DNA carries its own sunscreen — and it may explain a choice life made billions of years ago

Why does DNA use the letter thymine, when a simpler letter would do? Chemists studying how the building blocks of early genetic material react to ultraviolet light have found that thymine-based pairs act a little like molecular sunscreen, shedding damaging UV energy in ways that protect the code itself. The peer-reviewed findings, published in the Proceedings of the National Academy of Sciences, may also lead to faster tests for germs and better checks that UV disinfection of water has really worked.