Final answer:
Pyrimidine dimers are formed when adjacent pyrimidines on a DNA strand dimerize due to exposure to UV light. These dimers can cause problems during DNA replication but are repaired by mechanisms like nucleotide excision repair.
Step-by-step explanation:
Pyrimidine dimers are formed when adjacent pyrimidines, most commonly thymines, on a DNA strand dimerize due to exposure to UV light. These dimers can cause distortions in the structure of the DNA double helix, leading to problems during DNA replication. However, mechanisms, such as nucleotide excision repair, have evolved to repair these lesions by removing the pyrimidine dimer and replacing it with the correct nucleotides.