Question

individuals with the disorder xeroderma pigmentosum are hypersensitive to sunlight. what function is impaired in their cells?

○ they cannot replicate dna.

○ they cannot undergo mitosis.

○ they cannot exchange dna with other cells.

○ they cannot repair thymine dimers.

Explanation:

Xeroderma pigmentosum (XP) is a genetic disorder where cells have impaired DNA repair mechanisms, specifically the ability to repair DNA damage caused by ultraviolet (UV) light from sunlight. UV light causes thymine dimers (covalent bonds between adjacent thymine bases in DNA). In XP, the nucleotide excision repair pathway (which fixes thymine dimers) is defective.

• "They cannot replicate DNA" is incorrect because XP cells can still replicate DNA (though errors may accumulate).
• "They cannot undergo mitosis" is wrong as mitosis is not directly blocked by XP; the issue is DNA repair, not cell division itself.
• "They cannot exchange DNA with other cells" is irrelevant, as XP is about DNA repair, not horizontal gene transfer or DNA exchange.
• "They cannot repair thymine dimers" is correct because the key impairment in XP is the inability to repair UV - induced thymine dimers, leading to hypersensitivity to sunlight (more DNA damage accumulates, causing skin issues and cancer risk).

Answer:

D. They cannot repair thymine dimers.