Xeroderma Pigmentosum

Nucleotide excision repair

Xeroderma pigmentosum (XP) is a rare autosomal recessive disease characterized by photosensitivity, pigmentary changes, premature skin ageing, neoplasia and abnormal Nucleotide excision repair. 

Xeroderma pigmentosum (XP) is an autosomal recessive disease with sun sensitivity, photophobia, early onset of lentigines and freckling, and subsequent neoplastic changes on sun-exposed surfaces.

There is cellular hypersensitivity to UV radiation and to certain chemicals.

Xeroderma pigmentosm cells are defective in Nucleotide Excision Repair. In NER, the wrong base is removed by endonuclease. This option is not given here. So other enzymes involved in the NER are DNA polymerase I & DNA ligase.

Ref: Harper’s Biochemistry, 26th Edition, Page 337 ; Text book of Biochemistry By D M Vasudevan, 5th Edition, Pages 420, 421

A i.e. Formation of thymidine dimmers

A, B & C i.e. Xeroderma pigmentatosa, Bloom’s syndrome & Ataxia telangiectasia

Ataxia telangiectasia, Bloom’s syndrome, Fanconi’s anemia, Xeroderma pigmentosa, hereditary non polyposis colon cancer and few breast cancers are due to DNA repair defects.

Ans. is ‘a’ i.e., Xeroderma pigmentosa

o Defective DNA repair syndromes are : Xeroderma pigmentosa, Bloom syndrome, Fanconi syndrome and ataxia telongectisia.

Ans. is ‘b’ i.e., Nucleotide Excision Repair

o Exposure to UV rays damages DNA due to production of convalent linkages between adjacent pyrimidines. Normally the damaged DNA is repaired by excision and repair. In xeroderma pigmentosa the repair of UV damaged DNA is defective.

B i.e. Xeroderma pigmentosa

A i.e. Xeroderma pigmentosa

D i.e. Rheumatic fever