<prompt> Xeroderma pigmentosa is due to:Base excision defectNucleotide excision defectSos repair defectCross linking defect  <response>: 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. 