Final answer:
UV light's most well-characterized mutagenic effect on DNA is creating thymine dimers, where two adjacent thymine bases bond, potentially causing harmful mutations.
Step-by-step explanation:
The best characterized mutagenic DNA lesion induced by UV light is the formation of thymine dimers. This lesion occurs when adjacent thymine bases on the same strand of DNA bond with each other following exposure to UV light, particularly UV-B. This mutagenic effect can cause bulges in the DNA structure, leading to errors in DNA replication and transcription, potentially resulting in lethal mutations if left unrepaired. While organisms have mechanisms such as nucleotide excision repair to fix such damage, humans lack the specific enzyme (photolyase) that other species may use to directly reverse this type of damage through photoreactivation.