack DNA indirectly.
This oxidative damage tends to target a specific DNA base called guanine, converting it into a modified form. While this type of lesion is real, studies of skin cancer genomes show that the vast majority of sunlight-induced mutations trace back to pyrimidine dimers rather than oxidative damage. UVA and UVB also reach different depths in the skin. UVB penetrates only about 20 micrometers, barely past the outermost layer. UVA reaches roughly 50 to 60 micrometers on most of the body and even deeper on thicker skin like the palms, allowing it to affect cells that UVB cannot reach.
Damage That Keeps Forming After You Go Inside
One surprising discovery is that UV exposure keeps generating DNA lesions even hours after you’re out of the sun. In pigmented skin cells (melanocytes), UV light activates enzymes that produce nitric oxide and superoxide radicals. These radicals combine to form a powerful oxidant called peroxynitrite, which then excites the pigment melanin to a high-energy state. That excited melanin transfers energy directly to DNA, creating new pyrimidine dimers in complete darkness. This means the DNA damage from a day at the beach doesn’t stop accumulating the moment you step inside.
The UV Mutation Signature
When a cell tries to copy DNA that contains a pyrimidine dimer, it often makes a characteristic mistake: it reads a cytosine as if it were a thymine. This creates a C-to-T transition mutation that acts as a fingerprint of UV damage. More than two-thirds of mutations found in skin cancer genomes are these C-to-T transitions, and over 90% of them occur at sites where two pyrimidines sit side by side, exactly where dimers form. An even more distinctive signature is a tandem CC-to-TT double mutation, where both bases in a pair are changed at once. This double substitution is strongly associated with UVB exposure and is rarely caused by anything else.
How Your Cells Fix the Damage
Humans rely on a repair system called nucleotide excision repair to fix UV-induced DNA lesions. Many other organisms, from bacteria to fish to reptiles, have a simpler option: light-activated enzymes called photolyases that directly reverse pyrimidine dimers using visible light. Mammals lost these enzymes over the course of evolution, leaving nucleotide excision repair as the primary defense.
This repair system works in two modes. The first scans the entire genome for distortions in the DNA helix. Sensor proteins detect the bulge caused by a dimer, then recruit a team of proteins that pry open the double helix around the damage site. Specialized cutting enzymes snip out a short stretch of the damaged strand, and a DNA-copying enzyme fills in the gap using the undamaged strand as a template. The second mode is triggered when the cell’s gene-reading machinery physically runs into a lesion and stalls. This version prioritizes repairing genes that are actively being used, ensuring the most critical parts of the genome get fixed first.
Both modes use the same core machinery for the actual cut-and-replace step. The difference is how the damage gets flagged in the first place.
What Happens When Repair Fails
The clearest demonstration of what UV damage does when repair breaks down is xeroderma pigmentosum, a rare inherited condition in which one of the proteins in the nucleotide excision repair system is defective. People with this condition are extremely sensitive to sunlight and develop skin cancers at a dramatically elevated rate, often in childhood. Eight different gene defects can cause the condition, each disabling a different step in the repair process. Some forms affect only the skin, while others also cause neurological deterioration, depending on which repair protein is missing.
The most common form in the United States involves a defect in the protein responsible for initially sensing DNA damage. Without that sensor, lesions pile up across the genome with every sun exposure, and the mutations that result drive rapid tumor development. About 30% of cases involve a different gene that handles a backup copying process, allowing cells to replicate past damaged DNA without introducing errors. When this backup system fails, normal replication stalls at every lesion, forcing the cell to use error-prone workarounds that introduce mutations.
In people with functional repair systems, the sheer volume of damage from UV exposure, potentially hundreds of thousands of lesions per cell per hour, means some lesions inevitably slip through. Over decades of cumulative sun exposure, these unrepaired lesions accumulate into the mutational burden that underlies most skin cancers.