This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Molecular Genetics, Subject name: Biochemistry
Xeroderma pigmentosa is due to: 
Select and explain. 
A. Base excision defect, B. Nucleotide excision defect, C. Sos repair defect, D. Cross linking defect
B: Nucleotide excision defect
Answer: B: Nucleotide excision defect 
Explanation: Ans: b (Nucleotide excision defect) Ref: Vasudevan, 4th ed, p. 403; 5th ed, p. 421Steps in nucleotide excision repair:Repair: Excision endonuclease makes nick on both sides of thymine dimer and removes defective DNA. DNA polymerase fill the gap by synthesising DNA in 5'-3' direction. DNA Ligase seals the nick in the repaired strand .Most DNA repair occurs in G1 phase. Mismatch repair occurs in G2 phase.