# Texas A&M: thymine degrades faster than uracil under 265 nm UV but forms fewer irreversible lesions

- Published: 2026-11-14 21:11 UTC
- Section: Life & Health › Biology & biochemistry
- Tags: thymine vs uracil, DNA photostability, early Earth UV, 265 nm irradiation, cyclobutane pyrimidine dimer, (6-4) photoproduct, nonenzymatic repair, Raman spectroscopy, origin of DNA
- URL: https://realtech.media/news/rt-bb530b68c622f00d66e7ad0f_texas_a_m/

**In single-base experiments at 265 nm, thymine was damaged faster than uracil but steered damage toward reversible lesions. The authors link this to DNA's choice of thymine on early Earth.**

![Texas A&M: thymine degrades faster than uracil under 265 nm UV but forms fewer irreversible lesions](https://realtech.media/media/2026/11/8608be8354840baf.jpg)
*Earth.com*

A team at Texas A&M University led by Peter Rentzepis, with Keyvan Khosh Abady and Negar Karpourazar, compared thymine and uracil under controlled 265 nm ultraviolet light. According to a study in Proceedings of the National Academy of Sciences, thymine is damaged faster, but it directs the damage toward lesions that can be reversed [1]. The experiment used isolated nucleobases, not strands of DNA or RNA.

## Why thymine looked like a poor choice

DNA uses thymine, while RNA uses uracil; thymine is uracil with one added carbon group. Early Earth had no ozone layer, so intense ultraviolet light reached the surface, and thymine appeared to be the more vulnerable base [1].

Thymine absorbs most strongly at 265 nm and uracil at 259 nm. Models of the early Sun put the light at thymine's peak at about 2.5 times the intensity at uracil's peak, and thymine's absorption band is about 9 percent wider [1]. The 2.5-fold figure is modeled and depends on the assumed atmosphere.

Using steady-state absorption, fluorescence and Raman spectroscopy, the team found that thymine is inherently more photoreactive. Judged by how fast its absorption faded, it degraded nearly three times faster than uracil and fluoresced about 4.5 times more brightly. The authors read the glow as a sign that excited thymine lingers longer before relaxing, leaving more time for reactions [1].

## Where the damage goes

Each base was dissolved in water, frozen at -15°C in quartz cells and exposed for an hour to a 265 nm LED. Freezing pushes molecules into crowded pockets, which imitates neighboring bases in a strand, where the main lesions form [1].

Ultraviolet light produces two main lesions between neighbors. The cyclobutane pyrimidine dimer is reversible and can come apart without enzymes. The (6-4) photoproduct rearranges the bases permanently. In the fitted curves, thymine formed the (6-4) product at 1.56 per minute against 2.04 for uracil, so its damage went mainly into the reversible pathway [1].

The authors conclude that the canonical bases appear to have been selected not to minimize damage but to keep it localized and reversible, since repair enzymes did not yet exist [1]. "In that sense, thymine may have acted as a molecular sunscreen," Khosh Abady said.

The authors note that single bases in ice do not capture the full chemistry of real genetic molecules. They add that the measured spectral signatures before and after damage could help read ultraviolet damage in whole cells, for example to check whether a water-treatment lamp has inactivated microbes.

## Sources

1. [Proceedings of the National Academy of Sciences](https://doi.org/10.1073/pnas.2615278123)
2. [Earth.com](https://www.earth.com/science/scientists-learn-why-a-sensitive-part-of-our-dna-is-actually-a-built-in-shield-against-the-sun)

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Source: RealTech, realtech.media — https://realtech.media/news/rt-bb530b68c622f00d66e7ad0f_texas_a_m/