Final answer:
Nucleotide dimers are formed when adjacent pyrimidine bases dimerize due to UV light exposure. Thymine dimers can cause mutations during DNA replication and transcription. They can be repaired through nucleotide excision repair.
Step-by-step explanation:
Nucleotide dimers are formed when adjacent pyrimidine bases, typically thymines, on a DNA strand dimerize due to exposure to UV light. This can lead to frameshift or point mutations during DNA replication and transcription.
Thymine dimers can be repaired through nucleotide excision repair, where enzymes recognize the distortion caused by the dimer and replace the damaged strand with the correct bases.