Final answer:
Pyrimidine dimers, which disrupt DNA replication and can lead to mutations, are primarily caused by UV radiation. Nucleotide excision repair is a cellular mechanism that repairs such DNA damage.
Step-by-step explanation:
Pyrimidine dimers are caused by UV radiation. When DNA is exposed to UV light, adjacent pyrimidines on a DNA strand, commonly thymines, can form pyrimidine dimers. This kind of DNA damage hinders DNA replication and transcription, and can lead to mutations if the dimers are not repaired. Cells have mechanisms such as nucleotide excision repair to fix these errors, but individuals with conditions like xeroderma pigmentosa lack the ability to repair this DNA damage, increasing their risk for skin cancer.