hce_nchu
113年
普通生物及生化概論
第 33 題
A 10-year-old girl is brought by her parents to the dermatologist. She has many freckles on her face, neck, arms, and hands, and the parents report that she is unusually sensitive to sunlight. Two basal cell carcinomas are identified on her face. Based on the clinical picture, which of the following processes is most likely to be defective in this patient?
- A Removal of pyrimidine dimers from DNA by nucleotide excision repair
- B Removal of mismatched bases from the $3'$-end of Okazaki fragments by a methyl-directed process
- C Repair of double-strand breaks by error-prone homologous recombination
- D Removal of uracil from DNA by base excision repair
思路引導 VIP
當皮膚細胞暴露在強烈的紫外線下時，你認為 DNA 分子的化學構造最常發生哪一種特定的「物理形變」或異常共價鍵結？而細胞通常會動用哪一套具備「局部切割與重新合成」功能的工具箱，來修正這種造成螺旋扭曲的損害呢？
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AI 詳解
AI 專屬家教
恭喜你答對了！你能精準地從臨床病徵——特別是孩童時期就出現的基底細胞癌 (Basal cell carcinoma)、廣泛的雀斑以及對陽光高度敏感——正確判斷出這是典型的著色性乾皮症 (Xeroderma Pigmentosum, XP)，這展現了你對臨床生化與遺傳學的高度敏銳度。
紫外線損傷與 NER 修復機制
在正常生理情況下，陽光中的紫外線會導致 DNA 鏈上相鄰的嘧啶（如 Thymine）形成異常的共價鍵，產生嘧啶二聚體 (Pyrimidine dimers)。健康的細胞會透過核苷酸切除修復 (Nucleotide Excision Repair, NER) 機制，將這段受損並造成螺旋扭曲的序列精確切除並重新合成。然而，XP 患者因為參與 NER 途徑的特定蛋白（如 XP 系列蛋白）發生突變失能，導致 DNA 損傷無法被修復而持續累積，最終引發細胞癌變。
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