A COMMON MUTATION

Thymine, one of the four DNA building blocks, has a powerful chemical attraction to others of its own kind. Sometimes, as DNA is replicated, two adjacent thymines bases will bond as in the illustration, forming a "thymine dimer", and creating a glitch in the linear genetic information code. Thymine dimers are also formed by the interaction of ultraviolet light with DNA molecules – a good reason to limit exposure to bright sunlight. Fortunately, cells contain "proof reading enzymes" that scour the DNA, looking for thymine dimers to excise and replace, generally keeping the code accurate and up to date.