Final answer:
Nucleotide excision repair is a cellular mechanism used to repair thymine dimers, which are formed when thymines adjacent on a DNA strand bond due to UV exposure. Thymine dimers are excised and replaced with correct nucleotides by normal cells.
Step-by-step explanation:
Nucleotide excision repair is often employed when UV exposure causes the formation of thymine dimers. This kind of DNA damage occurs when thymines lying adjacent to each other on a DNA strand are covalently bonded due to UV light exposure. In a process critical for maintaining the integrity of the genome, these erroneous structures are recognized and removed by nucleotide excision repair mechanisms. Normal cells have the capacity to excise these dimers and then replace them with the correct nucleotides, thereby correcting the DNA sequence and preventing potential mutations that could lead to cancer, such as in the condition known as xeroderma pigmentosum.