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Short answer
Ultraviolet light damages DNA mainly by being absorbed by neighbouring pyrimidine bases (thymine and cytosine) on the same strand, which then bond to each other. The two main products are cyclobutane pyrimidine dimers (CPDs) and pyrimidine (6-4) pyrimidone photoproducts (6-4PPs). Both distort the helix and block replication and transcription unless the cell repairs them first. Unrepaired lesions can become mutations.
What UV bands reach DNA
The same answer applies whether you search for how UV rays, UV light, UV radiation or ultraviolet light damage DNA: the damage depends on the wavelength band, not on the name used for it. Two peer-reviewed reviews (listed under Sources) define three bands:
- UVC is absorbed directly by DNA, but it does not reach the earth’s surface because the ozone layer blocks it.
- UVB (280 to 315 nm) is absorbed directly by DNA and is the most mutagenic UV that reaches the ground.
- UVA also produces thymine dimers, plus oxidative damage through reactive oxygen species (ROS).
Why sunlight is mostly UVA yet UVB drives mutation
About 95% of the UV reaching the surface is UVA and about 5% is UVB. UVB still dominates mutation because DNA absorbs it far more efficiently than UVA (one review cites about tenfold). Photon count and DNA absorption are different quantities, which is the source of a common misreading of the 95/5 split.
Lesions UV leaves in DNA
The table compares the three lesion classes.
| Lesion | Formed by | Frequency and consequence | Repair |
|---|---|---|---|
| Cyclobutane pyrimidine dimer (CPD) | Direct absorption of UVB and UVC | Thymine-thymine CPDs form about ten times more often than thymine-thymine 6-4PPs; CPDs account for about 80% of UVB mutations | Removed more slowly |
| Pyrimidine (6-4) pyrimidone photoproduct (6-4PP) | Direct absorption of UVB and UVC | Less frequent than CPDs and less mutagenic | Removed with high efficiency |
| UVA-associated damage | Thymine dimers plus oxidative damage via ROS | Weaker at causing DNA damage; whether UVA damages DNA directly or only through oxidative routes is still debated | Thymine dimers are repaired as above |
The pairing of frequency and repair speed is the useful point. CPDs are the more common and more slowly removed lesion, and they are responsible for about 80% of UVB mutations.
How cells repair UV lesions
Human cells remove CPDs and 6-4PPs by nucleotide excision repair (NER). The review by Shah and He describes two entry routes: global genome NER (GG-NER), which is mainly responsible for removing CPDs and 6-4PPs from non-transcribed regions, and transcription-coupled NER (TC-NER), which handles lesions in transcribed regions. The later repair steps are shared.
Photolyases, enzymes that reverse these lesions directly and more efficiently, are absent in humans. Defective NER shows what happens when this system fails: xeroderma pigmentosum carries a high skin cancer risk, and Cockayne syndrome causes developmental and neurological problems and a shortened lifespan.
Where it matters in labs
The same chemistry is used deliberately when the goal is to kill organisms: the ultraviolet sterilization guide covers how UV-C forms pyrimidine dimers in microbial DNA and what that does and does not achieve for medical devices. In the other direction, DNA damage assays can measure UV lesions: the comet assay guide lists T4 endonuclease V as the enzyme used to reveal UV dimers. The gel imager guide explains the difference between UV and blue-light imaging of gels.
Common misunderstandings
UVC from the sun damages skin
No. The ozone layer blocks UVC, so none of it reaches the earth’s surface from the sun.
UVA is harmless because it is weaker
UVA is weaker at causing DNA damage, but it still produces thymine dimers and oxidative damage, and it makes up most of the UV that reaches the surface.
Only one kind of lesion matters
Both CPDs and 6-4PPs form, but they differ in how often they form, how fast they are repaired and how mutagenic they are, as the table shows.
FAQ
How does UV light damage DNA?
Adjacent pyrimidine bases absorb UV and bond together, forming CPDs and 6-4PPs that distort the helix and block replication and transcription.
How do UV rays damage DNA differently from UV radiation in general?
They do not differ; UV rays, UV light and UV radiation are the same thing. The wavelength band (UVA, UVB, UVC) determines which lesions form.
Which UV band is most damaging to DNA at ground level?
UVB. Although UVA is about 95% of the UV at the surface, DNA absorbs UVB far more efficiently, and it is the most mutagenic terrestrial UV.
Can cells repair UV damage?
Yes. Human cells use nucleotide excision repair; they lack photolyases. People with defective NER, as in xeroderma pigmentosum, have a high skin cancer risk.
Sources
Checked 2026-10-29.
- Shah P, He Y-Y. Molecular Regulation of UV-Induced DNA Repair. Photochemistry and Photobiology, 2015. https://pmc.ncbi.nlm.nih.gov/articles/PMC4355264
- Budden T, Bowden NA. The Role of Altered Nucleotide Excision Repair and UVB-Induced DNA Damage in Melanomagenesis. Int J Mol Sci 2013;14(1):1132-1151, doi:10.3390/ijms14011132. https://pmc.ncbi.nlm.nih.gov/articles/PMC3565312
