Background

Why DNA chose thymine, which is chemically similar to uracil but more susceptible to ultraviolet (UV) radiation, as the base for storing genetic information has long been a question in the biological community. Early Earth was a much harsher environment, with stronger UV radiation directly hitting the surface. At that time, organisms must have had the ability to protect themselves from physicochemical stimuli in order to stably preserve genetic information and pass it on to the next generation. However, the reason why thymine, which is more easily structurally altered when absorbing UV light, was chosen as a key component remains a mystery. Previous studies have focused only on the genetic benefits of reducing replication errors, but have not clearly explained the paradox of photochemical reactivity that occurs in strong UV exposure.

Key Findings

The research team, led by Professor Peter Lenzcepes of the Department of Chemistry at Texas A&M University, used a combination of steady-state absorption spectroscopy, fluorescence spectroscopy, and Raman spectroscopy to precisely track the UV reaction mechanisms of thymine and uracil at the molecular level. The analysis revealed that thymine has physical properties that strongly absorb sunlight UV radiation over a wider wavelength range than uracil. It was also confirmed that the rate of molecular structural change, or photoreaction, when exposed to UV energy is much faster in thymine than in uracil. If only the index of damage frequency is used as a criterion, thymine should be classified as an unsuitable substance that impairs the stability of genetic information.

However, the research team found the answer by identifying the type and structural characteristics of the final compounds produced when the molecule absorbs UV energy, beyond the frequency of molecular damage. According to precise optical analysis, uracil forms (6-4) photoproducts, which are irreversible bonds that are extremely difficult to restore to their original state, at a high rate when absorbing UV light, causing the molecular structure to bend. In contrast, thymine induces a chemical reaction that mainly forms cyclobutane pyrimidine dimers (CPDs) when exposed to UV damage. CPDs are compounds in which the double bond between the two bases breaks and a ring structure is formed, but the binding energy within the molecule is relatively unstable, so it is easily restored to its original state by changes in the surrounding environment or slight thermal energy. Thymine is frequently altered by UV light, but it induces damage in a form that is easily self-repaired, effectively protecting against permanent loss of genetic information.

Significance and Prospects

This study shows that early life evolved DNA bases not to simply avoid UV absorption, but to maximize the ease of post-repair. Thanks to the unique photochemical reaction of thymine, which localizes damage into a reversible form even when absorbing UV energy, the genetic material of early Earth was able to stably preserve the original form of genetic information even in the harsh cosmic radiation of the primitive sun. It is believed that this is the result of evolutionary selection pressure, in which substances that are easy to restore, even if they are frequently damaged, are much more advantageous for long-term survival than substances that are chemically inactive and appear to be safe. However, this experiment was conducted on a single nucleotide in an isolated solution state within a controlled physical device, so it does not perfectly reproduce the physicochemical interactions between bases that occur in a real, three-dimensional, dynamic double helix. Further research is needed to elucidate the effects of the three-dimensional chromatin structure and double helix structure on the efficiency of this photochemical reversible pathway.

This discovery opens the way for direct application to the design of biological therapeutics using artificial nucleic acids and the development of biosensors for extreme environments. It is suitable for use as a molecular design guideline for precisely controlling the UV sensitivity and self-repair efficiency of base sequences when designing artificial genomes. In particular, it is expected to provide useful clues for the development of molecular-level blocking devices or protective materials that reduce genetic damage in environments that are constantly exposed to cosmic radiation or strong UV radiation. Furthermore, it can also be applied to delivery technologies that help nucleic acid materials, which play a role as carriers in gene therapy, to safely reach target cells without being altered by external stimuli.